Temperature moderates the infectiousness of two conspecific Symbiodinium strains isolated from the same host population.

Symbioses between cnidarians and symbiotic dinoflagellates (Symbiodinium) are ecologically important and physiologically diverse. This diversity contributes to the spatial distribution of specific cnidarian-Symbiodinium associations. Physiological variability also exists within Symbiodinium species, yet we know little regarding its relevance for the establishment of symbiosis under different environmental conditions. Two putatively conspecific Symbiodinium strains (both ITS2-type A4) were isolated from the sea anemone Exaiptasia pallida and placed into unialgal culture. Thermal tolerance of these cultures was compared following heating from 26°C to 33.5°C over 18 days. Photosystem II function was negatively impacted by heating in one strain while PSII function in the other showed little response to elevated temperature. Additionally, infection of Symbiodinium cells into aposymbiotic anemones was assessed for both strains at 26°C and 30.5°C. The heat-sensitive strain had greater infection success at 26°C, while there was no difference in infection between the two strains at the higher temperature. Results from this work suggest that variability in thermal optima or -tolerance within Symbiodinium spp. has relevance for early stages of host-Symbiodinium interactions. Thus, varying infectiousness among differentially heat-sensitive Symbiodinium strains could provide a mechanism for the emergence of novel and potentially resilient cnidarian-Symbiodinium associations in a rapidly warming environment.

Partitioning of Respiration in an Animal-Algal Symbiosis: Implications for Different Aerobic Capacity between Symbiodinium spp.

Cnidarian-dinoflagellate symbioses are ecologically important and the subject of much investigation. However, our understanding of critical aspects of symbiosis physiology, such as the partitioning of total respiration between the host and symbiont, remains incomplete. Specifically, we know little about how the relationship between host and symbiont respiration varies between different holobionts (host-symbiont combinations). We applied molecular and biochemical techniques to investigate aerobic respiratory capacity in naturally symbiotic Exaiptasia pallida sea anemones, alongside animals infected with either homologous ITS2-type A4 Symbiodinium or a heterologous isolate of Symbiodinium minutum (ITS2-type B1). In naturally symbiotic anemones, host, symbiont, and total holobiont mitochondrial citrate synthase (CS) enzyme activity, but not host mitochondrial copy number, were reliable predictors of holobiont respiration. There was a positive association between symbiont density and host CS specific activity (mg protein(-1)), and a negative correlation between host- and symbiont CS specific activities. Notably, partitioning of total CS activity between host and symbiont in this natural E. pallida population was significantly different to the host/symbiont biomass ratio. In re-infected anemones, we found significant between-holobiont differences in the CS specific activity of the algal symbionts. Furthermore, the relationship between the partitioning of total CS activity and the host/symbiont biomass ratio differed between holobionts. These data have broad implications for our understanding of cnidarian-algal symbiosis. Specifically, the long-held assumption of equivalency between symbiont/host biomass and respiration ratios can result in significant overestimation of symbiont respiration and potentially erroneous conclusions regarding the percentage of carbon translocated to the host. The interspecific variability in symbiont aerobic capacity provides further evidence for distinct physiological differences that should be accounted for when studying diverse host-symbiont combinations.