Effects of high CO2 levels on growth and calcification of
the Caribbean coral Porites porites (Pallas, 1766)
Rossana Martins de Freitas
Masters in Marine Sciences – Marine Resources
Porto, 2011
UNIVERSIDADE DO PORTO
INSTITUTO DE CIÊNCIAS BIOMÉDICAS ABEL SALAZAR
Effects of high CO2 levels on growth and calcification of the
Caribbean coral Porites porites (Pallas, 1766)
Rossana Martins de Freitas
Dissertation application to the
Master‟s Degree in Marine Sciences
– Marine Resources submitted to
Instituto de Ciências Biomédicas de
Abel Salazar of Universidade do
Porto
Master‟s thesis co-ordinate by:
Prof. Dr. Eduardo Rocha (Instituto de
Ciências Biomédicas Abel Salazar –
Universidade do Porto)
Prof. Dr. Chris Langdon (Rosenstiel
School of Marine and Atmospheric
Sciences – University of Miami)
Investigation done in Landgon Lab,
Rosenstiel School of Marine &
Atmospheric Science – University of
Miami
Abstract
Ocean acidification is the increase in acidity (decrease in pH) of the ocean‟s
surface waters as result from oceanic uptake of atmospheric carbon dioxide (CO 2). The
concentration of this gas is projected to reach twice the preindustrial level by the middle of
the 21st century. In response to this increase, saturation state of surface seawater is
decreasing, causing concern for a global reduction in rates of reef accretion. This also
represents a major threat to coral reefs by reducing the calcification rate of framework
builders.
Here is proposed a study of the effects of high CO2 concentrations on skeletal
growth of the hermatypic coral Porites porites. An experiment was conducted using coral
nubbins at three CO2 levels: control (390 ppm), and two high CO2 scenarios, even if a little
bit higher than the projected ones to this century (1350 and 6000 ppm). In order to see
the effects of ocean acidification on these corals, growth rate was calculated every week
through two different methods: Buoyant Weight and Linear Extension. The Chlorophyll a
concentration and the Zooxanthellar density were also determined.
A decrease in the growth rate was observed for the higher values of CO 2 in the
end of the experiment. For the Buoyant Weight, the values recorded ranged from 17.7 ±
0.7 for the low level treatment (390 ppm) to 4.0 ± 1.7 mg d -1 cm-2 for the high level
treatment (6000 ppm). For the Linear Extension, the same answer was observed, i.e., a
decrease in the growth rate for the higher levels of CO2. For the low level treatment, the
growth rate was 98.5 ± 7.2 and for the high level this was 15.6 ± 2.7 µm d-1.
As the Chlorophyll a concentration and Zooxanthellar density, high levels of CO2
showed a decline both in numbers of zooxanthellae cells and chlorophyll per unit surface
area, as expected. Also, after 7 weeks of experiment, the corals that were in high CO 2
(6000 ppm) start to bleach, losing their pigments and/or their symbiotic algae.
Considering the results obtained, it can be concluded that Ocean acidification
affects the growth and the health of the hermatypic coral Porites porites. An estimate was
made using values of carbon dioxide concentration of 550 and 950 ppm using the AR4
best and worst CO2 projections for the years 2050 and 2100 respectively, based on
scenarios B1 and A1F1 projected by the Intergovernmental Panel on Climate Change
(IPCC 2007). Growth through buoyant weight is projected to decline by 18% by 2050 and
36% by 2100. The amount of Chla per cm2 of coral tissue is projected to decline 17% by
2050 and 33% by 2100. It is reasonable to expect that these changes will make this
2
species less competitive on the reef and will likely lead to a decline in its abundance on
Caribbean coral reefs.
Little is known about how acidification impacts the physiology of reef builders and
how acidification interacts with warming. So, the next step is to focus on the interaction
between temperature and saturation state, as accurate predictions of global change have
to include the effects of both on coral growth.
Keywords: Porites porites; Ocean acidification; Calcification; Growth rate; Climate
change.
3
Resumo
Acidificação dos oceanos é o aumento da acidez (diminuição do pH) das águas de
superfície dos oceanos como resultado da absorção oceânica do dióxido de carbono
atmosférico (CO2). Projecções feitas dizem que a concentração deste gás alcançará o
dobro do nível pré-industrial até meados do século 21. Em resposta a este aumento, o
estado de saturação da água do mar está a diminuir, causando sérias preocupações em
relação a uma redução nas taxas de acrecção de recifes. Isso representa também uma
grande ameaça para os recifes de coral pois causa uma redução na taxa de calcificação
dos coráis construtores de esqueletos.
O presente estudo foi realizado com o intuito de analisar os efeitos do dióxido de
carbono no crescimento do esqueleto do coral hermatípico Porites porites, quando este
(CO2) se encontra presente na água do mar em elevadas concentrações. Para isso, foi
conduzida uma experiência utilizando “nubbins” de coral e colocando estes em três
diferentes níveis de dióxido de carbono: controlo (390 ppm), e dois cenários de altas
concentrações, mesmo que um pouco superiores aos projectados para este século (1350
e 6000 ppm). Para uma análise dos verdadeiros efeitos da acidificação dos oceanos
nestes coráis, foi calculada a taxa de crescimento todas as semanas através de dois
diferentes métodos: Peso flutuante e Extensão linear. Foram também determinadas a
concentração de Clorofila a e a densidade Zooxantelar.
Em relação ao crescimento, uma diminuição na sua taxa foi observada para os
valores mais elevados de CO2 no final da experiência. Para o Peso flutuante, os valores
registados variaram entre 17,7 ± 0,7 para o tratamento de nível de CO2 baixo (390 ppm) e
4,0 ± 1,7 mg d-1 cm-2 referente ao tratamento de nível mais alto (6000 ppm). Para a
Extensão Linear, a mesma resposta foi observada, ou seja, houve uma diminuição na
taxa de crescimento para os níveis mais elevados de CO2. Para o tratamento de baixo
nível de dióxido de carbono, a taxa de crescimento foi de 98,5 ± 7,2 e para o nível mais
elevado, esta foi de 15,6 ± 2,7 µm d-1.
Quanto à concentração de Clorofila a e à densidade Zooxantelar, os níveis altos
de CO2 mostraram um declínio tanto no número de células de zooxantelas como na
Clorofila, por unidade de área de superfície; como seria esperado. Além destes
resultados, após sete semanas da experiência, os coráis que se encontravam nos
tanques com alta concentração de CO2 (6000 ppm), começaram a perder os seus
pigmentos e/ou algas simbióticas, ou seja, iniciaram o processo de branqueamento.
Considerando os resultados obtidos, pode-se concluir que a acidificação dos
oceanos afecta o crescimento e a saúde do coral hermatípico Porites porites. Uma
4
estimativa foi feita utilizando os valores de concentração de dióxido de carbono de 550 e
950 ppm com base no Quarto Relatório de Avaliação (AR4) - melhores e piores
projecções de CO2 para os anos de 2050 e 2100 respectivamente, de acordo com os
cenários B1 e A1F1 do Painel Intergovernamental de Alterações Climáticas (IPCC 2007).
A taxa de crescimento prevê-se decrescer em 18% até 2050 e 36% em 2100. A
quantidade de clorofila a por cm2 de tecido de coral deverá diminuir cerca de 14% até
2050 e 33% até 2100. Assim, é razoável prever que estas alterações irão tornar esta
espécie menos competitiva no recife, e irão provavelmente levar a um declínio na sua
abundância nos recifes de coral do Caribe.
Apesar de todos os estudos que começam a ser feitos nesta área, pouco se sabe
ainda sobre os impactos da acidificação na fisiologia dos coráis construtores de recifes e
como esta interage com o aquecimento da água do mar. Assim, o próximo passo a
realizar é concentrar-se na interacção entre temperatura e estado de saturação, como
previsões precisas das mudanças globais, que incluem os efeitos de ambas no
crescimento de coráis.
Palavras-Chave: Porites porites; Acidificação dos oceanos; Calcificação; Taxa de
crescimento; Alteração climática.
5
6
Acknowlegments
I would like to start this work with a few words of thanks to everyone who made
this work possible and that made me the person I am today.
Não deixando o português de lado, gostaria de começar este trabalho com
algumas palavras de agradecimento a todos aqueles que tornaram possível este trabalho
e que me tornaram na pessoa que sou hoje.
Thanks to my co-ordinates Professor Dr. Eduardo Rocha from ICBAS – University
of Porto and to Dr. Chris Langdon from RSMAS – University of Miami. Thank you both for
the possibility that you gave me to work with these fantastic animals, that I like and admire
so much. Also thanks for all the wisdom and dedication demonstrated, by your support
and especially for all the patience.
Also a special thanks to Nancy Muehllehner, who have sooo much patience with
me and teach me everything she can. There were many times that instead of being with
her beautiful daughter, she was helping me with my experiment and always with a big
smile in her face! I hope from now on you enjoy more time with Teah and your husband!
You saved me!! 
Thanks to everyone that were with me during these six months in Miami. A special
thanks to Carolina to be the fantastic friend she was and still is and to helped me so much
in this journey, and to Teddy that helped me to integrate myself here.
Thanks to everybody that worked with me in Rosenstiel School of Marine &
Atmospheric Science, but especially to Remy, Erica, Rachel, Jay and Evan, for all the
help and friendship.
Last but not least, thank you Gustavo, Rolando, Milan, Andy, Marc, Maria, Matt,
Krystyna, Henry, Bryan, Stephanie, Paul and CS people, that helped me having fun and
enjoyed Miami.
7
Antes de mais, um especial agradecimento à minha família. Aos meus pais e à
minha irmã, que tornaram possível este meu sonho e sempre me apoiaram, mesmo com
todas as adversidades e momentos menos bons. À Palmirinha, ao Jan, à Lina, à Maria
Manuela, à Cristina, à Maria, ao Betxinho por toda a amizade e carinho que me deram e
por acreditarem em mim. Ao resto da família pelos conselhos e toda a força, e
especialmente a força das saudades da minha pequenina, a minha Beatriz.
Ao pessoal todo que me acompanhou durante a licenciatura e fez os meus dias no
Algarve fantásticos e para nunca esquecer, em especial às minhas Marias, claro! Nós as
três éramos a força umas das outras e nada nem ninguém nos vai conseguir separar.
Nem com um oceano entre nós a nossa amizade vai acabar. Adoro-vos para sempre.
