Crustose Coralline Algae as a Proxy for Marine Nitrogen Cycling and Coral Trophic Strategies

The marine nitrogen (N) cycle plays a central role in regulating ocean productivity, ecosystem structure, and biogeochemical feedbacks on climate. However, our understanding of historical changes in the marine N cycle is limited by the short time span and spotty spatial coverage of direct observations. This study explores the use of crustose coralline algae (CCA) as a novel proxy for reconstructing past changes in the marine N cycle. We use the biomineral-bound nitrogen isotopic composition of tropical CCA (δ ¹⁵ N _CCA ) and compare it to the δ¹⁵N of subsurface nitrate (δ ¹⁵ N _Nitrate ) and corals (δ ¹⁵ N _coral ). Our results show that CCA reliably track subsurface nitrate δ¹⁵N and, therefore, can be used to reconstruct past changes in the N cycle. Additionally, anchoring δ ¹⁵ N _coral values to co-occurring CCA allows us to isolate the reliance on photosymbionts in coral species, revealing taxonomic and regional variation in trophic strategies. Therefore, δ ¹⁵ N _CCA also provides a tool to investigate coral trophodynamics in the modern ocean as well as in the historical record.