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Bleached Corals Compensate For Stress By Eating More Plankton

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In healthy corals, symbiotic algae housed within the coral photosynthesize during the day while the coral itself feeds on zooplankton, tiny shrimp and bug-like organisms, during the night.

It is estimated that corals receive more than half of their total nutrition from these symbionts, known as zooxanthellae.

"Together, corals and zooxanthellae are the world's most efficient light harvesters—far better than plants," says Roberto Iglesias-Prieto, professor of biology at Penn State. "They can absorb the same amount of light as a green plant but with an investment of an order of magnitude less chlorophyll, which is the most expensive thing for a primary producer to create."

But when corals bleach, they ditch their algal collaborators that normally live within their polyps, creating a nutrient deficit for the already stressed-out coral.

As Todd LaJeunesse, a biology professor at Penn State and expert in coral mutualism explains, "Corals occur in very nutrient-poor environments. They get around this problem by maintaining a symbiotic relationship with photosynthetic algae." So how do corals handle the sudden loss of energy when they bleach?

According to a a new study published in in Nature, the corals eat more. Researchers from Laboratoire d'Excellence in New Caledonia, France tested whether bleached corals ate more plankton than healthy corals. Specifically, they looked at whether corals ate more diazotrophs - planktonic bacteria and archaea that fix nitrogen and are potentially a great source of energy for corals. The consumption of diazotrophs by bleached corals was compared to healthy corals that had their algal zooxanthellae symbionts present.

Y. Tsukii

The scientists first fed a group of diazotrophs a diet with an altered form of nitrogen, known as an isotope, capable of being tracked and measured by scientists without changing how the nitrogen itself moves though the food chain. The nitrogen-altered diazotrophs were then fed to healthy and bleached corals, and the amount of isotopic nitrogen incorporated into the coral's tissues was measured.

Not only did the bleached corals incorporate 30-times more isotopic nitrogen than the healthy corals, but the researchers also found the bleached corals preferred to eat one particular diazotroph, Synechococcus, a particularly nitrogen-rich microbe.

"This specific feeding also represents a non-negligible source of carbon for corals devoid of [zooxanthellae]," say the authors in their publication. The amount of nitrogen-fixation performed by marine diazotrophs and the area of the world diazotrophs cover are both expected to increase as ocean temperatures continue to climb. So, increased consumption of nitrogen-rich plankton could be instrumental for coral survival and recovery from stressful bleaching.