Selective nutrient incorporation may underestimate heterotrophy of a mixotrophic reef-building coral.

Mixotrophic reef-building corals acquire nutrition via photosynthate translocation from endosymbiotic microalgae and by heterotrophic prey/particle capture. Heterotrophy promotes resistance to, and recovery from, environmental stress, but quantifying coral heterotrophy remains difficult due to complex resource exchanges within the coral holobiont. We interrogated the response of multiple biomarkers to coral heterotrophy using fatty acid profiling and stable isotope analysis of Stylophora pistillata grown along a controlled feeding gradient from pure autotrophy to pure heterotrophy. We found that fatty acids and nitrogen were effectively incorporated into both coral host and symbiont tissues, while carbon, which is the primary target for conventional heterotrophy measurements, was not. Our study underscores a functional purpose of heterotrophy to retain essential elements (e.g., N) and molecules (fatty acids) for mixotrophic corals. Selective nutrient incorporation of heterotrophic material also suggests that coral ecologists are likely underestimating the contribution of heterotrophy to a common reef-building coral using conventional carbon isotope offset approaches.