Physiology of Genotypes of Coral-Associated Endosymbionts (Breviolum antillogorgium) in Response to Bacterial Communities

Quantifying the effects of bacterial communities and temperature on coral-associated endosymbionts adds to our understanding of coral health. Genetic variation in endosymbionts may increase the fitness of the holobiont - coral and associated microbes - during periods of temperature stress by increasing the breadth under which symbiosis can be maintained. Increased temperatures and variation in the bacterial communities that endosymbiotic cells interact with were replicated in lab experiments with isolated strains of Breviolum antillogorgium, a photosynthetic coral endosymbiont. The novel bacterial communities were isolated from each endosymbiont strain, or "genotype", or sampled from the original marine environment. Every genotype was inoculated with each bacterial community in a full reciprocal transplant at two temperatures in order to assess the effects of bacterial community and temperature on the physiology of the endosymbionts. Each endosymbiont genotype was associated with a distinct bacterial assemblage, which had significant effects on some of the physiological traits generally assigned as endosymbiont/algal traits. The endosymbiont genotype had a significant effect on growth rate, but the bacterial community explained more of the variation in photosynthetic rate and efficiency, respiration rate, chlorophyll a, and nitrate. Temperature played a significant role in endosymbiont growth rate, photosynthetic rate, and respiration, independent of the bacteria present. The bacterial community associated with genotype 16-0630 was the most stable; the initial diversity did not change regardless of which endosymbiont genotype was present. The environmentally-collected bacterial community decreased in diversity throughout the experiment and did not alter the fitness of the endosymbionts. At elevated temperatures, neither the bacterial communities examined nor the genotype had a significant effect on physiological traits. Physiological traits that are important to the mutualism with coral are susceptible to change in the presence of different bacterial communities at ambient temperatures. Future work analyzing the physiology of these important dinoflagellates should control for the bacterial community; providing a beneficial bacterial community post-thermal stress periods could be a method of aiding coral survival.