The Role of Algal Symbiont Growth in Driving the Thermal Responses of a Widespread Photosymbiotic Ciliate

Understanding the impacts of climate change on ecosystems remains a major contemporary research theme in the biosciences. However, the role of mixotrophs, organisms that combine autotrophic and heterotrophic processes to grow and reproduce, as important determinants of the thermal responses of ecosystems and their services is largely unresolved. In particular, photosymbioses, ubiquitous mixotrophic associations in which autotrophs reside within heterotrophic hosts, are known to play key roles in global biodiversity and carbon cycling. While specific impacts, particularly in the coral-zooxanthellae interaction, are thoroughly documented, ecologists lack a general theoretical framework describing the impacts of temperature change on photosymbiotic interactions. Here, I apply principles of the metabolic theory of ecology to assess the metabolic basis of the temperature-induced disruption of photosymbiosis in the microbial <i>Paramecium bursaria - Chlorella</i> spp. association. In contrast to the general prediction that net autotrophy should decrease with temperature, this microbial photosymbiosis harboured larger algal symbiont populations and consumed fewer prey with warming, suggestive of increased net autotrophy with warming - a pattern that held across strains isolated from three different continents. This observation appeared to be a simple consequence of the response of symbiont growth rate. I conclude that a metabolic framework for photosymbiosis may prove insightful, but the ecological dynamics of the associations (e.g. mixotrophic strategy) must be considered in tandem.