Reefscape of diverse corals on the south side of Kiritimati (pronounced "Christmas") Island.
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.”
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
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.