Assessing production of the ubiquitous archaeal diglycosyl tetraether lipids in marine subsurface sediment using intramolecular stable isotope probing

Abstract

Summary The membrane lipids diglycosyl‐glycerol dibiphytanyl glycerol tetraethers ( 2G ‐ GDGTs ) in marine subsurface sediments are believed to originate from uncultivated benthic archaea, yet the production of 2G ‐ GDGTs from subseafloor samples has not been demonstrated in vitro . In order to validate sedimentary biosynthesis of 2G ‐ GDGTs , we performed a stable carbon isotope probing experiment on a subseafloor sample with six different 13 C ‐labelled substrates (bicarbonate, methane, acetate, leucine, glucose and S pirulina platensis biomass). After 468 days of anoxic incubation, only glucose and S . platensis resulted in label uptake in lipid moieties of 2G ‐ GDGTs , indicating incorporation of carbon from these organic substrates. The hydrophobic moieties of 2G ‐ GDGTs showed minimal label incorporation, with up to 4‰ 13 C enrichment detected in crenarchaeol‐derived tricyclic biphytane from the S . platensis ‐supplemented slurries. The 2G ‐ GDGT ‐derived glucose or glycerol moieties also showed 13 C incorporation (Δδ 13 C = 18–38‰) in the incubations with glucose or S . platensis , consistent with a lipid salvage mechanism utilized by marine benthic archaea to produce new 2G ‐ GDGTs . The production rates were nevertheless rather slow, even when labile organic matter was supplied. The 2G ‐ GDGT turnover times of 1700–20 500 years were much longer than those estimated for subseafloor microbial communities, implying that sedimentary 2G ‐ GDGTs as biomarkers of benthic archaea are cumulative records of past and present generations.

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