Genetic diversity in symbiont response to increased nutrients
Increased species diversity typically results in high ecosystem function and stability. However, recent evidence suggests that within-species diversity (e.g. genetic diversity) can play a similar role. Genetic diversity should increase a species’ ability to handle rapid environmental changes, through both ecological and evolutionary mechanisms. The aims of our experiments were (1) to determine whether genotypes of Symbiodinium differ in responses (physical and physiological traits) to nutrient environments (varying N:P ratios) and (2) whether increased genetic diversity affects these responses in the varying environments. Because we expect individual genotypes to differ in these traits, we also expect to find effects of diversity attributable to complementarity, facilitation or sampling effects. We examined nitrogen and phosphorus absorbance, chlorophyll content, photosynthesis (quantum yield and variable fluorescence), cell size and population growth rates for both monocultures and mixed cultures (2, 4, or 8 genotypes) of Symbiodinium genotypes grown in N:P ratios of 5:1, 15:1 and 30:1.
We found that genotypes of Symbiodinium differ in their responses to varying nutrient conditions. Genotypes showed significant differences in cell size and nitrogen absorbance. Nutrient treatment also significantly affected cell size, with the largest cells found at 15:1 N:P ratio. Additionally, we found significant interactions between genotype and nutrient treatment for phosphorus absorbance, variable fluorescence, quantum yield and chlorophyll content. High diversity cultures were more productive than monocultures. Photosynthetic algae of the genus Symbiodinium are taxonomically diverse and form resource mutualisms with many coral reef cnidarians. Our study shows that genotypes of Symbiodinium are also functionally diverse and this diversity affects the ecosystem function of the population. With this, we may begin to understand how the presence of specific symbionts, or a more diverse symbiont community, may make a host more or less capable of coping with a changing environment.