Research in the last decade has provided many examples of how evolution on ecological time scales affects interactions between species. The breakdown in the mutualism between coral reef species and their algal symbionts (Symbiodinium) that occurs as a result of increasingly warmer ocean temperatures is a major threat to coral reefs around the globe. Species may respond to this rapid environmental change through either acclimatization or adaptation, but the long generation times of most host species may result in slow rates of adaptation. However, high rates of somatic mutation have resulted in significant genetic variation within Symbiodinium populations, which could provide the raw material for natural selection and adaptation to increased temperature on contemporary time scales. Such genetic variation, coupled with strong selection pressure and short generation times of symbionts (Symbiodinium) afford the potential for rapid evolution and adaptation of the symbiont population. We measured trait variation among genotypes of Symbiodinium antillogorgium and exposed each genotype to ambient and increased temperatures to determine trait and fitness responses to future climate change.
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. We also found variation in how these traits and the fitness of each genotypes responded to increased temperature. This variation indicates a potential for selection on traits. Selection for particular symbiont genotypes may confer adaptation of the holobiont to increased temperatures on coral reefs and provide some hope for evolutionary rescue of reef species via adaptation of their symbionts.