THIOMARGARITA-LIKE MICROORGANISMS FROM DEEP COLD SEEPS OF THE BARBADOS ACCRETIONARY PRISM

Large sulfur-oxidizing bacteria in the Thiotrichaceae family are key players in the global sulfur cycle. Members of this group include the genera Beggiatoa, Thioploca, Thiomargarita, including the largest known bacterium, Thiomargarita namibiensis, with cell diameters up to 750 um. We discovered a population of small Thiomargarita-like microorganisms colonizing cold seep sediments at 5000 m water depth near Barbados. The Barbados microbes have spherical cells that are morphologically similar to Thiomargarita, but with much smaller diameters that range from 5 to 40 um. The cells contain elemental sulfur inclusions and have a large central vacuole. In order to identify and explore the ecophysiology of these novel sulfur bacteria, we used 16S rDNA amplicon sequencing and metatranscriptomics to characterize the sediment-hosted microbial community. A bacterial 16S rDNA amplicon library (V3 region) contained diverse Epsilon-, Delta-, and Gammaproteobacteria, and included Thiotrichaceae sequences. Assembly and analysis of rRNA transcripts from all three domains showed a taxonomic distribution generally similar to that of the rDNA library, but with a larger contribution from Gammaproteobacteria and with numerous Methanosarcinales-group Archaea. We identified a 16S rRNA sequence affiliated with the candidate genus Thiopilula in the Thiotrichaceae, which represents 3.7% of rRNA transcripts. Candidatus Thiopilula spp. are large vacuolated sulfur oxidizers, which is consistent with the morphology of the sulfur bacteria recovered from the Barbados seep sediments. Analysis of the mRNA transcripts offers new glimpses of the physiology and biogeochemistry of the cold seep community, and will help to reveal the ecological role of Ca. Thiopilula vis-à-vis their larger Thiomargarita cousins that have been identified at methane seeps in shallower water.