Microbial lipids reveal carbon assimilation patterns on hydrothermal sulfide chimneys.

Sulfide 'chimneys' characteristic of seafloor hydrothermal venting are diverse microbial habitats. ¹³C/¹²C 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 δ¹³C values (δ¹³ C(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 δ¹³C(lipid) values higher than biomass δ¹³C (total organic carbon), implicating autotrophy via the reverse tricarboxylic acid cycle. Our data also suggest δ¹³C(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 ¹³C fractionation during assimilation. In an inactive structure, δ¹³C(lipid) values lower than biomass δ¹³C 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 δ¹³C(lipid) analyses to elucidate carbon flow in hydrothermal structures when combined with other molecular tools.