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Reefscape of diverse corals on the south side of Kiritimati (pronounced "Christmas") Island.
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Hard corals are animals
that host symbiotic algae in their tissues called zooxanthellae. Corals obtain
most of their food from algal photosynthesis – the algae make sugars from
carbon dioxide, water, and sunlight, and some of this gets leaked to the coral
hosts, feeding them. Despite this effective relationship in
which a heterotrophic animal benefits from photosynthesis of microscopic algae,
additional nutrients such as nitrogen and phosphorus are necessary for plant
and animal growth, and must be attained by the coral ingesting zooplankton,
particulate matter, or dissolved compounds. In a perfect system the corals provide
shelter and nutrients like nitrogen and phosphorus to the algae, and the algae
provide the corals with sufficient food to grow. This symbiotic relationship
allows corals to create hard skeletons. Over long periods of time, corals can
grow into reefs large enough to view from space. However, chronic or episodic stress can
push this relationship out of whack, leading to the coral host expelling its
symbionts and becoming “bleached.”
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| This
white Acroporid coral colony is completely bleached, with its white skeleton now visible through clear tissues. If it has enough fat stores or is able to feed on zooplankton, it might survive this bleaching episode. |
The main
environmental stressor that causes large-scale coral bleaching is increased sea
surface temperature. But why do corals bleach? That’s an important question. Bleaching
is not as straightforward as it might seem, particularly because different
colonies of corals of the same species—even ones that live right next to each
other—might have very different responses to the same stress.
First, let’s get
a little technical: The current theory is that increased light and temperatures
cause direct damage to the photosystem II portion of the photosynthetic pathway
in coral symbionts. Excess oxygen radicals are produced that build up and
eventually become toxic to the coral host. This “oxidative stress” results in
the degradation and eventual expulsion of symbionts from host tissue. Interestingly, corals can host
different types of zooxanthellae, and these can differ in their thermal and
light tolerance. One theory suggests that stressed corals bleach to swap out
less tolerant for more tolerant symbionts.
However, once
symbionts are expelled, corals can starve or become more susceptible to disease. Corals that bleach and survive might either eat enough zooplankton, or live off stored-up fat, to survive these lean times. Increasing
seawater temperatures associated with global climate change are likely to result
in more frequent bleaching events.
We are joining
the Cobb and Baum labs on Kiritimati Island in March to help answer the questions:
What
factors influence coral survival through an extreme bleaching event?
Are there
characteristics we can identify that might predict coral survival in future
events? Understanding why
some corals resist or better recover from bleaching is crucial to better
protecting reefs into the future.
