TRACKING NUTRIENT ASSIMILATION IN CORAL USING STABLE ISOTOPES OF AMINO AND FATTY ACIDS: IMPLICATIONS FOR PAST, PRESENT, AND FUTURE REEFS (Invited Presentation)

Corals are the colonial species which form the world’s coral reefs - the most biodiverse marine habitat that is also severely threatened by numerous anthropogenic stressors. Beyond climate change, eutrophication of marine waters is a major threat to corals, one that has been shown to have compounding impacts to the animal. However, little is still known about nutrient assimilation and translocation by the coral animal and algal symbiont (holobiont), undermining our understanding of coral metabolics and predictions of coral health due to global-change. We address this gap by answering three questions: 1) What nutrient sources are preferentially assimilated by the holobiont; 2) What are these nutrients used for metabolically; and 3) Are there species-level differences in nutrient assimilation and allocation? We conducted feeding experiments where Porites sp. and Acropora sp. were simultaneously exposed to three nutrient forms: 1) inorganic (nitrate and bicarbonate) accessible by algal symbionts, 2) organic (urea and glucose) accessible to the holobiont, and 3) particulate food (phytoplankton Isochrysis galbana and zooplankton Artemia salina) accessible to the coral animal. In each treatment, one source was isotopically enriched in nitrogen (¹⁵N) and carbon (¹³C), allowing us to trace it from ingestion through assimilation and transfer. Bulk stable-isotope results showed both species were mixotrophic, although Porites sp. obtained a greater proportion of its nutrition from heterotrophy than Acropora sp. Compound-specific isotope analysis of amino and fatty acids (AA/FA) in Porites sp. revealed that nutrients were routed for different purposes: inorganic nutrients were preferentially synthesized into FA, whereas organic and heterotrophic nutrients were used in AA-synthesis. This work presents the first composite record of bulk, AA, and FA isotope data for corals and we discuss species-level metabolic differences that have implications for their responses to future environmental change as well as their utility in historical-reconstructions.