Biogeochemical evidence for chemosymbiosis in the fossil record

Chemosymbiotic invertebrates obtain nutrition from harbouring bacteria that oxidize reduced chemicals to produce energy for carbon fixation. This allows the animals to thrive in the extreme conditions of the deep sea, because the high concentrations of sulphide (thiotrophy) and methane (methanotrophy) at cold seeps and hydrothermal vents can be utilized by the symbiotic bacteria. This research investigates whether the key role of chemosymbiosis in shaping modern deep sea ecosystems can be traced through geological time, by using the stable isotope composition (δ13C, δ15N, δ34S) of organic matter in invertebrate shells. Shell-bound organic matter (SBOM) was isolated using various shell removal techniques, and method comparison suggests that the original isotopic signal is least affect by using EDTA or acetic acid. Multi-isotope analysis of SBOM obtained from (deep sea) molluscs and brachiopods confirms that the main types of chemosymbiosis can be differentiated from non-symbiotic heterotrophic nutritional strategies. In particular chemosymbiotic SBOM δ13C is characteristically depleted, with defined ranges for the presence of either methanotrophic or thiotrophic symbionts across environmental settings. In suspected thiotrophic taxa from ancient cold seeps, the preservation of this modern range (SBOM δ13C -35‰ to -29‰) is limited to young subfossil specimens, but the upper threshold is only exceeded in pre-Pliocene samples. Moreover, the protected intra-crystalline SBOM pool retains a distinct δ13C signal up to the Miocene, and available δ34S and δ15N data of intra-crystalline SBOM do not overlap between heterotrophy and thiotrophy. For methanotrophy (δ13C -65‰ to -36‰ at modern cold seeps) a residual δ13C biosignature does appear to be present in total SBOM from Miocene samples. This encouraging finding, together with the discovery of intra-crystalline original proteins in a fossil of Cretaceous age, suggests that future work on other well-preserved specimens could trace the evolution of chemosymbiosis deep into geological time.