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International audienceInteracting, diverse microbe-sediment systems exist in natural environments today but have not yet been recognized in the oldest records of life on Earth (older than 3.3 Ga) because of lack of distinctive biomarker molecules and patchy preservation of microbial paleocommunities. In an in-situ outcrop-to microbial-scale study, we have differentiated probable phototrophic, chemolithotrophic, and chemo-organotrophic fossil microbial signatures in a nearshore vol-canogenic sedimentary setting in 3.33 Ga rocks of the Josefsdal Chert, Barberton greenstone belt, South Africa, while demonstrating the importance of contemporaneous hydrothermal activity. Hydrothermal fluids, as a nutrient source, strongly controlled the development and distribution of the microbial communities and, as a silicifying agent, contributed to their rapid fossilization. We thus show that intricate microbe-sediment systems are deep-rooted in time and that at least some early life may indeed have been thermophilic. INTRODUCTION Microbial communities in natural environments exist as multispecies assemblages that interact directly with one another and with their surroundings, and thus can be viewed as distinctive systems (Nealson, 1997). For example, a wide diversity of organotrophic and lithotro-phic (chemotrophic) microorganisms has been described in present-day, carbon-rich hydrother-mally influenced basaltic sediments (Callac et al., 2013), similar to those described from early Earth. However, lack of specific biomarker preservation in very ancient rocks (Summons, 1993) and haphazard preservation of microbial communities in general (Campbell et al., 2001; Orange et al., 2009) make tracing such systems in fossilized form to the oldest records of life on Earth (early Archean, older than 3.3 Ga) very challenging. Nevertheless, phototrophic microbial communities in early to mid-Archean (ca. 3.5–3.2 Ga) strata of South Africa and Australia have been well documented (). On the other hand, no previous investigations have recognized and addressed the syngenetic diversity of Archean microbial paleocommunities—both phototrophs and che-motrophs —within their sedimentary habitats at the microbial scale and using in-situ methods. Here we describe a macroscopic to microscopic investigation of the sedimentary and geochemical settings of widespread, fossilized phototrophic and chemotrophic microorganisms in early to mid-Archean (3.33 Ga) coastal sediments from the Josefsdal Chert, Barberton greenstone belt, South Africa. We also emphasize the importance of contemporaneous hydrothermal activity both as a source of energy for biomass production and as the means of preserving the biosignatures

Archean (3.33 Ga) microbe-sediment systems were diverse and flourished in a hydrothermal context