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Name_________________
Coral bleaching and climate change
Featured scientist: Carly Kenkel from The University of Texas at Austin
Research Background:
Corals are animals that build coral reefs. Coral reefs are home to many species of
animals – fish, sharks, sea turtles, and anemones all use corals for habitat! Corals are
white, but they look brown and green because certain types of small plants, called
algae, live inside them. The algae produce food for the corals so they can grow big, and
the corals provide the algae a safe home. The algae and corals form a mutualism, or a
relationship between two species where both partners benefit each other and do better
together than they would alone.
When the water gets too warm, algae can no longer live inside corals. The corals turn
from green to white, called coral bleaching, because they do not have algae living in
them. Climate change has been causing the Earth’s air and oceans to get warmer.
With warmer oceans, coral bleaching is happening more often. If the water stays too
warm, bleached corals will die without their algae mutualists.
Carly is a scientist who wanted to study coral bleaching so she could help protect corals
and coral reefs. One day, Carly observed an interesting pattern. Corals on one part of a
reef were bleaching while corals on another part of the reef stayed healthy. She
wondered, why? Why can some corals and their algae still work together when the
water is warm while others cannot?
A Pacific coral reef with many corals
Carly observing a coral reef
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Ocean water that is closer to the shore (inshore) gets warmer than water further away
(offshore). Perhaps corals and algae from inshore reefs are used to warm water. She
wondered whether inshore corals were better able to work with their algae in warm
water because they are used to these temperatures. If so, inshore corals and algae may
bleach less often than offshore corals and algae. Carly designed an experiment to test
this. She collected 15 corals from inshore and 15 from offshore reefs in the Florida
Keys. She brought them into an aquarium lab for research. She cut each coral in half
and put half of each coral into tanks with normal water and the other half into tanks with
heaters. The normal water temperature was 27°C and is a temperature that both
inshore and offshore corals experience during the year. The warm water tanks were
31°C and are a temperature that inshore corals experience, but offshore corals never
experienced in the past but may experience with climate change in the future. After six
weeks she recorded the number of corals that bleached in each tank.
Check for Understanding: After reading the Research Background and looking at the
above model of the experiment students should be able to
• describe where the term “coral bleaching” comes from. Why do corals turn white?
• define mutualism and describe why the relationship between corals and algae
represents a mutualism. What do these partners provide for each other?
• describe how climate change could affect the coral-algae mutualism.
• recognize that the heat treatment represents climate change and the normal water
represents the control.
• describe how Carly used observations to decide what she wanted to study. Discuss the
many roles observation can play in the process of science.
Scientific Question: What are the differences in coral bleaching for inshore and offshore
reefs when grown in warm water?
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Teacher Copy, Level 1
What is the hypothesis? Find the hypothesis in the Research Background and underline
it. A hypothesis is a proposed explanation for an observation, which can then be tested
with experimentation or other types of studies.
Scientific Data:
Use the data below to answer the scientific question:
Corals from
Inshore reef
Offshore reef
Inshore reef
Offshore reef
Tank type
Normal
Normal
Heater
Heater
# bleached corals / total
0 / 15
0 / 15
5 / 15
10 / 15
What data will you graph to answer the question?
Independent variable: Where the corals were collected
(inshore or offshore)
Dependent variable: Number of bleached corals in heated
aquarium
Check for Understanding: After taking some time to look at the data table, have students
discuss the question, “What type of graph should you make?” There are many different
kinds of graphs, and each is appropriate for different types of data. What type of graph
would be most appropriate to make with these data?
• Bar graphs - suitable for when you have a categorical independent variable and
continuous dependent variable. Used to make comparisons among groups.
• Histograms - suitable for showing the distribution of continuous data. Breaks data into
equal intervals.
• Line graphs - suitable for when you have continuous independent and dependent
variables, like changes over time. Used to look at trends.
• Pie graphs - suitable for showing data that are parts of a whole.
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Draw your graph below: Identify any changes, trends, or differences you see in your
graph. Draw arrows pointing out what you see, and write one sentence describing what
you see next to each arrow.
•
•
Under the normal temperatures, no corals bleached. The number
was 0 bleached for both inshore and offshore corals (table).
In the heated temperatures, a greater number of offshore
corals bleached than did inshore corals (table or graph).
Interpret the data:
Make a claim that answers the scientific question.
In the warm water treatment, corals from offshore reefs bleach
more than corals from inshore reefs.
What evidence was used to write your claim? Reference specific parts of the table or
graph.
In normal water temperatures, there was no bleaching by either
the inshore or offshore corals. The number was 0 bleached for
both inshore and offshore corals (table). In warm water, 1/3 of
corals from inshore bleached, while 2/3 of corals from offshore
bleached.
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Explain your reasoning and why the evidence supports your claim. Connect the data
back to how warm water affects the mutualism between corals and algae.
The fact that offshore corals bleached more in the heated
treatment means that they were not able to tolerate the warmer
water as well as corals from inshore.
Overall, the offshore coral were not able to support a mutualism
with the algae when put into the warm water during the
experiment. This caused most of the corals to bleach in warm
water. Most of the inshore coral were able to support a
mutualism with their algae when placed into warm water during
the experiment, so fewer inshore corals bleached.
Teacher Note: Have a class discussion during the Interpret the data task on why your
students think inshore corals bleached less than offshore corals. What are potential
mechanisms (alternative hypotheses)? Carly shared with us two potential mechanisms that
she thinks could be driving this pattern and that she plans to explore next:
1. Because corals from inshore reefs live in water that gets warmer they may have
acclimated to warmer temperatures. Maybe inshore corals and their algae have more
experience living in warm water and so are better at working together when the water is
warm. This would be a physiological change that occurs within one generation, what
scientists call plasticity. Plasticity happens when a trait (like staying in a mutualism) is
determined by cues from the organism’s environment (like water temperature). In this
example, the corals and algae from the inshore reefs had been exposed to warm water
prior to the experiment, so they may have acclimated and be better suited to forming
mutualisms in this environment. This is similar to someone who grew up in Alaska being
better able to tolerate a freezing day than someone visiting from Florida who has not
acclimated to the extreme cold.
