The interaction between hosts and symbionts often varies along a continuum from parasitism to mutualism, depending on the environmental context in which the interaction occurs. Symbiodinium spp. are important symbionts on coral reefs; the breakdown in the mutualism with coral reef species that occurs as a result of increasingly warmer ocean temperatures is a major threat to coral reefs around the globe. Though coral-algal interactions are dependent on environmental context, they may also depend on the specific genotype of the symbiont. High rates of somatic mutation can lead to high levels of genetic diversity within and among Symbiodinium species. Here we examine the consequences of genetic variation in two species (Symbiodinium antillogorgium and Symbiodinium minutum). In culture, we measured symbiont traits likely to affect the strength of mutualism between symbiont and hosts: growth rate, chlorophyll content, and photosynthetic efficiency. We then examined how different genotypes responded to elevated temperature. Finally, we inoculated polyps and measured these same traits en hospite.
Results/Conclusions
We found significant genetic variation in Symbiodinium traits that are likely to affect the strength of the mutualism with host species, including chlorophyll content, quantum yield, and growth rate. When considering the fitness of symbionts in culture, we found that most genotypes responded negatively to increased temperatures predicted by climate change, but other genotypes responded positively. Such variation in traits implies that there is potential for natural selection in response to temperature. However, trait measures in culture did not correlate well with trait measures in polyps, suggesting that selection in the water column and selection within hosts are likely to differ. How symbiont populations respond evolutionarily to climate change will depend on much of their life cycle the algal cells spend in each environment.