Às minhas coisas boas do Puorto, Cláudia, Ângela, Sónia, Sara, Anas, Débora,
Ligia, Lucinha (companheira de quarto mais faladora do mundo) e aos meninos, Hermes,
Francisco, André, Jorge, Ricardo e Ildefonso, que se tornaram tão importantes no espaço
de um ano. Um muito obrigado por todo o tempo que passámos juntos, quer a estudar
quer a sair.
Ao meu pessoal de Aveiro, em especial ao meu Bitor, à Dani, ao Huguinho, ao
Ildefonso. Sem vocês não sou ninguém, são aquelas pessoas que por muito tempo que
estejamos separados, sei que posso sempre contar e vice-versa, são quem me apoia
quando estou em baixo e partem a loiça toda quando estou feliz. E por isso, adoro ter-vos
na minha vida!!
Finalmente quero agradecer a duas pessoas muito importantes na minha vida, a
Sofia e o Garcia. Cada uma à sua maneira, fazem a minha vida muito mais feliz e muito
mais alegre! Duas pessoas com quem a relação não foi nada fácil durante o meu tempo
em Miami, mas tudo se há-de recompor. Não vos quero nunca fora da minha vida e
obrigada por terem entrado nela e a fazerem melhor!
A todos (to all), Muito Obrigada, Thank You.
8
Aos meus pais e irmã,
Olga Maria Soares Martins de Freitas
Sérgio Martins de Freitas e
Isadora Martins de Freitas
9
10
Indice
1.
Introduction ...............................................................................................................13
1.1
Biology of Coral Reefs .......................................................................................13
1.2
Ecology and Biodiversity ....................................................................................15
1.3
Threats to reefs of the Caribbean ......................................................................15
1.3.1
Diseases.....................................................................................................16
1.3.2
Sediments ..................................................................................................17
1.3.3
Pollution......................................................................................................17
1.3.4
Overfishing .................................................................................................18
1.3.5
Coral Bleaching ..........................................................................................19
1.4
Porites porites: Biology ......................................................................................20
1.5
Ocean acidification ............................................................................................21
1.5.1
Ocean carbonate system ............................................................................23
1.5.2
Calcification and Dissolution response from Hermatypic Corals .................24
1.6
2.
Objectives ..........................................................................................................25
Material and Methods ...............................................................................................26
2.1 Preparation of corals ..............................................................................................26
2.2 Experimental setup.................................................................................................26
2.3 Measurements........................................................................................................28
2.3.1 Buoyant Weight ...............................................................................................28
2.3.2 Linear Extension ..............................................................................................30
2.3.3 Chlorophyll a concentration ..............................................................................32
2.3.4 Zooxanthellar density .......................................................................................32
2.3.5 Surface area determination ..............................................................................33
2.4 Statistical analyses .................................................................................................34
3.
Results .....................................................................................................................35
3.1 Chemical conditions ...............................................................................................35
3.2 Growth rates...........................................................................................................36
3.2.1 Buoyant Weight (B.W.).....................................................................................36
3.2.2 Linear Extension (L.E.).....................................................................................38
3.2.3 Combined data ................................................................................................39
3.3 Chlorophyll a concentrations and Zooxanthellae density ........................................39
3.4 Light data ...............................................................................................................43
3.5 Bleaching ...............................................................................................................44
4.
Discussion ................................................................................................................45
4.1 Growth rates...........................................................................................................45
11
4.2 Chlorophyll a concentrations and Zooxanthellar densities ......................................49
4.3 Bleaching ...............................................................................................................50
4.4 Projections .............................................................................................................52
4.5 Final Considerations ...............................................................................................53
5.
References ...............................................................................................................55
12
1.
Introduction
1.1
Biology of Coral Reefs
Corals are invertebrate animals, that is, without spine, belonging to the Order
Scleractinia, where all members have hard limestone skeletons, and one of the wonders
of the underwater world (Veron, 1986). They are divided into two groups: the ones that are
always polyp type and those that pass in their life cycle through a stage called jellyfish,
and may or may not also present a form of a polyp stage (Bacelar, 1997). Sexual
reproduction gives rise to a planula larva and this can or cannot settles onto the
substratum and grow into a benthic polyp phase. This polyp phase multiplies asexually
both to produce other polyps, which may lead an independent life or remain attached to
each to form a colony, as well as the planktonic medusa stage. This, in turn, develops
gonads and reproduces sexually. One of the two phases can be lost from the life history,
and if the medusa stage is absent the polyp stage develops the gonads as well as
multiplying asexually (Barnes, 1998).
Each polyp has a simple, tube-shaped body, with a circular mouth at the top which
also serves as anus. Around the mouth they have numerous tentacles to help capturing
food and inside there is a pharynx that connects the mouth to the coelenterons, and has a
number of mesenteries poking into it (Spalding, 2004). All exchanges of gases and fluids
are happen in coelenterates, because the water enters and leaves the body of the animal
by currents caused by tentacles.
They build exoskeletons, and these may be composed by organic matter or calcium
carbonate (Glynn & Wellington, 1983). They can also be divided in two types: Hermatypic
and Ahermatypic. The first ones can also be called stony corals build reefs and with the
help of the zooxanthellae, they convert surplus food to calcium carbonate, forming a hard
skeleton. The ahermatypic corals have no zooxanthellae and do not build reefs. These are
also known as soft corals and so they are flexible, undulating back and forth in the current,
and their skeletons are proteinaceous instead of calcareous as the stony corals (Wells &
Hanna, 1992). Both types suffer from environmental influences resulting in limiting the
quantity and quality of species.
13
Figure 1 – Basic anatomy of a hard coral polyp. Artwork Credit: NOAA/Gini Kennedy
Corals build colorful and amazing colonies when they find a supportive environment.
These great colonies are the most familiar forms of the animals that we know as corals,
and their skeletons are the ones that help building and defining coral reefs (Spalding,
2004). These clusters in colonies can reach millions of individuals (polyps), forming a
limestone formation, which can have thousands of kilometers. They are home to an
ecosystem with an extraordinary biodiversity and productivity.
The polyps have in their tissues a group of plant cells known as zooxanthellae,
which are the vegetative state of the dinoflagellate Symbiodinium microadriaticum (Wells
et al., 1983).
These zooxanthellae use the sunlight to convert carbon dioxide into carbohydrates,
lipids and aminoacids. These products are passed to the host, which in return, produce a
nitrogen source (ammonia) and phosphate to algae. Thus, the zooxanthellae takes up the
waste of the polyps at the same time that it feeds them, allowing corals to put all their
energy into their growth, and consequently into the formation of reefs (Delbeek & Sprung,
1994).
14
1.2
Ecology and Biodiversity
The diversity of coral reefs is comparable with the most rich and diverse terrestrial
habitats. These species are considered the base of diversity and despite the reefs occupy
a small area of the planet, there are possibly more species per unit area of coral reefs
than in any other ecosystem.
Coral reefs are home to a huge variety of organisms, where can be found more than
4000 species of fish. Among them are tropical or reef fish, such as parrotfish, angelfish
and butterfly fish, sponges, viruses, crustaceans (including prawns, lobsters and crabs),
echinoderms (like starfish, sea urchins and sea cucumbers), sea turtles and sea snakes
(Castro & Huber, 2000; Spalding et al., 2001).
Inside of the reefs, like other diverse communities, there is a huge variety of
interactions between species. Each one occupies a unique role, or niche, on the reef but
because there are so many species, they have to change and adapt. They have to find a
space to live and grow, to eat, to avoid being eaten, to hunt, to reproduce and to hide from
the hunters (Spalding, 2004).
Some organisms have become masters of camouflage to capture or escape. This
co-evolution has also led to a complex two-way interaction between species, including
mutualism, where both organisms benefit. One of the most important examples of this
partnership is the relationship between the corals and their algae, which has led to the
proliferation and success of coral reef builders. Algae use the sunlight and carbon dioxide
to produce organic compounds rich in carbon. These in turn are leaked to the polyp and
may provide almost all of its nutritional requirements (Allen, 2000).
1.3
Threats to reefs of the Caribbean
The coral reef ecosystems are very sensitive to external impacts that violate its
homeostasis, whether natural or human caused. As it is known, almost everywhere there
are far fewer corals today than there were even 10-20 years ago. The main causes for this
decrease are diseases, sedimentation and nutrient impacts, marine-based pollution,
overfishing, direct destruction, coral bleaching and coastal development (Spalding, 2004;
Sulu, 2007).
15
1.3.1
Diseases
There are reports of coral diseases since 1970 but it was only in the 1980s and
1990s that some of them had a dramatic impact. In the Caribbean there are over more
than 150 species infected, and new diseases continue to be added to the ones already
described (Côté & Reynolds, 2006). Infectious diseases can cause population declines or
even species extinctions.
Today most of the reefs in the Caribbean have lost 80-100% of their staghorn and
elkhorn coral through a disease known as white band disease. Some other coral diseases
have been reported and named according to their appearance: white plague, yellow band,
black band, dark spots and rapid wasting disease (Spalding, 2004).
Figure 2 – Some of the major coral diseases: 1) white plague; 2) yellow band; 3) white band; 4) black
band; 5) red band; 6) white pox; 7) dark spots. Image courtesy by Andy Bruckner, NOAA’s National Marine
Fisheries Service
Coral diseases appear everywhere, but they are more common and more
devastating in areas where the corals are already stressed by pollution or sedimentation
(Spalding, 2004). Many pathogens are also sensitive to factors as temperature, humidity,
and rainfall, and these factors can increase the pathogen development and survival rates,
disease transmission and host susceptibility. More research is needed to find if there are
points to potential linkages to other stresses on corals as possible instigators of disease
(Harvell et al., 2002).
16
1.3.2
Sediments
In the Caribbean, there are increasing levels of sediments in coastal waters. The
principal causes of this increase are clearance of forests and movement of agriculture on
the steep slopes, leading to soil erosion and these particles are carried by rivers to the
sea (Spalding, 2004).
Suspended sediments in the water column can affect coral populations and
community structure by suffocating adult corals and enforcing physiological costs by
reducing light availability for photosynthesis or increasing the energy needed for sediment
removal (Riegl & Branch, 1995). They are able to remove these sediments with their
tentacles (sloughing them off with a layer of mucus), but are not able to keep doing it
because it uses up energy and resources.
The real understanding of the effects of sedimentation is not easy because the
interspecific differences in morphology and sediment rejection ability (Stafford-Smith,
1993); not all the corals have the same ability to use heterotrophic modes of nutrition
(Anthony, 2000) and also because sedimentation rates vary with other environmental and
biological factors (Babcock & Smith, 2000).
