Microbial lipids reveal carbon assimilation patterns on hydrothermal sulfide chimneys

Sulfide chimneys' characteristic of seafloor hydrothermal venting are diverse microbial habitats. C-13/C-12 ratios of microbial lipids have rarely been used to assess carbon assimilation pathways on these structures, despite complementing gene- and culture-based approaches. Here, we integrate analyses of the diversity of intact polar lipids (IPL) and their side-chain C-13 values (C-13(lipid)) with 16S rRNA gene-based phylogeny to examine microbial carbon flow on active and inactive sulfide structures from the Manus Basin. Surficial crusts of active structures, dominated by Epsilonproteobacteria, yield bacterial C-13(lipid) values higher than biomass C-13 (total organic carbon), implicating autotrophy via the reverse tricarboxylic acid cycle. Our data also suggest C-13(lipid) values vary on individual active structures without accompanying microbial diversity changes. Temperature and/or dissolved substrate effects - likely relating to variable advective-diffusive fluxes to chimney exteriors - may be responsible for differing C-13 fractionation during assimilation. In an inactive structure, C-13(lipid) values lower than biomass C-13 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 C-13(lipid) analyses to elucidate carbon flow in hydrothermal structures when combined with other molecular tools.