Genotype-by-environment interactions and diversity-function relationships of the important reef symbiont, Symbiodinium antillogorgium, across a range of nutrient environments

Genetic variation and diversity provide the fuel for natural selection and evolution, but also play roles in ecological processes. The ability of organisms to respond to global change will depend on their genetic identity-they can migrate, acclimatize, or adapt. While adaptation was traditionally thought to be a slow and gradual process, rapid evolutionary change can occur within populations given adequate genetic variation. As such, it is important to know the responses of genotypes to various environments (genotype-by-environment interactions) to understand potential adaptive ability in the face of shifting selective pressures and potential ecological effects that these changes may initiate. In addition, understanding the role of genetic variation and diversity is especially important for species that play substantial roles in their ecosystems-Symbiodinium is a genus of photosynthetic dinoflagellate best known for its mutualistic association with coral reefs. In two laboratory experiments, I examined 1) how six genotypes of Symbiodinium antillogorgium performed across three nutrient environments (genotype-by-environment interactions) and 2) how genetic diversity (two, four, and six genotype treatments) of S. antillogorgium respond across three nutrient environments (diversity-function relationships). The measured responses were those most likely to influence the resource mutualism between S. antillogorgium and a host (growth rate, quantum yield, minimal chlorophyll fluorescence, in vivo chlorophyll fluorescence, and nitrogen consumption). I found significant genotype-by-environment interactions for a number of traits (quantum yield, minimal chlorophyll fluorescence and nitrogen consumption) and when responses were considered as a multivariate phenotype. Contrary to the expectation of seeing increasingly positive effects of genetic diversity in more stressful nutrient environments, I found that genetic diversity had significantly negative effects on S. antillogorgium growth rate, quantum yield, and minimal chlorophyll fluorescence, while nutrient treatment had a significant independent effect on only minimal chlorophyll fluorescence. These results suggest that genotypes of S. antillogorgium may perform better with fewer intraspecific competitors, but that the variation that exists among genotypes of S. antillogorgium may allow the mutualism to adapt to global change.