1.3.3
Pollution
Coral-reef ecosystems are highly vulnerable to environmental perturbations in
general. This sensitivity has three main factors, and they are: corals have small
physiological tolerance for environmental conditions (Johannes, 1975) and any variation
of physical-chemical outside these tolerance ranges could be decisive to coral growth and
survival (Johannes & Betzer, 1975; Endean, 1976; Pearson, 1981). Another factor is the
susceptibility of the interactions of key reef species (like algae-coral competition) to
pollutant stresses. The damage of these corals leads to an eventual end of many reef
species because these depend on living corals for many reasons like shelter, food and
refuge (Johannes, 1975). The third factor that affects corals sensitivity is that the effects of
most toxic substances are enhanced in water with high temperatures, which are easily
found in coral reef environments (Johannes & Betzer, 1975). These toxic wastes come
primarily from detergents, paints, herbicides and pesticides, industrial toxins and oils.
Despite the known toxic wastes, the most widespread form of pollution found on
coral reefs comes from simple nutrients like organic matter, nitrates and phosphates. Two
of the major sources of these are sewage and agriculture (Spalding, 2004). The potential
17
impacts of sewage effluent on coral-reef communities can be classified in three different
categories that represent separate but interacting processes, which are nutrient
enrichment, sedimentation and toxicity (Pastorok & Bilyard, 1985).
1.3.4
Overfishing
Humans have gathered and fished coral reef organisms for thousands of years to
use it as curios, jewelry and food. More recently, this exploitation has also increased as
novel medicinal compounds have been extracted and as reef-related tourism has
expanded. According to published work, in the last 30-40 years, increasing populations of
humans have been damaging coral reefs at an unprecedented rate (Luchavez & Alcala,
1988; Wilkinson et al., 1993; Munro & Munro, 1994; Grigg & Birkeland, 1997).
Overfishing is one of the major threats to the biological integrity of marine habitats
worldwide (Ginsburg, 1994; Dayton et al., 1995; NRC, 1995; Roberts, 1995; Safina,
1995). The removal of grazing species allows algae to overgrow and smother the corals
while the removal of small reef fish for commercial sale means that fewer individuals get
the chance to grow to reach the reproductive age and replenish the population (Karleskint
et al., 2009). Some fishing methods are very destructive, like the careless use of the traps
and nets which can smash corals, the use of explosives or bleach to drive fish out of the
reef and the use of cyanide (Rubec, 1986; McAllister, 1998, Spalding, 2004) to collect
tropical fish for the aquarium trade or fresh fish for local markets. Cyanide is a metabolic
poison, and this not only removes great number of fish from the reef but also poisons
corals and other invertebrates (Karleskint et al., 2009).
Although everybody knows what to do about overfishing, is incredibly difficult to
persuade fishers and politicians to change, because of all the money that is involved.
There are still a lot of fish, but not enough to reproduce and restock the reefs, which
therefore remain quite unproductive. Well managed, in long-term these same reefs could
supply more fish and would also be more attractive to tourists (Spalding, 2004).
18
1.3.5
Coral Bleaching
The phenomenon of coral bleaching affecting extensive reef areas across the
Pacific was first described by Glynn in 1984. This process is portrayed as the whitening of
corals due to loss of symbiotic algae (zooxanthellae) and/or their pigments and is a global
phenomenon that is probably correlated to global climate change and increasing ocean
temperatures (Glynn, 1993; Brown, 1997; Hoegh-Guldberg, 1999). Reef-building corals
are highly dependent on their zooxanthellae because these provide up to 95% of the
corals‟ carbon requirements for growth, reproduction and also maintenance (Muscatine,
1990).
Figure 3 – Partially bleached coral colony (Pocillipora sp.) on a Pacific Reef (A) and close up view of
bleached area (B). Photo courtesy by NOAA.
The most important stressor is the variation of sea surface temperature (Glynn,
1984; Cook et al., 1990; Fitt et al., 1993; Brown et al., 1995; Hoegh-Guldberg & Salvat,
1995; Anthony et al., 2008), however things like freshwater flooding (Goreau, 1964;
Egana & DiSalvo, 1982), pollution (Jones, 1997a; Jones & Steven, 1997), high turbidity
and sedimentation (Meehan & Ostrander, 1997), diseases (Kushmaro et al., 1997; Benin
et al., 2000), high levels of ultraviolet light, low light conditions and abnormal salinity
(Westmacott et al., 2000) can also cause bleaching on corals.
Most reef corals are very sensitive to environmental stressors that can interfere in
the symbiotic relationship with their algae and cause bleaching. Even when corals are
pale or white, that doesn‟t mean they are dead, but without its algae they are severely
weak and more susceptible to diseases. If conditions do not improve, the coral will
inevitably die (Spalding, 2004). Not all coral species are equally affected (Coles & Brown,
2003) and they have different resistance to bleaching (Loya et al., 2001). This difference
19
can be genetically determined or by distribution and environmental variability
(McClanahan et al., 2007).
1.4
Porites porites: Biology
Porites is a genus of sceleterian coral which is characterized by a finger-like
morphology. The members of this genus have widely spaced calices, a well-developed
wall reticulum and are bilaterally symmetrical. Porites porites is a branching species with a
more sprawling appearance than the other branching Porites. Branches may be curved,
hooked or project downwards and sideways, and are thicker with blunt, enlarged tips. The
three branching species are easily confused, and colony shapes overlap. This species
has thick branches (˃ 2cm) may be stubby and little developed with enlarged, blunt tips.
Developed branches upright or sprawling, often grey, occasionally bright blue. The
colonies of this species are common in shallow water near shore where they may form
extensive beds (Voss, 2002).
Figure 4 – Specimen of Porites porites
20
1.5
Ocean acidification
Over the past 250 years, the levels of atmospheric carbon dioxide (CO2) increased
by nearly 40%, from preindustrial levels of approximately 280 ppm (parts per million) to
nearly 384 ppm in 2007 (Solomon et al., 2007). This rate of increase in atmospheric CO2
is approximately 100x faster than has occurred for millions of years (Siegenthaler et al.,
2005) and the current concentration is higher than any level reached in the past 800,000
years (Lüthi et al., 2008). Approximately 25% of the CO2 emitted from all anthropogenic
sources, like human fossil fuel combustion, agriculture, cement production (Houghton,
2001) and deforestation, currently goes to the ocean, where it reacts with water to
produce carbonic acid (Hoegh-Guldberg et al., 2007). The CO2 uptake causes pH
alterations, has this already fallen by 0.1 unit, from approximately 8.21 to 8.10 (Royal
Society, 2005) compared to pre-industrial times, what means a 30% increase in the H+
concentration and is projected to drop another 0.3-0.4 pH units by the end of this century
(Mehrbach et al., 1973; Caldeira & Wickett, 2003; Orr et al., 2005; Loáiciga., 2006; Feely
et al., 2009; Anlauf et al., 2011).
Table 1 – Projected changes in surface ocean carbonate chemistry based on IPCC IS92a
CO2 emission scenario (Guinotte & Fabry, 2008).
-1
-1
Was assumed that PO4 = 0.5 µmol L and Si = 4.8 µmol L , and use the carbonic acid dissociation constants
of Mehrbach et al. (1973) as refit by Dickson & Millero (1987). pHT is based on seawater scale. Percent
change from preindustrial values are in parenthesis. After Feely et al. (2009).
21
When CO2 is absorbed by seawater, a series of chemical reactions happen. An
increase in atmospheric CO2 concentration lowers the pH and the carbonate ion
concentration (CO32-) available (Kleypas & Langdon, 2002). This reduction also leads to a
reduction in calcium carbonate saturation state (Ω), which has big impacts on marine
organisms (Guinotte & Fabry, 2008). Table 1 lists the projected changes in surface ocean
carbonate chemistry (carbon system parameters and temperature) based on the
Intergovernmental Panel on Climate Change (IPCC) IS92a CO2 emission scenario.
One way to predict climate changes is look backward, for example using ice cores.
A 2083-meter long ice core with 160,000 years was collected by the Soviet Antarctic
Expedition at Vostok, Antarctica (Barnola et al., 1987). There is a close correlation
between carbon dioxide in the air and global temperatures over this time. However, there
are some difficulties in extrapolating these correlations into the future (Krebs, 1994). The
present CO2 levels exceed the levels presented in nature during the last 160,000 years so
is not known if it‟s correct to extrapolate with the same relationship found in the past.
Another problem is the velocity at which CO2 levels are changing. Year to year they are
changing. Lessons learned about the relationship between CO2 and climate in the past will
have to be applied with caution in predicting the future climate because the rate of change
of climate forcing is very different today.
The rate of this change is really concerning biologically as well, since many marine
organisms, particularly the calcifying ones, may not be able to adapt quickly enough to
survive to these changes. Some of the most abundant life forms that will be affected are
plankton. These are very important because they form the base of the marine food chain
and are a major food source for fish and marine mammals (World Bank, 2010).
Acidification may decrease reef growth by reducing calcification rates but also
reproduction and recruitment (NRC, 2010).
Coral reefs are among the marine ecosystems most vulnerable to the changing
climate and atmospheric composition and are threatened by a combination of direct
human impacts and global climate change. Coral reefs are already being pushed to their
thermal limits by recent temperature increases (World Bank, 2009).
Diverse studies identify the ocean acidification as a threat to marine biota for
different factors such as: the calcification rates of shell-forming organisms in response of
supersaturation (Smith & Buddemeier, 1992; Kleypas et al., 1999); aragonite, a mineral
also very important to organisms that calcify, may become undersaturated in the surface
ocean in the early 21st century (Feely & Chen, 1982; Feely et al., 1988; Orr et al., 2005);
and the effects of the decreasing of pH go far beyond limiting calcification.
It is already known that calcification of corals and coral communities is controlled by
saturation of seawater with respect to aragonite (Gattuso et al., 1999; Langdon et al.
22
2000; Leclerq et al., 2000; Leclerq et al., 2002). Several papers have tried to predict the
future response of calcification by coral communities, using the experimental evidence of
previous works, to elevated pCO2 and its biogeochemical significance. Is consensual that
calcification rate will decrease by 14-30% by 2100 (Gattuso et al., 1999; Kleypas et al.,
1999).
