Amino acid δ(13)C and δ(15)N analyses reveal distinct species‐specific patterns of trophic plasticity in a marine symbiosis

Compound‐specific isotope analyses (CSIA) and multivariate “isotope fingerprinting” track biosynthetic sources and reveal trophic interactions in food webs. However, CSIA have not been widely applied in the study of marine symbioses. Here, we exposed a reef coral (Montipora capitata) in symbiosis with Symbiodiniaceae algae to experimental treatments (autotrophy, mixotrophy, heterotrophy) to test for trophic shifts and amino acid (AA) sources using paired bulk (δ(13)C, δ(15)N) and AA‐CSIA (δ(13)C(AA), δ(15)N(AA)). Treatments did not influence carbon or nitrogen trophic proxies, thereby not supporting nutritional plasticity. Instead, hosts and symbionts consistently overlapped in essential‐ and nonessential‐δ(13)C(AA) (11 of 13 amino acids) and trophic‐ and source‐δ(15)N(AA) values (9 of 13 amino acids). Host and symbiont trophic‐δ(15)N(AA) values positively correlated with a plankton end‐member, indicative of trophic connections and dietary sources for trophic‐AA nitrogen. However, mass balance of AA‐trophic positions (TP(Glx–Phe)) revealed heterotrophic influences to be highly variable (1–41% heterotrophy). Linear discriminant analysis using M. capitata mean‐normalized essential‐δ(13)C(AA) with previously published values (Pocillopora meandrina) showed similar nutrition isotope fingerprints (Symbiodiniaceae vs. plankton) but revealed species‐specific trophic strategies. Montipora capitata and Symbiodiniaceae shared identical AA‐fingerprints, whereas P. meandrina was assigned to either symbiont or plankton nutrition. Thus, M. capitata was 100% reliant on symbionts for essential‐δ(13)C(AA) and demonstrated autotrophic fidelity and contrasts with trophic plasticity reported in P. meandrina. While M. capitata AA may originate from host and/or symbiont biosynthesis, AA carbon is Symbiodiniaceae‐derived. Together, AA‐CSIA/isotope fingerprinting advances the study of coral trophic plasticity and are powerful tools in the study of marine symbioses.