The role of algal symbiont growth in driving the thermal responses of a widespread photosymbiotic ciliate

Photosymbioses, in which unicellular autotrophs reside within heterotrophic hosts, are critical ecological interactions in terms of global biomass, species diversity and primary production. Such associations are under threat from environmental warming; while specific impacts, particularlyin the coral-zooxanthellae interaction, arethoroughly documented, ecologists lack a general theoretical framework describing the impacts of temperature change on photo-symbiotic interactions. Here, i apply principles of the metabolic theory of ecology (MTE) to assess the metabolic basis of the temperature-induced disruption of photosymbiosis in the microbial Paramecium bursaria — Chlorella spp. association. In contrast to the general predictionthat net autotrophy should decrease with temperature, this microbial photosymbiosis harboured larger algalsymbiont populationsand consumed fewer prey with warming, suggestive of increased net autotrophywith 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.