One of the most obvious and well documented effects of ocean acidification is, as it
was said before, the decrease of calcification rates, affecting skeletal growth. There are
records that on the Great Barrier Reef, massive corals showed a decrease in calcification
rates by about 14% between 1990 and 2005 (De‟ath et al., 2009). In stony corals, most
studies indicate a 10-60% reduction in calcification rate for a doubling of preindustrial
atmospheric CO2 concentration. The differences between studies may be due to different
species or experimental setups (Doney et al., 2009; Kleypas et al., 2006; Langdon &
Atkinson, 2005).
Growth of the reef structures also rely on the successful recruitment of reef
organisms, which is determined by gamete production, fertilization rates, larval
development and settlement, and post-settlement growth. In general, there are a few data
on any of these aspects for reef-building species, making extrapolation to ecosystem
effects difficult (Jokiel et al., 2008; Albright et al., 2008).
1.5.1
Ocean carbonate system
The carbon system is one of the most important components of oceanography,
primarily because it acts as a governor for the carbon cycle and also because it controls
the acidity of seawater. This system also plays a very important role controlling the
pressure of carbon dioxide in the atmosphere, helping to regulate the temperature of the
planet (Emerson & Edges, 2008).
Seawater carbonate chemistry is a series of chemical equilibria, led mainly by DIC
(Dissolved Inorganic Carbon) and TA (Total Alkalinity) (Kleypas et al., 2006). The surface
water CO2 to atmospheric levels is equilibrated by exchanges between air and sea in a
timescale of approximately one year. When CO2 dissolves in seawater, it reacts with the
water and form carbonic acid (H2CO3), which will lose hydrogen ions to form bicarbonate
(HCO3-) and carbonate (CO32-) (Doney et al., 2009).The basic equations describing this
system are:
23
CO2 (g)
CO2 (aq)
CO2 (aq) + H2O
H2CO3
H2CO3
H+ + HCO3-
HCO3-
H+ + CO32-
Processes that increase DIC shift the equilibrium in a way to lower the pH and lower
CO32-
concentration; on the other hand, processes that increase TA shift the equilibrium in
a way to increase the pH and CO32- concentration. Calcification and dissolution affect both
DIC and TA, while respiration and photosynthesis primarily affect DIC (Kleypas et al.,
2006).
1.5.2
Calcification and Dissolution response from Hermatypic Corals
The reef-building corals, zooxanthellate, are a very important group with the
capacity to produce massive quantities of calcium carbonate (Stanley & Hardie, 1998). A
lot of experiments were performed to test the response of calcification rate to decreases in
aragonite saturation state, and some evidences indicate that the calcification rate of
tropical reef-building corals will be reduced by 20-60% at double preindustrial CO2
concentrations (Gattuso et al., 1998; Kleypas et al., 1999; Langdon et al., 2000; Kleypas &
Langdon, 2002; Langdon et al., 2003; Reynaud et al., 2003; Langdon & Atkinson, 2005;
Royal Society, 2005; Kleypas et al., 2006).
Calcification rates vary under different environmental conditions and the most
important are light, temperature and nutrients (Doney et al., 2009). Light increases the
primary productivity of coral reefs and provides the precipitation of CaCO3 in
zooxanthellate corals (Falkowski et al., 1990) and coralline algae (Johansen, 1981).
Marubini et al., 2001 found out that calcification increases with increasing light up to a limit
and then saturates. Temperature also affects the calcification rate, as proved in a lot of
literature. Most of species calcify faster with an increase in temperature until an optimal
temperature at or 1º - 2ºC below the normal peak summer temperature, after this point
calcification rates starts to decline (Clausen & Roth, 1975; Jokiel & Coles, 1977). Nutrient
concentration was also found to affect coral calcification in laboratory studies. The
24
principal studied nutrients were nitrate and ammonium. Enrichment of nitrate to levels of
5-20 µM resulted in increased zooxanthellae density and photosynthesis, and so
decreased calcification rate (Marubini & Davies, 1996; Marubini & Thake, 1999). Also an
increase in the ammonium concentration produces similar results (Ferrier-Pages et al.,
2000; Hoegh-Guldberg & Smith, 1989; Stambler et al., 1991).
1.6
Objectives
This study aims to investigate the effects of global environmental changes on the
physiology of scleractinian corals in order to reach better predicting capabilities on their
response to future changes. The effects of increases of CO2 were investigated using
concentrations of 390 ppm (actual concentration found in seawater), 1350 and 6000 ppm.
The experimental coral chosen for this study was the ecological significant species Porites
porites. To achieve that, specific objectives were drawn:
1. To evaluate the effects of high CO2 in corals growth using:
a. Buoyant Weight;
b. Linear Extension;
2. To determine and analyze the zooxanthellar density for corals in high CO2
levels and in control;
3. To calculate the concentration of Chorophyll a for exposed organisms.
25
2.
Material and Methods
2.1 Preparation of corals
Branches of Porites porites were brought from the Smithsonian laboratory in Fort
Pierce, FL to RSMAS (Rosenstiel School of Marine & Atmospheric Science) in January
2011. There they were placed in a tank, 380 L, inside the Experimental Corals and
Climate Change Facility and kept at a temperature of 27ºC and a salinity at 36.5. In April
2011, tips were removed from the branches and their cut faces were ground flat. They
were then prepared into coral „nubbins‟ as described in Davies (1995) and glued with All
Game Epoxy II-Part putty on PVC plastic tiles (35 x 37 mm). After the „nubbins‟ were
done, they were placed in a smaller tank with 262 L of seawater, which was called control,
since the CO2 level was 390 ppm. They were kept there for one week so they could
recover and then were switch to the different CO2 levels.
2.2 Experimental setup
An experiment was performed to evaluate the effect of different CO 2 levels on
growth and calcification of Porites porites. The experiment was done in the Experimental
Corals and Climate Change Facility of RSMAS and lasted for eight weeks, from April 21st
to June 15th.
This facility is composed by twelve fiberglass tanks measuring 0.6 x 0.5 m filled to a
depth of 0.22 m. The tanks are inside a shadehouse with a polypropylene clear plastic
roof that admits natural sunlight but at the same time, protects the equipment from the
elements. The nominal amount of sunlight reaching the organisms varies seasonally but is
typically 6-8 mol photons m-2 d-1. The tanks are supplied by high quality low nutrient
concentration natural seawater from the nearby Bear Cut entrance to Biscayne Bay. The
salinity also varies seasonally but usually is between 33 and 38. The water from the Bear
Cut is pumped into a 50,000 gallon settling tank and then through a series of filters that
are intended to remove particulates down to a size of 10 µm. This seawater flows to each
of the experimental tanks at a rate of approximately 150-200 ml min-1. These experimental
tanks contain 60 L of seawater and are associated with a 200 L sump tank. The seawater
from the sump tank into the experimental tank is continuously circulated by a magnetically
coupled 500 GPH pump which is in the sump tank. This seawater pumped sprays out
26
through some holes made in a length of ½” PVC pipe that is in the bottom of the
experimental tank, generating lots of turbulence within the tank in proximity to the corals.
The seawater in the experimental tanks is exchanged with the seawater in the sump
approximately every 10 minutes and through a standpipe.
Each sump tank is fitted with a 1500-watt tungsten heater and heat exchanger coil
made of 1” polypropylene irrigation tubing. A solenoid valve is used to block or permit the
circulation of chilled freshwater (20ºC) through the heat exchange coil. There is also an
Omega CN7500 digital temperature controller that is able to hold the average daily
temperature to ±0.1ºC.
The CO level in the experimental tanks is controlled by bubbling the seawater in the
sump tanks with a CO2-enriched air mixture. An air compressor delivers air with a pCO2 of
390 ppm to a centrifugal drier. The dried air at a pressure of 14 psi is connected to several
Sierra Instruments 810C Mass Flow Controllers (MFC). These deliver a steady stream of
dried air at a precisely controlled rate that is adjusted to 6-14 L per minute. A second set
of MFCs are connected to a cylinder of instrument grade CO2 gas. These are adjusted to
a supply stream of CO2 gas at flow rates ranging from 5-100 ml per minute. The outputs of
air and CO2 MFCs are connected in pairs to gas mixing tubes. Adjusting the setting n the
two MFCs can precisely control the pCO2 of the mixture. The flow rate of gas mix going to
each tank is monitored using a rotameter, and by determination a flow rate of 3-5 L per
minute was found to be adequate for controlling the pCO2 in the experimental tanks. From
the rotameter the gas mix goes to a venturi injector. This produces a strong stream of
bubbles that promotes the exchange of CO2 in the gas mix with the seawater in the sump
tanks. To achieve a desired CO2 level, the pCO2 of the gas mix needs to be adjusted
empirically to compensate for the gas exchange and the rate that CO2 is added and
removed from the system by the flow of Bear Cut seawater. An equilibrator/IRGA system
monitors the pCO2 of the seawater actually achieved on the tanks and an Onset Computer
HOBO data logger acquires the temperature of each of the experimental tanks, the tank
pCO2 and the incoming photosynthetically available irradiance every 5 minutes.
In this experiment, three different levels of CO2 were used to test the effects of this
on corals, which were 390 ppm (low), 1350 ppm (medium) and 6000 pm (high).
27
Figure 5 – Tanks inside of the Experimental Corals and Climate Change Facility at RSMAS,
where corals were placed. Photo by the author
The corals were feed every Thursday with a special recipe composed by 134 ml of
phytoplankton, 0.6 g of oystereggs, 0.6 g of larval food supplement 100-150 microns, 0.6
g of larval food supplement 250-450 microns and 50 ml of seawater. Before the feeding
the water flow was turned off and the temperature was take down to 24.8º C.
2.3 Measurements
2.3.1 Buoyant Weight
Coral growth was measured by buoyant weighing (Davies, 1989) once every 2
weeks until May 15th and then weekly. An electronic balance was used, reading to 0.1 mg.
The balance was mounted on a wooden box whose interior formed the weighing chamber.
The front was fitted with a transparent door, which could be closed to exclude air
movements during weighing. After all setup, the coral was handled by the tile to minimize
damage to the tissues and hold inside of a container filled with seawater maintained at the
same temperature and salinity so the density wouldn‟t change.
Final skeletal growth rates were obtained by linear regression of weight against time
over the last 7 weeks of the experiment only, because the first measurement was relative
28
to the week they were under control. At the end of the experiment the surface area of live
tissue of each colony was determined and the growth rate was normalized to the cm2 of
live tissue.
Buoyant weight is the weight of the coral in water, so it was necessary to calculate
the weight of coral in air. The equation used was the one that is below (Eq. 1) and has
been explained in more detail by Jokiel et al., 1978.
