Photosynthesis and respiration
• Collectively, photosynthesis and
respiration control bioelemental cycling.
Sunlight
Photosynthesis:
6CO2 + 12H2O
C6H12O6 + O2 + 6H2O + heat
Respiration:
C6H12O6 + 6O2
6CO2 + 6H2O + heat
•
Photosynthesis is
initiated by the
absorption of light
energy by pigments or
photochemically
reactive proteins. The
absorbed light energy
is passed from the
light harvesting
pigments (antenna) to
the photosynthetic
reaction centers via
the flow of electrons.
Sunlight
Photosynthesis:
6CO2 + 12H2O
C6H12O6 + O2 + 6H2O + heat
Phytoplankton taxonomy
• Phytoplankton include single cells or organized
colonies; the vast majority are very small (<10
µm):
– Picoplankton: 0.2-2.0 µm
– Nanoplankton: 2.0-20 µm
– Microplankton: 20-200 µm
The picoplanktonic photosynthetic cyanobacteria
comprise a substantial component of plankton
biomass and production in low nutrient systems.
Identifying phytoplankton
•
•
•
•
Cell biomarkers (pigments)
Microscopy/Flow cytometry
Nucleic acids (rDNA)
Antibody probes
Major divisions and classes of
photosynthetic plankton in the ocean
•
Prokaryotes
– Cyanophyta (includes
Prochlorophytes)
•
Eukaryotes:
– Chlorophyta (green algae); include
the following classes:
• Chlorophyceae
• Prasinophceae
• Euglenophyceae
– Chromophyta (brown algae);
include the following classes:
• Chrysophyceae
• Pelagophyceae
• Prymnesiophyceae
• Bacillariophyceae (diatoms)
• Dinophyceae (dinoflagellates)
• Cryptophyceae (crytophytes)
• Phaeophyceae (phaeophytes)
– Rhodophyta (red algae)-mostly
macrophytes
1 µm
Micromonas
Pelagomonas
Marine cyanobacteria
Prochlorococcus
• Cyanobacteria: major
groups of cyanobacteria in the
oceans include:
Prochlorococcus,
Synechococcus,
Trichodesmium, Crocosphaera,
Richelia
Synechococcus
– Characteristics: unicellular,
filamentous, colonial
– 2 layer cell wall-inner
murein and outer
lipoprotein
– Some species fix N2
Richelia
Many images from: http://www.sb-roscoff.fr/Phyto/gallery/main.php?g2_itemId=19
Trichodesmium
Rhodophyta (Red Algae)
•
Rhodophytes
– In addition to Chl a, also
contain phycoerythrin,
phycocyanin,
allophycocyanin
– Most marine
representatives are
macrophytes
Porphyridium
Rhodella
Chlorophyta (green algae)
• Chlorophytes
– Contain Chl b
– Uncommon in
open ocean;
mostly
freshwater.
– In marine
environment,
mostly tropical
(Hawaiian
coastal waters)
– Can be single
cells or colonies,
coccoid or
flagellated
• Prasinophytes
– Contain Chl b
– Predominately
unicellular
– Relatively
common, but not
abundant in
ocean
– Can be single
cells or colonies,
coccoid,
biflagellated, or
quadri-flagellated
Nannochloris
Prasinophyceae
Chromophyta (brown algae)
• Pelaophytes
– Contain Chl c
– Very common in
open ocean.
– Coccoid or
monoflagellated
• Chrysophytes
– Contain Chl c
– Relatively rare in
open ocean
– Mostly biflagellated
(flagella of
unequal length)
• Cryptophytes
– Contain Chl c
– Contain
carotenoid
alloxanthin
– Contain
phycoerythrin or
phycocyanin
– Flagellated
unicells
Pelagomonas
Dictyocha
Images from: http://planktonnet.sb-roscoff.fr/index.php#search
Rhodomonas salina
Chromophyta (brown algae)-Cont.
• Prymnesiophytes
– Mainly biflagellates
– Very common in
open ocean
– 2-5 µm
– Some species form
CaCO3 plates
(coccoliths)
• Bacillariophytes
– Ubiquitous
– All contain Chl c and
carotenoid
fucoxanthin
– Rigid silicaimpregnated cell wall
– Many form colonies
– 2 prominent cell
morphologies:
centric and pennate
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Emiliana huxleyi
Coscinodiscus
Rhizosolenia
Chromophyta (brown algae)-Cont.
• Dinophytes
– Possess the
carotenoid
peridinin
– Widely distributed
(estuaries, open
ocean)
– Mostly unicellular
and autotrophic,
but colorless
heterotrophs can
also be abundant
– 2 flagella
– Many are
bioluminescent and
some case toxic red
tides blooms
Armored dinoflagellate-cellulose theca
Ceratium
Naked dinoflagellate
Gymnodinium
Active site of photosynthesis
in eukaryotes is the
chloroplast. Chloroplasts
contain light harvesting
pigments, electron carriers
that capture energy for
reducing power (NADPH2) and
ATP, and enzymes that use
NADPH2 and ATP to fix CO2
and H2O to carbohydrate.
