An anaerobic ∼3400 Ma shallow-water microbial consortium: Presumptive evidence of Earth’s Paleoarchean anoxic atmosphere

Abstract

A microbial consortium preserved in shallow water carbonaceous chert of the Paleoarchean (∼3400Ma) Strelley Pool Formation (Pilbara Craton, northwestern Western Australia) is composed of copiously abundant globular “cobweb-like” swirls of randomly oriented filaments intermingled with laterally extensive flat-laminated masses of subparallel filaments. The shallow-water (possibly mudflat-like) habitat of the biocoenose is established by its geological setting and intimately intermixed association with millimetric quartz radiating laths and rosettes interpreted to be pseudomorphs after evaporitic gypsum/anhydrite. The indigenousness and syngenicity of the permineralized fossils to the encompassing chert; their uncompacted minimally altered “life-like” spatial distribution; and their biogenicity and bacterial affinity are shown by optical microscopic, Raman spectroscopic and confocal laser scanning microscopic analyses in situ that document the mineralogy of their embedding matrix and their cylindrical filamentous morphology, cellularity, and kerogenous composition.By comparison with mid-Precambrian and modern sulfur-cycling ecosystems, and consistent with sulfur isotopic data and their intimate co-existence with copious microgranular pyrite and pseudomorphs evidently after sulfate, the swirled aggregates are interpreted to be anaerobic sulfuretums. In contrast, the intermixed flat-laminated components, like microbes of fossil and modern stromatolitic mat communities, are interpreted to be co-occurring light-requiring phototrophs. Whether such phototrophs were anoxygenic photosynthesizers, like extant photosynthetic bacteria (e.g., chloroflexaleans) or facultatively anaerobic cyanobacteria (cf. Oscillatoria limnetica), their anoxygenic metabolism seems plausibly established. Given its great age, shallow water habitat, and anaerobic/microaerophilic physiology, this evidently first reported occurrence from the fossil record of anoxygenic phototrophs and of a microbial sulfuretum-phototroph consortium is consistent with the supposition that Earth’s Paleoarchean environment was essentially anoxic and suggests that O2-producing photosynthesis may have been a later evolutionary innovation.