Eq. 1 – Equation to calculate the weight of object in air (Davies 1989)
where wt in water = weight (g) of the object in water, Dwater = density (g/cm3) of the
seawater, Dobject = density (g/cm3) of the object being weighed. For the density of the
object was used the value of 2.822 g/cm3, which was direct determined for Porites porites
by Edmund & Davies, 1986. The Dwater was accurately determined from Eq. 2 by weighing
an inert reference object of known air weight and density in the seawater. The object used
was glass and its density was calculated from Eq. 3.
Eq. 2 – Equation to calculate the Density of water (Davies 1989)
29
Eq. 3 – Equation to calculate the Density of object (Davies 1989)
The temperature and the salinity of the seawater were always controlled during the
buoyant weighing and kept at 27ºC and 35 respectively, so the density didn‟t change.
2.3.2 Linear Extension
Another way to measure the coral growth was through linear extension using a
micrometer. The height of the corals was measured almost every week during all
experiment.
The linear extension was measured using an optical micrometer, capable of
measuring linear extension rates to a precision of 0.5 µm. During each measurement
period, the corals were placed into bins filled with seawater and taken to the lab. There
they were briefly removed from the bin, fitted onto the stage of the micrometer, and
advanced through the path of the optical micrometer. Each measurement was conducted
in no longer than 4 minutes, thereby minimizing the time they were out of water.
The average growth rate was obtained from regression equations of maximum point
(maximum height) against time over 5 weeks of the experiment only, because like in the
buoyant weight one of the weeks the corals were still in controls and the other week that
there‟s no data, was because the Micrometer wasn‟t not on the lab.
30
Figure 6 – Micrometer used to the Linear Extension (L.E.). Photo by the author
Figure 7 – Coral being measured by L.E. Photo by the author
10
5
April-20
Height (mm)
0
-5
0
10
20
30
40
May-10
May-20
-10
May-28
-15
June-6
-20
June-13
-25
Distance (mm)
Figure 8 – Example of a graph for growth rate by Linear Extension
31
2.3.3 Chlorophyll a concentration
After the end of the experiment, all the corals were brought to the Baker lab in
RSMAS where they were blasted, that is, the tissues were removed from the skeletons
with a jet of filtered seawater using a WaterPikTM (Johannes & Wiebe, 1970). The volume
of the blastate was made up to 50 ml using filtered seawater. Subsamples of the blastate
were taken for analysis of zooxanthellar densities and chl a concentration.
For the chlorophyll a concentration, 1 ml of the blastate was placed in a glass tube,
9 ml of 90% acetone was added with a graduated cylinder and the sample was put in an 80ºC freezer for about 30 hours. After this time, the sample was brought to room
temperature and vortexed to remove filter fibres from suspension. 0.5 ml of the sample
was removed to a tube and 3 ml of 90% acetone was added (Nakamura et al. 2003). The
sample was put into the Fluorometer TD-700 Turner Designs, and the first read of
absorbance was taken. After this, 1 drop of 10% HCl was added to acidification and the
second read taken.
The follow equation was used to calculate the concentration of Chlorophyll a
Chla = (1x10-3) * D * (K * T) * (Fo-Fa)
where Chla is the amount of the pigment in the coral tissue sample in µg, the factor
-3
10 converts from liters to milliliters, D is the overall dilution (50:1)*(10:1)*(7:1) or 3500, K
is the calibration constant (K = 0.0000892904 µg/mL), T is the maximum ratio of
chlorophyll a fluorescence before and after adding acid (T = 1.885908293), Fo is the first
fluorescence reading, Fa is the second fluorescence reading after adding the HCl. Finally
the pigment results were divided by surface area of the coral to obtain the µg Chla per
cm2 of coral tissue.
2.3.4 Zooxanthellar density
1.5 ml of the 50 ml blastate solution was put into Eppendorf tubes and 30 ml of
Lugols added (i.e. 1.5:31.5 dillution). The tubes were stored out of the light. The samples
32
were then vortexed and a portion pulled up into a Pasteur pipette. A haemocytometer was
positioned on edge of microscope platform with coverslip, and filled by capillary action.
A counter was used to count all cells falling within or on the lines defining the 9
squares of the haemocytometer. The counts were recorded in a table under the sample
number and some repeats were done loading the haemocytometer at least 3-5 times. The
volume of liquid counted (9x10-4 ml), the volume of the blastate counted (1.5 ml), the
dilution of the blastate (1.5:31.5) and the surface area of the coral were used to calculate
the density of zooxanthellae cells per cm2.
The formula used to calculate the density of zooxanthellae was the following one:
Z = (N/9x10-4)*D/SA
Where Z is the zooxanthellae density in cell per cm2 of coral tissue, N is the average
number of zooxanthellae counted in the haemocytometer slide, 9x10-4 ml is the volume of
Lugol solution counted, D is the overall dilution factor (50:1)*(31.5:1.5) and SA is the
surface area of the coral.
2.3.5 Surface area determination
Measurements of coral growth rate were made and determinations of zooxanthellae
densities and chlorophyll concentrations. However, the results were taken from different
sizes of coral “nubbins”, which required a method of „normalizing‟ the amount of coral in
the analyses. The most common method is to use the surface area of the colonies to
account for the different amount of biomass in each measurement.
The method used in this specie to measure the surface area was the Aluminum foil
(Marsh, 1970) since it works well for “boulder” corals like Porites porites. To determinate
it, after removal of tissue from the coral, an Aluminium foil was layed over the coral
surface, using scissors to trim the foil to cover the entire blasted area. Then the foil was
stretched and a picture was taken. In the program “Image J” the image was treated and
the surface was calculated.
33
2.4 Statistical analyses
The effect of high levels of CO2 on coral growth rate was tested for each experiment
by separate One-way ANOVAs. Calcification rate by buoyant weight, chlorophyll a
concentration and zooxanthellae density per nubbin were positively correlated to final
surface area, so this was used to normalize the data. Statistical differences were
considered for P ˂ 0.001.
All the statistical analysis were performed using the “data analysis package” from
Microsoft Office, Excel.
34
3.
Results
3.1 Chemical conditions
The chemical conditions for the treatment are summarized in Table 2. The
temperature was maintained at 27.1 ± 0.5ºC and the salinity at 36.4 ± 0.2. The Total
Alkalinity (TA) ranged from 2147 ± 115 for the higher level of CO2 to 2282 ± 79 µmol kg-1
for the lower level. The values of pH and Ωaragonite were between 7.1 ± 0.1 and 0.6 ± 0.1 for
the lower level respectively, and 8.1 ± 0.04 and 3.9 ± 0.4 for the higher level.
Table 2 – Physical and chemical conditions during the laboratory experiment 1. Data
given as means ± SD
Salinity
Temp.
(ºC)
CO2 (ppm)
TA (µmol/kg)
pH
Ωaragonite
36.1 ± 0.64
26.8 ± 0.62
329.4 ± 46.94
2068.8 ± 61.82
8.09 ± 0.08
3.58 ± 0.40
36.5 ± 0.33
27.0 ± 0.73
1325.5 ± 134.24
2182.4 ± 43.30
7.57 ± 0.03
1.37 ± 0.08
36.5 ± 0.26
27.1 ± 0.69
4316.2 ± 671.63
2364.3 ± 96.40
7.72 ± 0.14
0.57 ± 0.06
35.7 ± 0.91
27.1 ± 0.49
263.3 ± 7.70
2137.2 ± 59.07
8.15 ± 0.02
3.96 ± 0.18
36.8 ± 0.35
27.1 ± 0.30
1477.0 ± 156.84
2115.5 ± 45.30
7.52 ± 0.06
1.22 ± 0.15
36.6 ±0.45
27.2 ± 0.39
4155.8 ± 224.42
2254.2 ± 25.41
7.12 ± 0.02
0.54 ± 0.02
35.6 ± 0.82
27.1 ± 0.06
295.1 ± 17.33
2333.5 ± 148.21
8.15 ± 0.002
4.42 ± 0.31
36.8 ± 0.33
27.1 ± 0.08
2274.1 ± 1202.61
2242.7 ± 19.90
7.39 ± 0.22
1.06 ± 0.43
36.9 ± 0.26
27.3 ± 0.11
3775.3 ± 905.01
2248.9 ± 63.48
7.16 ± 0.11
0.62 ± 0.16
April
Low
Medium
High
May
Low
Medium
High
June
Low
Medium
High
35
3.2 Growth rates
The effects of different levels of CO2 on growth of corals were determined by
buoyant weight and linear extension. The data related to that effect is presented in Tables
3 and 4, for buoyant weight and linear extension respectively.
3.2.1 Buoyant Weight (B.W.)
Table 3 – Mean initial weight, surface area and growth rate for Porites porites calculated
by buoyant weight. Data given as means ± SE
Treatments
CO2
level
(ppm)
Ωaragonite
n
Initial
weight (g)
Surface area
2
(cm )
Growth rate
-1
-2
(mg d cm )
Low
390
3.6
6
14.7 ± 0.9
27.2 ± 3.6
17.7 ± 0.7
Medium
1350
1.3
5
15.9 ± 0.9
27.7 ± 3.5
11.2 ± 0.7
High
6000
0.4
5
15.1 ± 0.8
22.0 ± 2.1
4.0 ±1.7
In the Buoyant weight, the average initial weight of nubbins was 15.2 ± 0.5 g and the
surface area was 25.9 ± 1.9 cm2. A decrease in the growth rate was observed for the
higher values of CO2 in the end of the experiment. The values recorded ranged from 17.7
± 0.7 for the low level treatment (390 ppm) to 4.0 ± 1.7 mg d-1 cm-2 for the high level
treatment (6000 ppm). In a 95% confidence, really small error bars were presented, which
mean that the results are really reliable (Figure 9).
36
g of weight change in 8 weeks
0,35
0,3
0,25
0,2
390
0,15
1600
0,1
6000
0,05
0
390
1600
CO2 Levels (ppm)
6000
Figure 9 – Growth rate by buoyant weight for Porites porites in the end of experiment. Error bars are 95%
confidence
Another very important variable is the aragonite saturation state (Ω). In the 390 ppm
treatment, Ω was kept at 3.6, in the 1350 ppm kept at 1.3 and finally, in the 6000 ppm
maintained at 0.4. The relation between the growth rate and the saturation state it‟s
almost linear, corresponding the lowest growth rate to the lower Ω and the highest growth
rate to the highest Ω (Figure 10).
