Investigating biogeochemical evidence for chemosymbiosis at fossil cold seeps

Cold seeps are sites in the ocean where fluids rich in methane or other hydrocarbons seep onto the seafloor and fuel a series of biogeochemical reactions. One of the most intriguing features of the cold seep environment is the occurrence of dense animal communities in areas of the deep sea where animals are normally rare because of scarce food resources. However, unlike most marine life, which is part of an ecosystem based on photosynthesis, many cold seep animals depend on the highly unusual nutritional strategy of chemosymbiosis.  This means that they live in symbiosis with bacteria that obtain energy for carbon fixation by oxidising compounds derived from the seeping fluid (chemosynthesis).

Modern cold seeps communities are populated mainly by chemosymbiotic tubeworms, mussels, clams and other bivalves, together with some non-chemosymbiotic species that consume organic detritus. Cold seep mussels can live in symbiosis with methane-oxidising (methanotrophic) or sulphide-oxidising (thiotrophic) bacteria, or both, whereas clams and other bivalves contain thiotrophic symbionts only. This difference in symbiont-type has been shown to correspond to differences in the carbon and sulphur stable isotope composition of the animals’ body tissues. In modern animals, carbon and sulphur stable isotope values can be used to distinguish between chemosymbiotic and non-chemosymbiotic animals, and between methanotrophic and thiotrophic chemosymbiosis. 

Since the discovery of the first cold seeps, many fossil accumulations of previously uncertain origin have been identified as ancient cold seep communities. For the past 65 million years, cold seeps have been populated mainly by bivalves from groups with modern chemosymbiotic seep representatives. Older seep ecosystems also contained bivalves from now-extinct groups, as well as rhynchonellid brachiopods (another type of two-shelled marine animal physiologically distinct from bivalves), whose relatives are still alive today but do not live at modern seeps. Fossils with modern chemosymbiotic relatives are assumed to be chemosymbiotic, but whether older seep animals were also chemosymbiotic is currently unknown.  The aim of this project is to investigate chemical evidence for chemosymbiosis in seep fossils, in order to better understand the biochemistry of extinct seep animals.

Chemosymbiosis has been extensively studied in modern seep animals but most of the methods used cannot be applied to fossil specimens. However, the shells of bivalves and rhynchonelliform brachiopods are formed of calcium carbonate minerals crystallised within and around an organic, protein-rich framework, which can form up to 5% of the shell by weight. This organic component of shells, or “shell-bound organic matter” (SOM) is secreted by the soft tissues of the animal and therefore can be expected to have similar stable isotope values. This has been confirmed in a small number of non-seep bivalve species, and previous research has demonstrated that SOM can be preserved in fossil shells, providing a record of soft tissue isotopic composition that persists even when no soft tissues are preserved. Analysis of the carbon and sulphur stable isotope composition of SOM preserved in cold seep fossil shells, therefore, has the potential to reveal methanotrophic and thiotrophic chemosymbiosis in extinct animals.