Components of the
chloroplast:
•Membranes
•Thylakoids (lipids,
chlorophyll, and protein)contain the pigments and
electron carriers
•Stroma: gel-like matrix
containing enzymes needed to
fix CO2 (RUBISCO).
• Different algal groups distinguished
by arrangement of thylakoids
– Red algae: single thylakoids
– Cryptophyta: thick pairs of thylakoids
– Other groups tend to have 3 thylakoids
stacked together
– The green algae (and higher vascular plants)
have 5-20 small diameter thylakoids stacked
together
Cyanobacteria have thylakoids free in the
cytoplasm with phycobilisomes along the
thylakoid surface.
Light harvesting
photosynthetic pigments
• Chlorophylls
• Carotenoids
• Biliproteins
• Recently discovered photoreceptor proteins
(Proteorhodopsin and Bacteriorhodopsin)
serve as proton pumps, but do not appear to
harvest energy for oxygenic photosynthesis.
Chlorophylls
• Cyclic
tetrapyrole
with a
magnesium
atom
chelated in
the center of
the ring
Chlorophyll c
lacks the phytol
group
Phytol
•
•
•
•
All oxygen evolving
photosynthetic plankton
contain Chlorophyll a (peak
absorption 450 and 660 nm)
Chlorophyll b (peak absorption
500 and 640 nm) found mostly
found in Chlorphytes,
Euglenophyta, and
cyanobacteria
Chlorophyll c found in diatoms,
dinoflagellates, chrysomonads,
cryptophytes
Chl a, b, and c all absorb
strongly in the red (between
440-450 nm) and blue
wavelengths (~645-660 nm).
The presence of Chl b and c
increases the absorption
between 450-650 nm.
Carotenoids
• Isoprenoid compounds
(lipids)-not water
soluble, embedded in
hydrophobic
membranes.
• Almost all chlorophyll
and carotenoids occur
as complexed proteins.
• β-carotene is found in
all algal classes except
the Cryptophytes
The double bonds absorb strongly in the
short wavelength region of the visible
spectrum
Carotenoids
• Photosynthetic carotenoids participate
in photosynthetic electron transport .
• Non-photosynthetic carotenoids appear
to protect photosynthetic reaction
centers from oxidation via free radical
scavenging
Phycobiliproteins
• Water soluble pigments that capture light energy
and pass the energy to chlorophyll.
• Found in Rhodophytes (red algae),
Cryptophytes, and Cyanobacteria
– Red: phycoerythrin (red algae, often in
cyanobacteria); phycoerythrocyanin
– Blue: phycocyanins, allophycocyanins
• Occur as aggregates, termed phycobilisomes,
composed of 3 or more phycobiliproteins, that
attach to the thylakoid.
Summary of photosynthetic pigments;
pigments in bold are diagnostic
Group
Major pigments
Prokaryotes
Prochlorococcus
Dinvinyl Chlorophyll a and b, monovinyl
chlorophyll b, zeaxanthin
Synechococcus
Monovinyl chlorophyll a, zeaxanthin, phycoerythrin
Eukaryotes
Diatoms
Monvinyl chlorophyll a, chlorophylls c1 and c2,
fucoxanthin
Prymnesiophytes
Monovinyl chlorophyll a, chlorophylls c2 and c3, 19hexanoyloxyfucoxanthin
Pelagophytes
Monovinyl chlorophyll a, chlorophylls c2 and c3, 19butanoyloxyfucoxanthin
Chrysophytes
Monovinyl chlorophyll a, chlorophylls c1 and c2,
fucoxanthin and violaxanthin
Cryptophytes
Monovinyl chlorophyll a, chlorophyll c2, alloxanthin
Dinoflagellates
Monovinyl chlorophyll a, chlorophyll c2, peridinin
Prasinophytes
Monovinyl chlorophyll a and b, prasinoxanthin
Chlorophytes
Monovinyl chlorophyll a and b, lutein
Carotenoids and biliproteins extend the spectral
region able to support plankton growth; thus light
forms an important environmental control on
microorganism evolution.
Stomp et al. (2004)
Stomp et al. (2007)
In vivo absorption spectra
Through variations in pigment
composition, phototrophic
microorganisms differentiate
their light harvesting
capabilities, providing stable
coexistence of multiple
competing species
•
•
•
Reaction center: the site in a photosystem where energy from
absorbed light is transferred (via an electron) from one molecular
(donor) to another (acceptor).
Reaction center chlorophyll becomes excited with the receipt of
light energy.