2. The second potential mechanism is evolution. If previous generations of inshore corals
(their parents and grandparents) lived in warm water, then individuals that were better
able to resist bleaching would survive better and produce more offspring. Over
generations, mutations that helped the corals to not bleach would accumulate in the
population. The ability to not bleach when the water gets warm is called an adaptation,
or a trait that helps an organism do better in its environment. These traits would be
inherited (just as you inherit eye color from your parents, corals may inherit the ability to
not bleach when the water gets warm from their parents) and passed down to the next
generation.
Did the data support Carly’s hypothesis? Use evidence to explain why or why not. If
you feel the data were inconclusive, explain why.
These data supports the idea that inshore corals and the algae
within them are more resilient to warmer ocean temperatures and
may be better able to tolerate warmer waters caused by climate
change.
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Teacher Note: You can expand your students’ understanding of this study system by
including a discussion of how evolution might play out for corals facing warmer waters from
climate change. Ask students to discuss which traits of the corals might be under selection
from the environment. Once students have identified traits under natural selection, bring in
the other things that must be present for evolution to occur. Remember VIST – Variation,
Inheritance, Selection, and Time. Ask students if they see evidence of variation in the traits.
Students can refer to the fact that not all offshore corals bleached, and some inshore corals
bleached to serve as evidence that there is variation in bleaching traits in each population.
From this study we cannot determine if coral bleaching traits are heritable, but students
could ask the scientist if she knows if corals that bleach leave behind offspring that bleach as
well. We see evidence that selection is acting on the traits related to coral bleaching
because of the differences in coral bleaching between the inshore and offshore populations
when grown in normal or heated waters. Finally, time, in the form of several generations,
would be needed to observe evolution. Over generations, corals that did not bleach would
survive and leave behind more offspring, and traits that helped corals not to bleach
(adaptations) would become more common in the population.
Your next steps as a scientist: Science is an ongoing process. What new question do
you think should be investigated? What future data should be collected to answer your
question?
Students may come up with a variety of answers here. See the
Teacher Note for ideas on a classroom discussion on this topic.
Teacher Note: This study was the first step towards addressing why some coral
populations bleach in warm water while others do not. Take a moment before the Your
next steps as a scientist task to have a class discussion and ask your students whether
they think coral bleaching is caused by traits of the corals or the algae. Do the data from
this experiment give us any indication of whether it is the algae or the corals from inshore
reefs that are more tolerant to warm temperatures? Because the term is called coral
bleaching, students may assume that the data is about corals only. But the data actually
represents the mutualism between corals and algae. Independent of issues of plasticity or
adaptation, scientists have still not determined which mutualist partner is responsible for
coral bleaching or tolerance to warm temperatures by some populations.
Carly’s research will address two main questions in the future:
• Knowing that inshore corals are more tolerant of warm water, Carly will next try to
determine what caused this difference. She thinks that maybe one partner, the algae or
the animal, could be responsible. Is it because the algae themselves are less stressed
by warm water, or because the coral animal is stronger and can shield the algae?
• She will also explore whether plasticity or evolution was responsible for the difference.
Do corals at inshore reefs keep working with their algae because they are better
adapted to warm water compared to offshore corals (evolution)? Or could any coral
from an inshore or offshore reef do better in warm water if given enough time to
practice (plasticity)?
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Name_________________
Teacher Note: Once students have completed this activity, you can have a class
discussion where you ask them to apply this research to a new scenario. How does this
experiment help scientists understand the effects of climate change?
Carly’s research helps our understanding of how the warming of ocean waters as a result of
climate change may affect coral reefs. This experiment was designed to examine how the
mutualism between corals and algae might respond to warmer water. Carly’s data show
that some corals and their mutualists were able to remain unbleached whether they were
from inshore or offshore habitats. However, many more corals from the inshore remained
unbleached because these populations had either adapted or acclimated to warm water.
This means that while some corals may be able to tolerate or adapt to warm water, the
overall performance of reefs is likely to decrease if ocean warming happens rapidly. This
has implications beyond the coral and algae mutualism. Many species use coral reefs for a
habitat and if coral reefs decline globally, we will likely see a loss of some of these species.
Additional teacher resources related to this Data Nugget
There are two scientific papers associated with the data in this Nugget. The citations and
PDFs of the papers are below:
• Kenkel, C. D., Meyer, E., Matz, M. V. (2013). Gene expression under chronic heat
stress in populations of the mustard hill coral (Porites astreoides) from different
thermal environments. Molecular Ecology, 22:4322-4332
o http://datanuggets.org/wp-content/uploads/2014/03/mec12390-final.pdf
• Kenkel, C. D., Goodbody-Gringley, G., Caillaud, D., Davies, S. W., Bartels, E., Matz,
M. V. (2013). Evidence for a host role in thermotolerance divergence between
populations of the mustard hill coral (Porites astreoides) from different reef
environments. Molecular Ecology, 22:4335-4348
o http://datanuggets.org/wp-content/uploads/2014/03/mec12391-final.pdf
A video in BioInteractive’s “Scientist at Work” series showing researchers working on the
same hypothesis in another part of the world: Steve Palumbi & Megan Morikawa Study
Coral Reef Damage in American Samoa:
• https://www.hhmi.org/biointeractive/steve-palumbi-megan-morikawa-study-coral-reefdamage-american-samoa
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