Crustose coralline algae biomineral-bound nitrogen isotopes provide a baseline to reconstruct coral trophic strategies

Coral resilience is shaped by trophodynamic flexibility – the balance between photosymbiont-derived energy and feeding-based heterotrophy – yet quantifying this balance across taxa and through time remains difficult. Nitrogen (N) isotopes are a powerful tool to investigate trophic strategies, but their use requires information on local isotopic baselines. Here, we introduce the biomineral-bound N isotopes of crustose coralline algae (CCA) as an archive that closely tracks the N isotopic composition of the reef nitrate supply, making it a proxy for a reef’s isotopic baseline. Coupled N isotope measurements of co-occurring CCA, symbiont-bearing, and symbiont-barren corals further enable us to quantify a coral’s “trophic enrichment factor,” reflecting the efficiency of the internal N recycling between the coral and its photosymbionts. From this framework, we derive a Reliance on Symbionts Index (RSI) that captures taxonomic and regional variation in mixotrophy, enabling reconstruction of coral trophodynamics in modern and fossil reef systems.