Corals could survive climate change by getting fat

When it comes to climate change, some shall live and some shall die. If we can make educated guesses as to which species will be in the first group and which in the second, we can put more of our conservation efforts where they will be more likely to succeed. That’s especially important for corals, a group of critters particularly susceptible to the effects of a warming world and an increasingly acidic ocean.

Corals themselves are colorless. Their beautiful hues come from the algae that live symbiotically inside the cells of the individual corals. When seawater gets too warm, the corals become stressed, and the algae are ejected from inside their cells. Lacking the vibrant brown, yellow, green, or red algae, the corals appear “bleached” and remain a pale white-ish brown-ish tone. Of course, some corals are more resistant to bleaching than others. And while corals can recover from single bleaching events, or from recurring bleaching events that are separated by multiple years, it is unknown whether they’ll be able to sustain the annual bleaching that is predicted by climate change.

By 2025, the Caribbean is expected to be hot enough that the corals will bleach once each year. The rest of the tropics are expected to catch up by 2050. If scientists can figure out what makes those corals stronger than others, they might be able to better identify which corals are more likely to weather the oncoming climactic storm, and which are better left to creep their way closer to extinction.

New research conducted by Andréa Grottoli of The Ohio State University provides some of the first evidence that some corals can, it turns out, weather annual bleaching, as well as some insight into how they’re able to do it. She and her colleagues started by collecting chunks of coral from 9 healthy reefs off the eastern coast of Mexico and, in a laboratory, subjecting them to carefully controlled water temperatures. The experiment was repeated the following year. It’s the first time that research on coral bleaching has subjected the same corals to bleaching year after year. She discovered that the corals best able to survive the annual bleaching were fatter. They had more energy stored in their cells.

In order to recover following a bleaching event, corals need to acquire new algae. The strongest ones, though, were able to switch the type of algae that served as their symbiotic partner. Algae get most of their nutrients from the coral in which they live, and the corals retrieve the sugars that the algae produce in photosynthesis. Excess sugars are stored as fat, just like in humans.

The corals that proved most resilient to repeated bleaching events were ones that stored up extra fat during the good times, so that they could more easily survive the bleaching until the waters began to cool again. They were also algae generalists – once the waters did cool, they could begin working together with whatever algae was present, rather than waiting around for a preferred species. Corals that didn’t store fat or were too finicky about their algal partners didn’t survive.

Together, the study provides researchers and conservationists with two important pieces of information. The first is insight into which species of coral are more likely to survive repeated annual bleaching. That provides a target for conservation efforts.

The second is the realization that just because a given coral species can survive a single bleaching event doesn’t mean they’re capable of the withstanding climate change. “The capacity of corals to resist and recover from single isolated bleaching is not a reliable predictor for resistance and recovery potential from annually recurring bleaching,” Grottoli writes. – Jason G. Goldman | 09 July 2014

Source: Grottoli, Warner, Levas, Aschaffenburg, Schoepf, McGinley, Baumann, and Matsui (2014). The cumulative impact of annual coral bleaching can turn some coral species winners into losers. Global Change Biology.

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