The interaction between coral feeding and bleaching

Corals are able to thrive in nutrient-poor tropical seas due to their nutritional partnership with algae; the algae harness autotrophic nutrients (inorganic carbon and nitrogen) through photosynthesis whereas the coral is a heterotrophic consumer that feeds on organic particles in seawater, including plankton. These partners exchange nutrients, maximizing their sources of nutrition and recycling carbon and nitrogen between them so that these nutrients are conserved. Given that corals have access to both autotrophic and heterotrophic forms of nutrition, they are defined as mixotrophs. However, mixotrophy is different across coral species, where some are more dependent on autotrophic nutrition whereas others are more dependent on heterotrophic nutrition. Further, evidence indicates that corals more reliant on autotrophy are more susceptible to bleaching in response to warming temperatures.

My previous work applied a novel stable isotope-based approach to rapidly assess the relative reliance on autotrophy vs. heterotrophy across seven coral genera and demonstrated a correlation between reliance on autotrophy and resistance to bleaching; corals that rely more on nutrition from their algae are the first to bleach during thermal events, while corals that consume more plankton persist longer under elevated temperatures without bleaching. Despite the clear link between trophic strategy and bleaching, it is unknown if and how the abundance of planktonic food affects coral bleaching resistance. Further, current understanding of the genes involved in feeding is limited and there have been no investigations into the genetic underpinnings of the interaction of heterotrophy and temperature stress.

In a controlled experiment, this project will manipulate the abundance of plankton and the thermal stress experienced by two coral species: Acropora samoensis, a predominant autotroph highly susceptible to bleaching, and Platygyra carnosa, a predominant heterotroph that exhibits resistance to bleaching. Samples will be collected at multiple time points during the experiment for stable isotope analysis to assess trophic status and RNA extraction to assess gene expression.

This work will inform how corals with different trophic strategies respond to the combination of food abundance and thermal stress, and reveal the genetic mechanisms behind delayed bleaching in heterotrophic species. These results will have critical ramifications for predicting the responses of coral communities to future climate scenarios as well as identifying the most resilient coral species and individuals for use in reef conservation and restoration activities.