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Coral symbioses are predicated on the need for mutual nutrient acquisition and translocation between partners. Carbon translocation is well-studied in this classic mutualism, while nitrogen (N) has received comparatively less attention. Quantifying the mechanisms and dynamics of N assimilation is critical to understanding the functional ecology of coral organisms. Given the importance of symbiosis to the coral holobiont, it is important to determine what role photosynthetic symbionts play in N acquisition. We used the facultatively symbiotic temperate coral  Astrangia poculata  and  15 N labeling to test the effects of symbiotic state and trophic status on N acquisition. We tracked assimilation of 2 forms of isotopically labeled dissolved inorganic N (DIN: ammonium,  15 NH 4  +  and nitrate,  15 NO 3  - ) by fed and starved colonies of both symbiotic and aposymbiotic  A. poculata.  Coral holobiont tissue was subsequently analyzed for δ 15 N and changes in photosynthetic efficiency. Results suggest that corals acquired the most N from DIN via their symbiont  Breviolum psygmophilum  and that NH 4  +  is more readily assimilated than NO 3  - . Photosynthetic efficiency increased with the addition of NH 4  + , but only for fed, symbiotic treatments. NO 3  -  adversely affected photosynthetic efficiency among starved corals. Our results suggest that symbiosis is advantageous for DIN acquisition, that dysbiosis inhibits corals’ mixotrophic strategy of nutrient acquisition, and that either feeding or symbiosis alone does not fully provide the energetic advantage of both. This study lends support to the emerging hypothesis that symbionts are mutualists in optimal conditions but shift to a parasitic paradigm when resources or energy are scarce.

marine ecology. progress series

Testing assumptions of nitrogen cycling between a temperate, model coral host and its facultative symbiont: symbiotic contributions to dissolved inorganic nitrogen assimilation