The isotopic biosignatures of photo‐ vs. thiotrophic bivalves: are they preserved in fossil shells?

Symbiont‐bearing and non‐symbiotic marine bivalves were used as model organisms to establish biosignatures for the detection of distinctive symbioses in ancient bivalves. For this purpose, the isotopic composition of lipids (δ 13 C ) and bulk organic shell matrix (δ 13 C , δ 34 S , δ 15 N ) from shells of several thiotrophic, phototrophic, or non‐symbiotic bivalves were compared (phototrophic: F ragum fragum , F ragum unedo , T ridacna maxima ; thiotrophic: C odakia tigerina , F imbria fimbriata , A nodontia sp.; non‐symbiotic: T apes dorsatus , V asticardium vertebratum , S cutarcopagia sp.). ∆ 13 C values of bulk organic shell matrices, most likely representing mainly original shell protein/chitin biomass, were depleted in thio‐ and phototrophic bivalves compared to non‐symbiotic bivalves. As the bulk organic shell matrix also showed a major depletion of δ 15 N (down to –2.2 ‰) for thiotrophic bivalves, combined δ 13 C and δ 15 N values are useful to differentiate between thio‐, phototrophic, and non‐symbiotic lifestyles. However, the use of these isotopic signatures for the study of ancient bivalves is limited by the preservation of the bulk organic shell matrix in fossils. Substantial alteration was clearly shown by detailed microscopic analyses of fossil (late P leistocene) T . maxima and T rachycardium lacunosum shell, demonstrating a severe loss of quantity and quality of bulk organic shell matrix with time. Likewise, the composition and δ 13 C‐values of lipids from empty shells indicated that a large part of these compounds derived from prokaryotic decomposers. The use of lipids from ancient shells for the reconstruction of the bivalve's life style therefore appears to be restricted.