Microbially induced precipitation of silica by anaerobic methane- oxidizing consortia and implications for microbial fossil preservation

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 et al., Sci. Rep. 4, 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 et al., Appl. Environ. Microbiol. 88, e02109- 21 (2022)] resulted in precipitation of amorphous silicate particles (similar to 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 et al., Geology 48, 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.