• Ellisman, Mark
• Metcalfe, Kyle S.
• Yu, Hang
• Deerinck, Tom
• Grotzinger, John P.

Abstract

<jats:p>Authigenic carbonate minerals can preserve biosignatures of microbial anaerobic oxidation of methane (AOM) in the rock record. It is not currently known whether the microorganisms that mediate sulfate-coupled AOM—often occurring as multicelled consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB)—are preserved as microfossils. Electron microscopy of ANME-SRB consortia in methane seep sediments has shown that these microorganisms can be associated with silicate minerals such as clays [Chen<jats:italic>et al</jats:italic>.,<jats:italic>Sci. Rep.</jats:italic><jats:bold>4</jats:bold>, 1–9 (2014)], but the biogenicity of these phases, their geochemical composition, and their potential preservation in the rock record is poorly constrained. Long-term laboratory AOM enrichment cultures in sediment-free artificial seawater [Yu<jats:italic>et al</jats:italic>.,<jats:italic>Appl. Environ. Microbiol.</jats:italic><jats:bold>88</jats:bold>, e02109-21 (2022)] resulted in precipitation of amorphous silicate particles (~200 nm) within clusters of exopolymer-rich AOM consortia from media undersaturated with respect to silica, suggestive of a microbially mediated process. The use of techniques like correlative fluorescence in situ hybridization (FISH), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), and nanoscale secondary ion mass spectrometry (nanoSIMS) on AOM consortia from methane seep authigenic carbonates and sediments further revealed that they are enveloped in a silica-rich phase similar to the mineral phase on ANME-SRB consortia in enrichment cultures. Like in cyanobacteria [Moore<jats:italic>et al</jats:italic>.,<jats:italic>Geology</jats:italic><jats:bold>48</jats:bold>, 862–866 (2020)], the Si-rich phases on ANME-SRB consortia identified here may enhance their preservation as microfossils. The morphology of these silica-rich precipitates, consistent with amorphous-type clay-like spheroids formed within organic assemblages, provides an additional mineralogical signature that may assist in the search for structural remnants of microbial consortia in rocks which formed in methane-rich environments from Earth and other planetary bodies.</jats:p>

Topics

• impedance spectroscopy
• mineral
• cluster
• amorphous
• phase
• scanning electron microscopy
• precipitate
• precipitation
• Energy-dispersive X-ray spectroscopy
• spectrometry
• secondary ion mass spectrometry