The growth rate by the technique of buoyant weighing for the control and the two
levels of CO2 was statistically different (P˂0.001), so big differences were found within or
between experiments.
0,02
0,018
0,016
0,014
0,012
0,01
0,008
0,006
0,004
0,002
0
Growth rate (g d-1)
0,0177
6000
0,0112
1350
390
0,004
0
1
2
3
4
Aragonite saturation state
-1
Figure 10 – Scatterplot of average growth rate by Buoyant Weight per tank (g d ) to Aragonite saturation
state (Ω)
37
3.2.2 Linear Extension (L.E.)
Table 4 – Mean initial height and growth rate for Porites porites calculated through linear
extension. Data given as means ± SE
Treatments
CO2 level
(ppm)
n
Initial height
(mm)
Growth rate
(µm/d)
Low
390
6
16 ± 2
99 ± 7
Medium
1350
5
17 ± 2
45 ± 8
High
6000
5
19 ± 1
16 ± 3
Regarding the linear extension, the results were really similar to the ones obtained
from buoyant weight. A decrease in the growth rate was observed for the higher levels of
CO2. For the Low level treatment the growth rate was 99 ± 7 and for the High level this
was 16 ± 3 µm d-1.
Statistically, the growth rate calculated by linear extension for the control and the
two levels of CO2 was also very different (P˂0.001), which means that big differences
were found within or between experiments.
0,12
Growth rate (mm d-1)
0,1
0,09852
0,08
6000
0,06
1350
0,04486
0,04
390
0,02
0,01556
0
0
1
2
3
4
Aragonite Saturation State (Ω)
-1
Figure 11 - Scatterplot of average growth rate by Linear Extension per tank (mm d ) to Aragonite saturation
state (Ω)
38
3.2.3 Combined data
To a better understanding of the effects of CO2 on growth rate of the corals, the two
different methods were analyzed together.
Comparing both methods of growth (Figure 12) was understandable that the growth
rate for the linear extension was higher than the one for the buoyant weight. However, for
all the corals, the growth rate by buoyant weight was much more linear and constant.
140
14
120
12
y = 2,52x + 2,54
R² = 0,81
Growth Rate
100
10
8
80
6
y = 25,35x + 8,13
R² = 0,87
60
4
40
Linear
Extension
(um/d)
Buoyant
Weight
(mg/d/cm2)
2
20
0
0
-2
0
1
2
3
Aragonite Saturation State (Ω)
4
Figure 12 – Growth rates through the two methods: Linear extension (blue) and Buoyant weight (red) for the
different levels of CO2
3.3 Chlorophyll a concentrations and Zooxanthellae density
High levels of CO2 showed a decline both in numbers of zooxanthellae and
chlorophyll concentration per unit surface area. The Chlorophyll concentration in 390 ppm
ranged from 4.7 to 9.1 µg cm-2 (Table 5), whereas those exposed to 6000 ppm ranged
from 1.0 to 2.5 µg cm-2. Zooxanthellae densities for the 390 ppm were between 3,500,000
and 3,800,000 cells cm-2 and for the 6000 ppm between 310,000 and 1,100,000 cells cm2
.
39
Table 5 – Chlorophyll a concentration and number of zooxanthellae per area. Means ± SE
CO2 Level
(ppm)
390
1350
6000
# Coral
Chlorophyll a
2
(µg/cm )
70
73
74
79
85
88
72
75
76
80
86
77
78
83
84
87
4.7
6.5
5.9
7.5
9.1
6.7
2.1
5.0
5.0
4.1
3.6
2.5
1.0
2.5
2.4
1.4
Average
6.7 ± 0.6
3.9 ± 0.5
2.0 ± 0.3
Zooxanthellae
2
cells/cm
Average
3,700,000
3,500,000
3,600,000
3,500,000
3,800,000
3,600,000
2,600,000
3,600,000
3,300,000
3,300,000
3,200,000
960,000
310,000
1,100,000
1,000,000
1,100,000
6
4
6
5
5
5
3.6 x 10 ± 4.5 x 10
3.2 x 10 ± 1.5 x 10
9.1 x 10 ± 1.5 x 10
For a better understanding of the chlorophyll concentration and number of
zooxanthellae in corals during the experiment, the average per surface area was
calculated for the three different levels of CO2 and represented by the Figure 13 and 14,
respectively.
A One-way ANOVA found that carbon dioxide concentration has a highly significant
Chlorophyll a concetrations (µg cm-2)
effect on chlorophyll a concentration and zooxanthellae density, being P˂0.001.
9
8
7
6
390
5
1350
4
6000
3
2
1
0
390
1350
6000
CO2 levels (ppm)
2
Figure 13 – Average concentration of Chlorophyll a per unit of area (cm ) for the different levels of CO2. Error
bars are 95% confidence.
40
8
7
ug Chla cm-2
6
y = 101,02x-0,453
R² = 0,9993
5
4
3
2
1
0
0
1000
2000
3000
4000
5000
6000
7000
CO2 (ppm)
2
Figure 14 – Concentration of Chlorophyll a per unit of area (cm ) for the different levels of CO2 and
respectively power regression.
4000000
Zooxanthellae cells cm-2
3500000
3000000
2500000
2000000
390
1500000
1350
1000000
6000
500000
0
390
1350
6000
CO2 Levels (ppm)
2
2
Figure 15 – Number of zooxanthellae/cm for the different levels of CO . Error bars are 95% confidence.
41
4,00E+06
3,50E+06
y = -473,64x + 4E+06
R² = 0,9986
Zoox cm-2
3,00E+06
2,50E+06
2,00E+06
1,50E+06
1,00E+06
5,00E+05
0,00E+00
0
1000
2000
3000
4000
5000
6000
7000
CO2 (ppm)
2
Figure 16 – Number of zooxanthellae/cm for the different levels of CO2 and respectively linear regression
The chlorophyll concentrations in zooxanthellae, presented a different result from
the other analysis. A higher value of µg chlorophyll per zooxanthella was observed for the
highest CO2 level (6000 ppm) instead for the control (Figure 17). The values ranged from
1.2 x 10-6 for the medium level (1350 ppm) to 2.3 x 10-6 for the highest level of CO2.
µg Chlorophyll a/Zooxanthellae
0,0000035
0,000003
0,0000025
0,000002
390
0,0000015
1350
0,000001
6000
0,0000005
0
390
1350
6000
CO2 Levels (ppm)
Figure 17 – Chlorophyll a concentrations per number of zooxanthellae for each treatment. Error bars are 95%
confidence.
42
3.4 Light data
Keeping the amount of light received as close as possible with the natural conditions
on ocean is one of the things to have in attention in an experiment. Light has a central role
in primary productivity of coral reefs and provides the precipitation of CaCO3 skeletons in
hermatypic corals (Falkowski et al., 1990). Figure 18 represents the values obtained for
16
14
12
10
8
6
4
2
0
04.21
04.23
04.25
04.27
04.29
05.01
05.03
05.05
05.07
05.09
05.11
05.13
05.15
05.17
05.19
05.21
05.23
05.25
05.27
05.29
05.31
06.02
06.04
06.06
06.08
06.10
06.12
06.14
Total light mol photons m-2 per day
everyday during the experiment, which ranged from 6.0 to 15.1 mol photons m-2 d-1.
Dates
2
Figure 18 – Total of light (mol photons/m ) emitted per day
43
3.5 Bleaching
After 7 weeks of exposure to high levels of CO2 (6000 ppm) all the corals that were
in this treatment became white (loss of pigmentation), starting to bleach, as shown in the
picture below (Figure 19). The corals that were in the other treatments didn‟t show any
signal of bleaching.
Figure 19 – The left coral belonged to the control tank (390 ppm), while all the other belonged to the high level
of CO2 (6000). A big difference in color can be seen. Photo by the author.
44
4.
Discussion
4.1 Growth rates
In this experiment, the effect of increased CO2 concentration in seawater was tested
for the species Porites porites. The results showed that this increase causes a decrease
in the growth rate of this hermatypic coral skeleton, which means that calcification in this
species depend not only but also on dioxide carbon concentration.
Three different levels of CO2 were used, being one the control (390 ppm) and
another two levels, 1350 and 6000 ppm. These two values are higher than the ones
predicted to the next century, but they were chosen out of curiosity about what may
happen in coming centuries if human society does not bring CO2 levels under control. The
390 ppm corresponded to a saturation state of 3.6, the 1350 ppm to a Ω = 1.3 and finally,
the 6000 ppm to a Ω = 0.4.
Growth rates in this experiment are similar to previous estimates for nubbins of
Porites compressa and Porites porites grown in the laboratory (Marubini & Davies, 1996,
Marubini & Atkinson, 1999, Marubini & Thake, 1999, Marubini et al., 2001, Gattuso et al.,
1999) and in the field (Edmunds & Davies, 1986, Edmunds & Davies, 1989). Since weight
gain is a function of colony size and this is artificially determined when nubbins are cut off
the mother colony, it‟s important to control the size for when measuring growth rate
(Marubini et al., 2001). There are many ways to estimate colony size, being the more
important, surface area (Barnes & Taylor, 1973, Rinkevich & Loya, 1984), skeletal weight,
protein, chlorophyll and tissue content (Marubini et al., 2001). In this study the interest
was the growth response to increased levels of carbon dioxide and decreased saturation
state, as chlorophyll a concentration and zooxanthellae density. For this reason, surface
area was used, since it is linearly correlated to growth rate and controls for colony size.
That is a lot of literature that provides evidence of detrimental effects on corals of
increased pCO2 alone or in combination with other factors (Gattuso et al., 1999, Ohde &
van Woesik, 1999, Leclerq et al., 2000, 2002, Langdon et al., 2003, Marubini et al., 2003,
Reynaud et al., 2003, Renegar & Riegl, 2005). However none of these studies was done
with the species Porites porites neither with such high values of CO2 and low values of
saturation state.
For the buoyant weight, the values recorded ranged from 17.7 ± 0.7 for the control to
4.0 ± 1.7 mg d-1 cm-2 for the high level treatment, which shows a really big decrease in the
growth rate. Results obtained for the species Porites compressa by Marubini et al., 2001,
showed that nubbins grown in conditions of the Last Glacial Maximum (target values: Ω =
45
5.6; pCO2 = 200 µatm) in shallow water had the highest growth rate, 1.8 ± 0.2 mg CaCO3
d-1 cm-2, while the ones grown in conditions of the year 2100 (target values: Ω = 2.4; pCO2
= 700 µatm) at 4 m depth had the lowest growth rate (0.6 ± 0.1 mg CaCO3 d-1 cm-2).
