Calibrating the mixotrophy spectrum 

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 suggests that corals more reliant on autotrophy are more susceptible to bleaching in response to warming temperatures.

My recent work developed a novel approach to rapidly assess the relative reliance on autotrophy vs heterotrophy across seven coral genera, the largest number in one study to date. I found that coral trophic strategies fall along a “mixotrophy spectrum” that ranges from high dependence on heterotrophy to high dependence on autotrophy. However, it is unknown if differences across species are a result of variation in the input of heterotrophic vs autotrophic nutrients or variation in the amount of nutrient recycling with algal symbionts.

This project uses a combination of conventional and cutting-edge approaches to determine which of these mechanisms underlies differences in coral trophic strategy and thus “calibrate” the mixotrophy spectrum. Using three coral species that span this spectrum (high heterotrophy, high autotrophy, and middle), my team is first using classic measurements of feeding, respiration, and photosynthesis to quantify the contribution of heterotrophic and autotrophic nutrients to coral respiration. Second, we are applying a stable isotope labeling technique to quantify the contribution of heterotrophic and autotrophic nutrients to coral and algal growth. Finally, we are conducting an elongated isotope labeling experiment to measure the amount of nutrient recycling between the coral and algae.

This project will determine the physiological mechanism that underpins trophic strategy differences across corals, improving existing assessment approaches. Given the link between trophic strategy and bleaching resistance, this work has critical ramifications for the ability to predict bleaching responses, identify corals resilient to climate change, and implement effective conservation and restoration strategies to protect valuable coral reef ecosystems.