Simulation of the impacts of diagenesis or low-grade metamorphism on neutrophilic microaerobic Fe(II)-oxidizing biofilm

The microaerobic iron-oxidizing bacteria in circumneutral environment produce extracellular polymeric substances (EPS) with unique morphologic features, such as stalks or sheaths, which can be regarded as geobiological signatures. The Archean and early Palaeoproterozoic oceans were anoxic with high soluble Fe(II) that were suggested to have been oxidized through the metabolism of Fe(II)-oxidizing bacteria. The precursor of the ultrafine hematite in banded iron formation (BIF), e.g., ferrihydrite, was suggested to be the mineral record of microbial Fe(II)-oxidation at that time. However, both the biological materials and primary iron minerals were prone to being altered by diagenetic or low-grade metamorphic processes. This makes it difficult to interpret the genesis of Precambrian BIFs. Here, we report experimental simulation on the effects of diagenesis or low-grade metamorphism on neutrophilic microaerobic Fe(II)-oxidizing bacteria and their biomass. Stalks, sheaths, and iron oxide spheroidal aggregates are partially preserved after the 100 MPa/300°C treatments, which indicates the mixed organic matters and iron oxides could survive the diagenetic or low-grade metamorphic processes. Some organic-mineral mixing structures carry information on microbial processes, though they appear similar to pseudomorphs of fossilized bacteria.