Marubini et al., 2001 studied not also the carbonate ion concentration but also the
influence of the light, using four different depths: 0.5, 2, 4 and 6.5 m. They found that
depth has a strong effect on growth rate, with the nubbins growing at 0.5 m depth having
the fastest calcification rate in all cases.
Marubini & Thake, 1999 tested the hypothesis that the supply of DIC (Dissolved
Inorganic Carbon) in seawater limits the calcification and if limitation of calcification by
nitrogen can be compensated for by DIC enrichment in the hermatypic coral Porites
porites. They found that the nubbins in all treatments in seawater with added DIC have a
higher slope of growth rate. They also reported that in the first 17 days of experiment
there was no significant effect of nitrogen treatment on the total amount of calcium
carbonate deposited. The results were not the same for the last 15 days (which phase
they called established growth rate), when the effect of N on calcification becomes
significant among corals growing in seawater. Corals that were in enriched DIC did not
show a decreased growth rate in response to increased N. The values without DIC were
11.5 ± 1.6 for the control, 5.5 ± 0.8 for NO3- and 8.6 ± 0.9 mg d-1. With the addition of DIC
all these values increased, ranging from 18.6 ± 2.6 for the control + DIC, to 16.0 ± 1.8 for
NO3- + DIC and to 19.0 ± 2.0 mg d-1 for NH4+ + DIC.
Another study was made to test the effect of nutrient enrichment and elevated CO 2
partial pressure for the coral species Acropora cervicornis by Renegar & Riegl, 2005.
Their results showed a growth rate of 1. ± 0.7 for the control period and a growth rate of
0.3 ± 0.1 for the period where the nutrient concentrations were raised to 10 µM NO3- and 4
µM P-PO4-3, and the pCO2 level increased to ~700 to 800 µatm. Even if it is not
appropriate to discount the effects of increased nutrient concentrations on corals, they
found that elevated pCO2 has the most negative effect on growth rate.
We showed that calcification in Porites porites correlates well with saturation state;
as Ω decreased from 3.6 (low concentration of CO2) to 0.4 (high concentration of CO2) the
estimated maximum calcification rate decreased by around 23%. Furthermore, the change
from the medium concentration of CO2 (Ω = 1.3) to those of high treatment (Ω = 0.4)
caused a reduction in calcification.
Looking at the graph in Figure 10, it can be seen the true relationship between the
state of saturation and the rate of coral growth in the different treatments. It can be
concluded that is a linear relationship, being for the lowest saturation state, the lowest
growth rate value and for the highest saturation state, the highest growth rate value.
46
Through the shape of the graph, it can also be seen that the growth rate is directly
proportional to the saturation state and inversely proportional to the concentration of CO2
present in the seawater.
Linear extension is a method of calculation the growth rate that is very new and little
known. However, the results obtained through this process are very reliable and very
clarifying. For this study, it can be seen that, as to the buoyant weight, there was a linear
relationship between the saturation state and the growth rate. The values of growth rate
obtained in this work, ranged from 99 ± 7 for the lowest level of CO2 to 16 ± 3 µg d-1 for the
highest level, with the average of initial height of 17.4 mm. This proves that high
concentration of CO2 not only affect dramatically growth in terms of weight but also in
terms of height of the coral. The observed reductions in linear extension of Porites porites
are consistent with the hypothesis that calcification and, ultimately, growth decline as
pCO2 increases and saturation state decreases (Figure 11). The same results were
obtained by Albright et al. 2010 for the species Acropora palmata. These authors studied
different steps in corals recruitment, concluding that a slowed juvenile growth will translate
into increased juvenile mortality, given that the risk of mortality is inversely proportional to
juvenile growth rate and colony size (Vermeij & Sandin, 2008; Babcock, 1991).
One factor that should also be considered is the temperature increase. Despite not
having been manipulated in this experiment, this factor along with the increase in CO 2
concentrations causes further changes in coral growth. Calcification in corals is known to
be enhanced with increasing temperatures (Clausen & Roth, 1975; Reynaud-Vaganay et
al., 1999; Bessat & Buigues, 2001) up to an optimum temperature of ~27ºC (Clausen &
Roth, 1975; Marshall & Clode, 2004; Cooper et al., 2008). Over this temperature,
calcification rates tend to decrease, but dry-weight there is some data that reveal
increased calcification at higher temperatures in the primary polyps of Porites
panamensis. This suggests that this may not be the case in all corals (Anlauf et al., 2011).
Reynaud et al. (2003) showed that the calcification in colonies of Stylophora
pistillata maintained at elevated temperature (28.2ºC), declined by 50% in response to
increased pCO2. However, in colonies maintained at normal temperature (25.2ºC),
calcification was not affected by elevation of pCO2. This last result is not in agreement
with several papers that found a negative relationship between calcification and CO2, or a
positive relationship with the aragonite saturation state (Gattuso et al., 1998; Marubini &
Atkinson, 1999; Langdon et al., 2000, 2003; Leclercq et al., 2000, 2002; Marubini et al.,
2003). These results show that pCO2 and temperature significantly interact to control
calcification in scleractinian corals, which could explain the large range of responses to
elevated pCO2 reported in corals and coral communities.
47
Albright et al., 2008 found that the growth rate of juvenile Porites astreoides
decreased between 45 and 56% when pH was lowered by just 0.07 units. Likewise,
Cohen et al., 2009 reported reduced skeletal growth of the cross-sectional area in newlyreared primary polyps of Favia fragum under lowered aragonite saturation and calculated
a nearly 80% decrease in the precipitated aragonite.
Marubini & Atkinson (1999) and Langdon et al. (2000) found that the rate of
calcification declined immediately after the rise in pCO2 and did not change afterwards,
demonstrating that no acclimation process occurred. Marubini & Atkinson (1999) reported
for the scleractinian coral Porites compressa that the decrease of calcification in
responses to changes in the carbonate system is immediate and reversible. Also,
Langdon et al. (2000) found that the response of the community calcification rate of the
Biosphere 2 ocean is not significantly different during short-term (days) and long-term
(months) changes in aragonite saturation state.
Reef-building corals can exhibit several responses to reduced calcification, having
all of them deleterious consequences for reef ecosystems. The most direct response is
considered the decreased linear extension rate and skeletal density of coral colonies.
Cooper et al., 2008 found that the massive coral Porites on the Great Barrier Reef has
shown reductions in linear extension rate of 1.02% year-1 and in skeletal density of 0.36%
year-1 during the past 16 years. This is equivalent to a 20.6% drop in growth rate, the
product of linear extension rate and skeletal density, over the 16-year period.
Marubini et al. (2001) made an extrapolation for the effect of seawater Ω on coral
calcification in controlled laboratory setup to the global scale, and concluded that CaCO 3
deposition rate in scleractinian corals is expected to decrease around 10% over the next
100 years, as pCO2 will double. Several studies were made to date about the relationship
between Ω and calcification (Gattuso et al., 1999; Schneider & Erez, 2006; Leclercq,
2000) and all of them were consistent with the hypothesis that biogenic calcification is
influenced by the saturation state of seawater. However, the predictions for reduction in
calcification between 2001 and the year 2100 range between 3% (Gattuso et al., 1998)
and 40% (Langdon et al., 2000). This large range in response could be artificially inflated
by the high variability in experimental protocols (combination of many factors like
organismal scale studies with ecosystem scale studies, different taxa of calcifying
organisms, different coral families, different type of chemical manipulation and
experimental duration) or can be due to physiological differences between organisms.
48
4.2 Chlorophyll a concentrations and Zooxanthellar densities
The exposure to high concentrations of CO2 under laboratory conditions severely
affected chlorophyll a concentrations and zooxanthellae densities in Porites porites. The
corals presented in the tanks with high CO2 concentration, showed really low numbers
both in zooxanthellae density and chlorophyll a concentration per unit surface area in the
end of the experiment. The chlorophyll a concentration in control corals ranged from 4.7 to
9.1 µg cm-2 coral surface area, whereas those exposed to high concentrations of carbon
dioxide contained 1.0 – 2.5 µg cm-2. Zooxanthellae densities of the control were 3.4 – 3.8
x 106 cells cm-2. In contrast, densities fell to 3.1 x 105 – 1.1 x 106 cells cm-2 when exposed
to high concentrations. However, the chlorophyll concentration per zooxanthella
decreased as an inverse function, being the highest value to the high CO2 level (2.3 x 106
) and the lowest to the medium level (1.2 x 10-6 µg Chl per zooxanthella).
µg Chlorophyll a/Zooxanthellae
0,0000035
0,000003
0,0000025
0,000002
0,0000015
0,000001
0,0000005
0
0,00E+00
1,00E+06
2,00E+06
3,00E+06
4,00E+06
Zooxanthellae cm-2
Figure 20 – Relationship between zooxanthellar chl a concentration and zooxanthellar density for all
corals submitted to low, medium and high CO2 concentrations
The results in this study suggest that, for this specie, loss of zooxanthellae can
occur without decreases in zooxanthellar chlorophyll concentrations when exposed to high
levels of CO2. The same results have already been reported in corals exposed to elevated
seawater temperatures during laboratory manipulations (Hoegh-Guldberg & Smith, 1989a;
Glynn et al., 1992; Fitt & Warner, 1995) and in corals sampled after natural bleaching
events (Hoegh-Gulberg & Smith, 1989a; Fitt et al., 1993; Jones, 1997b).
49
Le Tissier & Brown (1996) also found increases in zooxanthellar chla concentrations
in corals (Goniastrea aspera) during stress-related loss of zooxanthellae. In their opinion,
the increase in zooxanthellar chlorophyll concentration occurred because of a greater loss
of zooxanthellae from the apical tissues (leaving “dark-adapted” zooxanthellae located in
the lower tissues) or because a breakdown products of chlorophyll and other pigments
interfering with the absorption peak used to compute chla concentrations.
The inverse relationship between zooxanthellar density and zooxanthellar chl a
concentration (Figure 20) is the opposite of that expected from a photoadaptive response
to zooxanthellar self-shading as zooxanthellar densities increase (Heogh-Guldberg &
Smith, 1989b; Fitt et al., 1993). The Chla concentration of unicellular algae is also a
reliable indicator of nutrient status (Rees, 1991), and the relationship can signify nutrient
limitation at higher zooxanthellar densities (Hoegh-Guldberg & Smith, 1989b; Fitt et al.,
1993; Jones & Steven, 1997). So, the increase in zooxanthellar chla concentration can
represent a response of the remnant zooxanthellar population to increased nutrient ability
through decreased competition.
