By Charlotte Evans
The corals are dying! They’re bleaching in over-warm waters! Why does this happen? Well, corals are actually animals that live in the rocky structures that they make, and they have a symbiotic relationship with tiny, colorful algae called zooxanthellae that live in their “stomach lining”. When corals are stressed, they kick out these algae and the corals look white. Beyond looking creepy, coral bleaching is also really dangerous for the corals because the zooxanthellae provide much of the nutrients that their hosts need by acting like an extra digestive system. Just like plants, these algae photosynthesize sugars and nutrients from carbon dioxide and sunlight, making food for the corals in exchange for protection (NOAA, 2020).
However, the zooxanthellae are not the only place a coral can get food. Corals are mixotrophs, similar to human omnivores, and can change their reliance on various forms of food based on changes in resource availability. Corals can eat tiny plankton and can also absorb inorganic nutrients like phosphorus and nitrogen from the water to supplement their diet. This ability to take advantage of multiple food sources makes corals flexible and adaptable to many environments, but very little research has been done on the non-zooxanthellae aspect of a coral’s diet in a real-world setting. A recent paper by Fox et al. (2018) used new techniques to model food availability and determine the proportion of nutrition that came from various sources in a particular coral population in the Southern Pacific.
The researchers used an island archipelago as a case study upon which they could test their methods and then extrapolate their findings to other coral reefs around the globe. The crux of their analysis was to collect corals, separate them from their endosymbiont algae, and compare the isotope signatures of carbon and nitrogen in these samples to the surrounding water’s nutrient concentrations. If the isotope signatures of the coral were more similar to one food source, then it was assumed that the coral was relying more on that one food source for its diet.
Further, they looked at satellite data of the amount of chlorophyll in the water as a proxy for plankton, and verified these approximations with more water samples (Fox et al., 2018). Higher levels of nutrients and sunlight allows for more growth, so chlorophyll levels make a good estimate of the productivity of different areas and depths of the ocean. The researchers found that increasing levels of chlorophyll were associated with higher levels of non-algae-based nutrition. On the flip side, the isotope signatures of the algae and their corals matched more in areas where chlorophyll, and therefore plankton, levels were lower, like in deep water farther from the equator (Fox et al., 2018; Wiedenmann & D’Angelo, 2018). What this means is that the corals eat the food that’s abundant in their environment, and maybe fall back on their symbiotic relationship in areas where there’s less “other food” (Fox et al., 2018).
In sum, these results support the idea that corals increase their intake of plankton intentionally in environments where they are more available. This may not seem like a stunning finding, because this is exactly how snacking works for humans too - if there’s a box of donuts available, I’ll choose that over the sandwich I brought from home. But, this was the first study showing a direct link between trophic strategy and location-specific primary production for corals. Potentially more important, it also showed that use of satellite data for estimating chlorophyll is a good predictor for what corals are eating and can be applied to any region.
It’s crazy to think that we can tell, or at least predict, what animals are eating from hundreds of miles away – here, corals chowing down on carb-rich plankton! This will become increasingly important in predicting the ability of corals to recover from bleaching as temperatures change, nutrient gradients change, and ocean levels rise because reliance on zooxanthellae may be detrimental to survival. This study offers an amazing way to study itty-bitty creatures with satellites, uses one archipelago to successfully extrapolate findings to multiple reefs around the world, and shows the power of publically-available data to do fundamental research on ecological trends thousands of miles away.
Fox, M. D., Williams, G. J., Johnson, M. D., Radice, V. Z., Zgliczynski, B. J., Kelly, E. L. A., Rohwer, F. L., Sandin, S. A., & Smith, J. E. (2018). Gradients in primary production predict trophic strategies of mixotrophic corals across spatial scales. Current Biology, 28(21), 3355-3363.e4. https://doi.org/10.1016/j.cub.2018.08.057
National Oceanic and Atmospheric Administration. (2020, January 7). Corals: Zooxanthellae what’s that? Ocean Service Education. https://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html
Wiedenmann, J., & D’Angelo, C. (2018). Symbiosis: High-carb diet of reef corals as seen from space. Current Biology, 28(21), R1263–R1265. https://doi.org/10.1016/j.cub.2018.09.056