To reduce this energy, the excited chlorophyll can transfer an
electron to an acceptor.
•
The majority
of energy
captured by
reaction
center
chlorophyll is
transferred via
antenna
pigments.
• Summary of light reactions:
2H2O + 2 NADP+ + 3 ADP +3 Pi
8 hν
O2 + 2 NADPH2 + 3 ATP
• Summary of dark reactions:
CO2 fixation via the Calvin-Benson cycle
CO2 + 2 NADPH2 + 3 ATP
H2O + (CH2O) + 3 ADP + 3Pi + 2 NADP+
CO2 + 2 H2O
8 hν
(CH2O) + H2O + O2
Key enzymes for photosynthesis
• RUBISCO (1,5-bisphosphate
carboxylase/oxygenase)-key enzyme in the
Calvin-Benson cycle, incorporates CO2 into 3phosphoglycerate. Most abundant protein on
Earth.
• Carbonic anhydrase: converts CO2 to
bicarbonate (HCO3-) and vice versa. HCO3- is
the most abundant form of inorganic carbon in
seawater; most marine microorganisms take up
HCO3- and carbonic anhydrase dehydrates to
CO2 near RUBISCO.
Net and Gross Photosynthesis
• Net photosynthesis (PN): rate of photosynthesis
excluding material lost to respiration in the light (RL).
• Gross photosynthesis (PG): The total amount of light
energy converted into biochemical energy. Measured by
evaluating total CO2 fixed or O2 evolved, including that
lost to respiration.
PG = PN + RL
• Note that respiration also occurs in the dark (RD).
• Relatively straightforward to measure net or gross
photosynthesis in pure cultures, but the oceans contain
many organisms that contribute to respiration (e.g.,
bacteria, zooplankton).
Primary Production
• Gross Primary Production: The rate of
organic carbon production via the reduction of
CO2 due to photosynthesis.
• Net Primary Production: Gross primary
production, less photoautotrophic respiration,
integrated over time and depth.
• Net Community Production: Gross primary
production, less all autotrophic and heterotrophic
losses due to respiration.
• Units for all measures of production: mgC m-2 d-1
What methods are used to
measure aquatic primary production?
•
Typical rates of photosynthesis range between 0.2-2 µmol C L-1 d-1 or
0.3-3 µmol O2 L-1 d-1
•
Analytical sensitivity of measurements:
–CO2 by coulometry = 1 µmol C L-1
–O2 by Winkler titration = 0.01 µmol O2 L-1
Most Common methods used to measure
photosynthesis:
• CO2 assimilation: radio- or stable isotopes of carbon
(14C or 13C) – developed by Steeman-Nielsen 1951.
• Changes in O2 concentration - developed by
Gaarder and Gran 1927)
• Oxygen isotope techniques (18O, 17O, 16O)
O2 light-dark bottle approach
Measures changes in oxygen
concentrations in light and
dark bottles following
incubation period:
•Light bottle = net community
production (photosynthesis
and respiration).
•Dark bottle: community
respiration. Light + Dark =
Gross production
Several caveats:
1) assumes rate of respiration
in dark = light.
2) requires conversion from
O2 to carbon units
(photosynthetic quotient, PQ).
3) Requires incubation and
confinement of samples.
PQ values can vary from 1.1
to 1.4 depending on nutrient
environment and products of
photosynthesis.
14C-bicarbonate
assimilation
Examine assimilation of 14C (as
bicarbonate) by photosynthetic
plankton in the light.
•Add 14C labeled HCO3- to bottles
containing seawater; incubate
and harvest plankton biomass by
filtration.
•Count (using scintillation
counter) the amount of
radioactive 14C assimilated into
plankton biomass during
incubation period.
Several caveats:
1) Always returns a positive result, even
if net community production is negative.
2) Does not discriminate light and dark
respiration.
3) Unclear whether net or gross
photosynthesis is measured.
4) Generally ignores organic carbon
produced and excreted or lost during
incubation.
5) Requires incubation and confinement
of samples
Oxygen isotopes
• Addition of 18O labeled H2O: light bottle/dark
bottle incubation approach; traces splitting of
H2O by photosynthesis yielding 18O2. By
simultaneously examining changes in O2
concentrations, can determine gross primary
production.
• Triple oxygen (16O, 17O, 18O): aquatic
photosynthesis has a characteristic fractionation
of H2O in production of O2; can use the ratio of
the different isotopes to derive photosynthetic O2
production.
What factors limit primary
production?
• Nutrients
– Protein (~50% of cell biomass): H, C, O, N, S,
metal co-factors
– Lipid (~10% of cell biomass): H, C, O, P
– Nucleic acids: H, C, N, O, P
– Carbohydrates (~20% cell biomass): H, C, O
• Light
• Temperature