Bacteria affect the performance of many species, but their presence may also have indirect effects on interspecies interactions. Corals and their associated photosynthetic algal symbionts (Symbiodinium spp.) are critical foundation species. The mutualism between coral and symbionts, along with associated bacteria, collectively forms the “holobiont”. Bacteria alter nutrient dynamics within the host, which may have an indirect effect on the interaction between corals and their symbionts. The octocoral species Antillogorgium bipinnata associates strongly with Symbiodinium antillogorgium, and the symbionts flourish in vitro making them an ideal study system. Preliminary data show that different genotypes of S. antillogorgium vary in their physiology, but the extent to which these differences are driven by dissimilarities in the associated bacterial community are unclear. Bacterial communities may play a vital role in holobiont fitness by affecting the symbiont’s ability to contribute photosynthates to the coral host. This study aimed to discern if the abundance of bacteria affects symbiont physiology, and how bacteria influence different S. antillogorgium genotypes. Four S. antillogorgium genotypes were isolated from the coast of the Florida Keys. They were cultured at 26°C, then treated with an antibiotic cocktail to reduce bacterial presence below measurable levels. All symbionts were innoculated with native bacteria communities at relative high, medium, and low densities, and measured for a response in photosynthesis, respiration, and nitrogen uptake, standardized for cell density. We compared trait measurements before and after treatment with antibiotics to control for antibiotic impact.
Results/Conclusions
The effects of bacteria on the physiology of S. antillogorgium were dependent on the algal genotype. Each genotype interacts uniquely with bacteria to amplify or diminish the effects on physiology. As S. antillogorgium is a host-specific dinoflagellate, associating with bacteria that enhance their physiology is important for their survival. Bacteria are likely to indirectly affect the mutualism between coral and symbionts by altering nutrient dynamics and decoupling the strong resource mutualism between host and symbiont. Diazotrophs, or nitrogen fixing bacteria, influence the supply of nitrogen through a system, and other bacteria may alter the available forms of nitrogen. Performance traits of symbionts are dependent on the specific form of nitrogen, with ammonia being the preferred form. Understanding the bleaching response of corals in response to new environmental conditions from warming oceans will also require understanding how bacteria affect nitrogen cycling and symbiont performance.