Microbial Lipids Reveal Carbon Assimilation Patterns on Hydrothermal Sulfide Chimneys

Sulfide ‘chimney’ structures characteristic of seafloor hydrothermal activity are extreme microbial habitats. 13C/12C ratios of microbial membrane lipids have rarely been used to investigate bacterial and archaeal carbon assimilation pathways on these structures, despite complementing existing gene- and culture-based investigations. Here, we integrate analysis of IPL diversity and d13C values (d13Clipid) with 16S rRNA gene based taxonomy to examine microbial carbon flow patterns on active and inactive chimney structures from the Manus Basin. Surficial crusts of active structures, dominated by epsilonproteobacterial communities, all yield bacterial d13Clipid values higher than biomass (total organic carbon) d13C, implicating CO2 fixation via the reverse tricarboxylic acid cycle. Our data also suggest d13Clipid values vary on individual active structures without accompanying changes in microbial diversity. Temperature and/or dissolved substrate effects on 13C fractionation during assimilation may be responsible, likely due to variable advective/diffusive fluid fluxes to chimney exteriors. In an inactive structure, d13Clipid values lower than biomass and a distinctive IPL and 16S rRNA gene diversity suggest a shift to a more diverse community and an alternate carbon assimilation pathway after venting ceases. We discuss here the potential of IPL and d13Clipid analyses to elucidate microbial carbon flow in hydrothermal structures when combined with other molecular tools.