Project: New insights into the response of reef corals to climate change using the symbiont to host cell ratio as a metric of bleaching susceptibility

Description from NSF award abstract:
The long-term future of reef ecosystems depends on the continued persistence of corals as essential habitat builders. Environmental threats facing coral reefs globally include chronic and acute thermal stress (which can result in "coral bleaching" as a result of the expulsion of their symbiotic algae), increasing CO2, and nutrient pollution. This project will assess the independent and interactive effects of these stressors using a new molecular based technique showing that coral bleaching susceptibility depends on the symbiont:host (S:H) cell ratio in coral tissues. Corals with higher ratios (more symbionts) are predicted to be more susceptible to thermal stress, suggesting that abiotic factors that affect symbiont densities can directly influence thermal tolerance. This new metric of symbiont density is of physiological relevance and has already revealed patterns that differ from currently used metrics, thus providing new insights into the dynamics of these symbioses. This project will also continue the development and application of these methods to further our understanding of the ecology of coral-algal symbioses, while helping to elucidate the responses of these critical ecosystem engineers to complex changes in their environment. The proposed activities will help us understand how tradeoffs in the regulation of symbiotic partners respond to the environment and relate to the differential susceptibility of corals to environmental stress.

From project proposal:
In the laboratory, we will experimentally test the effects of light, temperature, nutrients, and CO2 on the S:H cell ratio in five coral species from the Caribbean and Indo-Pacific. We will then expose these corals to thermal stress to test whether conditions which elevate the S:H ratio, such as elevated nutrients, also increase coral susceptibility to bleaching. We will also test whether conditions which decrease the ratio, such as elevated CO2, decrease bleaching susceptibility. We further hypothesize that higher S:H cell ratios result in higher levels of oxidative stress when exposed to bleaching stressors, and will test this in collaboration with a leading scientist in coral immunity. Complementing these laboratory experiments, we will also assess the degree of spatial and temporal variability in S:H ratios in the field by monitoring seasonal changes at several sites in the northern Florida Keys, and in a new educational coral nursery at the University of Miami’s field station on Broad Key. In addition, we will also apply our molecular assay to existing long-term datasets in order to further test these hypotheses in a collaborative, cost-effective way.

These combined activities will help us understand how tradeoffs in symbiosis are regulated in response to the environment. By identifying the links between environment, S:H cell ratios, and stress susceptibility, we will be better able to understand and predict coral bleaching and mortality in response to climate change, while addressing the critical question of why there is so much variability in bleaching susceptibility among corals in space and time. Testing the hypothesis that the S:H ratio drives bleaching susceptibility may help us properly interpret a large body of previous and ongoing work testing the interactive effects of multiple stressors on corals.

Project-related links:
The Coral Reef Conservation Research Lab
Additional information about Dr. Andrew Baker