The fingerprint of chemosymbiosis: origin and preservation of isotopic biosignatures in the nonseep bivalve Loripes lacteus compared with Venerupis aurea

Endosymbionts in marine bivalves leave characteristic biosignatures in their host organisms. Two nonseep bivalve species collected in Mediterranean lagoons, thiotrophic symbiotic Loripes lacteus and filter-feeding nonsymbiotic Venerupis aurea, were studied in detail with respect to generation and presence of such signatures in living animals, and the preservation of these signals in subfossil (late Pleistocene) sedimentary shells. Three key enzymes from sulfur oxidation (APS-reductase), CO2 fixation (RubisCO) and assimilation of nitrogen [glutamine synthetase (GS)] were detected by immunofluorescence in the bacterial symbionts of Loripes. In Loripes, major activity was derived from GS of the symbionts whereas in Venerupis the host GS is active. In search of geologically stable biosignatures for thiotrophic chemosymbiosis that might be suitable to detect such associations in ancient bivalves, we analyzed the isotopic composition of shell lipids (δ13C) and the bulk organic matrix of the shell (δ13C, δ15N, δ34S). In the thiotrophic Loripes, δ13C values were depleted compared with the filter-feeding Venerupis by as much as 8.5‰ for individual fatty acids, and 4.4‰ for bulk organic carbon. Likewise, bulk δ15N and δ34S values were more depleted in recent thiotrophic Loripes. Whereas δ34S values were found to be unstable over time, the combined δ15N and δ13C values in organic shell extracts revealed a specific signature for chemosymbiosis in recent and subfossil specimens.