Nitrogen cycling in the temperate Northern star coral, Astrangia poculata: distinguishing autotrophic from heterotrophic nutrient contributions
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Abstract
Coral reefs are in a global state of decline due to coral bleaching, which is a state of dysbiosis between the coral host and its algal endosymbionts that can bring about a nutritional crisis potentially leading to mass mortality. In this study, I used a temperate coral, Astrangia poculata, to determine the effects of symbiotic state and trophic status on nitrogen acquisition. The facultatively heterotrophic nature of A. poculata allows for the decoupled analysis of host and symbiont without the induced stress of a dysbiotic event. In this study, I used δ15N labeled DIN to track the assimilation and translocation of ammonium and nitrate by fed and starved colonies of both symbiotic and aposymbiotic A. poculata. I also analyzed tissue samples for δ13C, %N, %C, and C:N. Using photosynthetic efficiency as a proxy, I analyzed changes in symbiont health before and after being treated with DIN. Stable isotope analysis suggested that corals acquire more of their nitrogen from DIN than from heterotrophy, that their symbiotic algae, Breviolum psygmophilum, are responsible for a greater amount of DIN assimilation than the host, and that ammonium is more readily assimilated than nitrate. Protein analysis was inconclusive in determining any potential advantages to symbiotic state. Photosynthetic efficiency analysis suggested that ammonium boosts symbiont activity and nitrate may adversely affect symbionts. Overall, both symbiotic state and nutritional condition influenced holobiont health, and symbionts were found to be the driving force behind nitrogen acquisition. These results suggest that dysbiosis not only inhibits corals’ mixotrophic strategy of nutrient acquisition but suggests that symbiosis has advantages across nitrogen acquisition pathways that are augmented by feeding. As such, starved, aposymbiotic corals suffer from energetic double jeopardy, and having either feeding or symbiosis alone does not fully equivocate to the energetic advantage of both.