In general, algal densities in corals are remarkably constant, independently of
genus, depth, growth form or location (Muscatine & Porter, 1977; Porter et al., 1984).
Drew (1972) measured an algal density of 1.45 x 106 cells cm-2 in ten genera of
hermatypic corals and two genera of soft corals. Based on these observations, he
suggested that the area available for the zooxanthellae to occupy directly determines their
densities under steady-state conditions, i.e. a space limitation theory. Dubinsky & Jokiel,
1994 has pointed out that in corals at depths the undersides of the tissues often have no
symbiotic algae. This means that light availability may also limit algal densities and restrict
zooxanthellae from occupying deeper tissues in corals with highly perforate skeletons.
4.3 Bleaching
Bleaching has been described as the dissociation of the symbiotic relationship
between algae and their respective hosts (Iglesias-Prieto et al., 1992). It is important to
understand how each symbiotic partner (both individually and within the intact association)
perceives the environmental stressors that elicit bleaching and how these signals are
transmitted into the observed response. Many authors observed in laboratory experiments
that the response to high temperatures is the paling of the coral through either loss of
zooxanthellae (Hoegh-Guldberg & Smith, 1989a; Glynn & D‟Croz, 1990; Lesser et al.,
50
1990; Fitt & Warner, 1995) and/or the loss of photosynthetic pigment per zooxanthella
(Sharp, 1995; Fitt & Warner, 1995).
It is also known as a generalized stress response resulting from a variety of
environmental conditions outside the normal local range. This can be induced by
unusually high or low temperatures, low light levels or constant light, high levels of UV
radiation, salinity shock or may other chemical factors (Cook et al., 1990; Gates, 1990).
Despite
apparent
environmental
adaptations,
different
species
have
different
vulnerabilities to stress-related bleaching and associated mortality at a particular site
(Cook et al., 1990; Hoeksema, 1991). Prevalence of bleaching seems to be higher in
species that have high rates of recruitment and grow rapidly than in other corals in the
same general environment (Gates, 1990; Jokiel & Coles, 1990), and mortality after the
bleaching is higher in some branching species (particularly members of Acropora) than in
many massive corals (Brown & Suharsono, 1990).
The results in this study showed that after 7 weeks of exposure to high
concentrations of CO2, the corals started to bleach (loss of pigmentation). These results
indicate that future predictions of bleaching in response global warming must also take
account of the additional effect of acidification and suggests that any potential adaptation
and acclimatization by coral reef organisms to thermal stress (Baker et al.,
2004;
Berkelmans & van Oppen, 2006) may be compensate or overridden by CO2 effects.
Previous studies of CO2 enrichment and also warming in corals and in algae have not
observed a bleaching response (Reynaud et al., 2003; Schneider & Erez, 2006). One
possible explanation is that this experimental period of CO2 dosing in this study was
longer than for example the one in the experiment made by Reynaud et al. 2003, thus
giving time for the buildup of physiological stress. The process by which high CO2 induces
bleaching is still unknown, but according to some authors can involve a number of
possible mechanisms such as photorespiration (Kim et al., 2004), changes to the carbonconcentrating mechanism (Leggat et al., 1999), and/or direct impacts of acidosis (Gattuso
et al., 1999). According to Anthony et al., 2008, A. Crawley, S.D., & K.R.N.A., unpublished
data, obtained some results indicating that high CO2 and/or lowered pH disrupt the
photoprotective mechanisms of coral symbionts or algal chloroplasts by lowering rates of
photorespiration and the capacity for thermal dissipation.
Anthony et al., 2008 found that, under warm conditions, productivity in Porites
dropped dramatically at the highest CO2 level (1000-1300 ppm), but only 20% (and
nonsignificantly) lower than at control conditions, suggesting a generally weaker response
to acidification than, for example, Acropora.
The productivity responses of the corals to CO2 dosing are likely to be a result of a
series of opposing mechanisms. With a bleaching event, a decline in the symbiont
51
population (or the chlorophyll pool) overrides the increased photosynthetic efficiencies,
leading to a drop in areal productivity. CO2 is the substrate for photosynthesis, and
increasing its supply may increase rates of photosynthesis in organisms that are CO2
limited (Anthony et al., 2008). The potential effects of increasing CO2 and/or impacts of
acidosis on inorganic carbon acquisition are likely to be highly variable in different
organisms, as are the thresholds in temperature that dictate whether this thermal increase
leads to a negative or positive response.
4.4 Projections
Using the data obtained, an estimate was made of the predicted decline in growth in
terms of buoyant weight and linear extension, in chlorophyll a/cm2 and in zooxanthellar
density at CO2 levels 550 and 950 ppm. These values were used because they represent
the levels projected in Fourth Assessment Report (AR4) best and worst CO2 projections
for the years 2050 and 2100 respectively, based on scenarios B1 and A1F1 projected by
the Intergovernmental Panel on Climate Change (IPCC 2007). Table 6 shows the results
of those calculations.
Table 6 – Predicted changes in growth rates, Chlorophyll a and zooxanthellar densities for
the years 2050 and 2100, based on IPCC 2007 B1 and A1F1 CO2 emission scenarios.
Year
pCO2
(ppm)
Ωaragonite
Buoyant
Weight
(mg d-1
cm-2)
Present
390
3.91
12.4
107.2
3.82 x 106
6.8
2050
550
3.04
10.2
85.2
3.74 x 106
5.8
2100
950
2.14
7.9
62.4
3.55 x 106
4.5
% Decrease by 2050
-17.7
-20.6
-2.0
-14.4
% Decrease by 2100
-36.0
-41.8
-7.0
-33.2
Linear
Zooxanthellae
Extension
cm-2
-1
(µm d )
µg Chl a
cm-2
Growth through buoyant weight is projected to decline by 18% by 2050 and 36% by
2100, while through linear extension is expected a decline around 21% by 2050 and 42%
by 2100. Marubini et al., 2001 showed that calcification in Porites compressa is correlated
52
with saturation state; as Ω decreased from 5 (Last Glacial Maximum) to 2.3/2.5 (Year
2100), the estimate maximum calcification rate decreased by 30%. These results are
consistent with the ones obtained in this experiment. Anthony et al., 2008 also showed a
significant reduction in calcification for the species Acropora intermedia and Porites
lobata. For the first species, the rate of calcification in the high-CO2 dosing regime (10001300 ppmV), was ~40% lower than at control conditions (380 ppmV). The calcification
response of Porites was almost identical to that of Acropora. In this experiment, these
authors also studied the effects of high temperatures. However, for these two species, the
temperature effect was not very significant. Albright et al., 2010 also showed reductions in
growth rate, but by linear extension, of Acropora palmata juveniles. Comparing with the
control, there was a significant reduction of 39% at the mid-CO2 level (560 µatm) and 50%
at the high-CO2 level (800 µatm). Reynaud et al., 2003 demonstrated that the rate of
calcification of Stylophora pistillata could decrease by 50% between 2000 and 2100.
The amount of Chla per cm2 of coral tissue is projected to decline 14% by 2050 and
33% by 2100. These predictions are quite similar with other studies. So, even if the CO2
levels used here are higher than the ones used in previous studies, the predicted changes
for the years 2050 and 2100 are in line with these. Reynaud et al., 2003 obtained an
increase of photosynthesis with increasing temperature under normal pCO2, which is in
agreement with previous studies performed on scleractinian corals (Coles & Jokiel, 1977,
Coles & Jokiel, 1978, Kajiwara et al., 1995). These last authors shoed that elevated pCO2
did not stimulate photosynthesis, which even decreased slightly. Langdon et al., 2003 also
proved that net community production did not change in response to elevated pCO2.
4.5 Final Considerations
The scientific knowledge of the biological effects of ocean acidification is in its
infancy and the long-term consequences of changing seawater chemistry on marine
ecosystems can only be theorized. However, we know most about the calcification
response for shallow-water scleractinian corals. There are some data sets allowing the
identification of a threshold seawater carbonate chemistry below which ocean acidification
will cause a net calcification rate less than zero (i.e. dissolution) (Hoegh-Guldberg et al.,
2007).
This study did not support the existence of a threshold for Porites porites. Growth
rate decreased with increasing CO2 but remained greater than zero up to the highest level
53
tested, i.e. 6000 ppm. We also observed a decrease in the Chla concentration per unit
area of coral tissue. The zooxanthellae density did not change significantly so the amount
Chla per zooxanthellae cell declined. This could presumably lead to a decrease in the
amount of photosynthate available to the coral host under high CO2 conditions and this
could lead to reduced coral fitness and might explain the observed decrease in
calcification rate. However, photosynthetic rates were not measured in this study so this
hypothesis cannot be confirmed. Anthony et al., 2008 reported a decrease in the pigment
content (based on a semi-quantitative luminance scale) of two Pacific coral species,
Acropora intermedia and Porites lobata at a CO2 level of 1010-1350 ppm. The net
photosynthetic rate of P. lobata declined 30% under normal temperature conditions (27ºC)
relative to the ambient CO2 control while the net photosynthetic rate of the A. intermedia
declined to essentially zero.
Considering the results of this study, the next obvious step is to focus on the
interaction between temperature and saturation state, as accurate predictions of global
change have to include the effects of both saturation state and temperature on coral
growth. Besides the temperature, light and water motion there are other environmental
parameters that should be investigated together with pCO2 to predict the response of
corals calcification and coral communities to global environment changes. Another
important challenge that has received little attention is whether long-term (years to
decades) physiological acclimation and adaptation have the potential to mitigate acute
physiological responses (Gattuso & Buddemeier, 2000).
There is also a need to further investigate the ability of corals to acclimatize and/or
adapt to elevated pCO2 given prolonged exposure, as well as the possibility of taxonomic
differences in sensitivity. Focusing efforts on the protection and cultivation of more
adaptable species may improve the effectiveness of coral preservation and restoration
efforts.
Some priorities must be studies which increase the ecological understanding of the
mechanisms and interactions that structure marine communities; also some experiments
that quantify the capacity of animals to adapt and evolve must be done. To be able to
predict the effects of not only ocean acidification but other, as yet unforeseen,
environmental challenges faced by marine ecosystems, an investment in multidisciplinary
research that brings together community and evolutionary biology, and physiology with
population is necessary.
54
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