Hitching a ride: A gastropod-associated microbiome community at a Hydrate Ride methane seep and impacts on local biogeochemical cycling

Abstract

Exploration of cold seeps from geological, chemical, and biological perspectives has grown exponentially in recent decades since the first discovery of cold seeps in the 1980s. Symbiotic relationships, often rooted in geochemistry of the environment, have proven to be ubiquitous at cold seeps. However, our understanding of symbiotic relationships at extreme environments is limited. In this thesis, we first review two kinds of cold seep systems—gas hydrate-forming seeps, using Hydrate Ridge as an example, and brine-influenced seeps, with a focus on the Gulf of Mexico—from geological, microbiological, and biogeochemical perspectives. We then report on the composition of microbial communities associated with provannid gastropods as characterized using 16S rRNA gene amplicon and clone libraries as well as Fluorescence In Situ Hybridization. The gastropod shells, collected from seeps at Hydrate Ridge and the Gulf of Mexico are covered with filamentous epibionts on their shells. Large filamentous epibionts were identified as Candidatus Marithrix, Thiomargarita nelsonii, and a previously undescribed Chloroflexi. Our analysis of three incomplete Chloroflexi’s genomes leads us to hypothesize that the Chloroflexi is an acetogen. Environmental samples from a previous sample revealed that the gastropod-associated community differed from the surrounding microbial communities, implying a selection mechanism for gastropod habitation and that the gastropod shell potentially serves as a unique niche. The diverse community of microbes on the shell of these seep-dwelling gastropods may represent a symbiotic relationship made possible by the gastropods motility that provides the attached microbial community with essential metabolites, while the attached community may serve the gastropod by providing it with a source of nutrition, and potentially detoxifying hydrogen sulfide.