Hundreds of studies have now demonstrated that species diversity affects ecosystem function. More recent research has demonstrated similar effects of genetic diversity on stability and productivity. 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. High rates of somatic mutation can lead to increased genetic diversity within Symbiodinium species. Here we examine the consequences of such variation in one species (Symbiodinium antillogorgium). We measured symbiont traits likely to affect the strength of mutualism between symbiont and hosts: growth rate, chlorophyll content, photosynthetic efficiency, and nitrogen use. We then examined how different genotypes respond to different temperature and nutrient regimes. Finally, we measured these same traits in replicate cultures composed of two, four, or six genotypes.
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, growth rate, and nitrogen use. 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. Similar, some genotypes responded positively to the high nitrogen environments expected within hosts, but others responded negatively. Surprisingly, unlike most studies on species diversity, we found that increasing genetic diversity reduced traits likely to benefit hosts. Such variation in traits implies that there is potential for natural selection in response to temperature and nutrients, but that such selection may depend on which other genotypes are present in the population. Evolution in response to such selection may confer adaptation of the holobiont to climate change and provide some hope for evolutionary rescue of reef species via adaptation of their symbionts.