Isotopic and molecular characterization of shell organics in modern brachiopods: Implications for palaeocological reconstructions of fossil brachiopods

Brachiopod shells are valuable biomineral archives for reconstructing environmental and climatic changes through deep time due to their widespread fossil record and the high preservation potential of their low-Mg calcite shells. Although representing only a minor component of the shell, the intracrystalline and intercrystalline organic matrix preserved within the shell structure may retain a geochemical signature of great importance for palaeoecological interpretations. Carbon and nitrogen isotope compositions of shell organics (δ13C and δ15N org org) have been proposed as proxies for reconstructing trophic strategies and symbioses in fossil marine invertebrates. Studies on the giant Mississippian brachiopod species of Gigantoproductus interpreted low δ13C and intermediate δ15N values as org org evidence for a mixotrophic lifestyle involving suspension feeding and photosymbiosis, allowing these marine invertebrates to attain an exceptional size and thick shell at the onset of the Late Paleozoic Ice Age (Angiolini et al., 2019; 2026). These studies also demonstrated the preservation of amino acids within the shell organic matrix through GC-MS and 1H-NMR analyses. However, this interpretation mainly relies on geochemical data obtained from modern mollusks, whereas a comparable dataset for extant brachiopods is lacking. Here, we present carbon and nitrogen isotope compositions of shell organic matrix extracted from nine extant brachiopod species collected from different geographic and environmental settings worldwide to establish a modern isotopic reference for brachiopod shell organics. The aim is to investigate trophic strategies in modern brachiopods and compare them with published data from modern mollusks and fossil brachiopods to refine palaeoecological interpretations based on shell organic matrix isotopic signatures. In addition, the molecular composition of the extracted organic matrix is being investigated through GC-MS analysis to characterize organic compounds. The analyzed shell organic matrix of modern brachiopods, all known suspension feeders, shows δ13C org values ranging between −23‰ and −16‰ and δ15N org values between +6‰ and +11‰, closely matching the range reported for modern non-symbiotic filter-feeding mollusks. These results support the applicability of shell δ13C and δ15N org org as proxies for identifying trophic strategies and provide the first modern comparative framework focused on brachiopods. Compared with the lower δ13C and δ15N org org values reported for fossil Gigantoproductus species, the modern dataset strengthens the interpretation that gigantoproductids occupied a trophic and ecological niche distinct from that of modern non-symbiotic brachiopod filter feeders. More broadly, this study provides a calibration for refining palaeoecological interpretations based on preserved fossil shell organic matrix and highlights the importance of modern analogues for reconstructing past ecosystems and ecological interactions. Angiolini L. et al. (2019) - The giants of the phylum Brachiopoda: a matter of diet? Palaeontology, 62(6), 889-917, https://doi.org/10.1111/pala.12433. Angiolini L. et al. (2026) - Brachiopod giants from the Mississippian (Asbian) of western Ireland: Fossil bioarchives of seasonality and symbiosis and far-field harbingers of climate change. Palaeogeogr. Palaeoclimatol. Palaeoecol., 683, 113418, ISSN 0031-0182, https://doi.org/10.1016/j.palaeo.2025.113418.