With Dr. Julia McGonigle, Bigelow Laboratory for Ocean Sciences on “Formate metabolism by Chloroflexi is key in unlocking deep carbon for the Lost City chimney ecosystem.” Abstract: The Lost City hydrothermal field on the Mid-Atlantic Ridge supports dense microbial life on towering calcium carbonate chimney structures. This microbial life is fueled by chemical reactions between the ultramafic rock under the chimneys and ambient seawater. These serpentinization reactions provide reducing power (as hydrogen gas) and organic compounds that can serve as microbial food. Previous studies have characterized the interior of the chimneys as a single-species biofilm inhabited by the Lost City Methanosarcinales, but genomic evidence indicating this methanogen is able to metabolize the most abundant carbon source (formate) is lacking. I will present recent metagenomic results that suggest the non-formate utilizing species inhabiting Lost City chimney biofilms might rely on carbon-cycling activity of a Chloroflexi population. I will also present current comparative genomic work on a distantly-related Chloroflexi population, obtained through hydrothermal fluid sampling, suspected to reside in the subsurface habitat under the Lost City chimneys.