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

Abstract

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

Earth as a Tool for Astrobiology—A European Perspective

“…3). Volcanoclastic sediments deposited in shallow water-at depths similar to those of crater lakes on Mars-and hosting abundant hydrothermal vents contain the remains of early, anaerobic life forms (Westall et al , 2015a(Westall et al , 2015b. Both chemotrophic and phototrophic colonies are preserved in the strongly hydrothermally-influenced sediments.…”

“…lichens, microcolonial fungi and tardigrades) have also been studied, and shown a high resistance against the extreme conditions Jänchen et al 2015, Onofri et al 2004, 2012, Sancho et al 2007, de la Torre et al 2010, 2014b. In addition, fossil traces of primitive prokaryotes in well-preserved rocks from the early Earth constitute ideal analogues for potential primitive life forms on the early Mars (Westall et al 2015a(Westall et al , 2015b.…”

“…In addition, bioleaching of rocks and soils (Cockell 2010) due to metabolic activity and release of acidic substances as well as possible deposits of secondary metabolites can change mineral composition or enrich soil with organic material. The analysis of such environmental patterns as a biosignature of life can be studied in the field as well as in the laboratory, which would lead to the identification of fossilization processes on Earth and support the search for extinct life on other planets (Orange et al 2011;Westall et al , 2015aWestall et al , 2015b.…”

“…Prokaryotes (e.g. bacteria, bacterial spores, cyanobacteria) have been studied in such extreme planetary field analogue environments due to the fact that early life forms on Earth were prokaryotes and the assumption that if any extra-terrestrial life exists in the Solar System, it must be simple cellular organisms (Hansen 2007;Westall et al 2015aWestall et al , 2015b. For example, microbial cryptoendolithic communities that colonize the pore spaces of sedimentary rocks in Antarctica and other deserts to avoid stressful environmental conditions (e.g.…”

Early Microbial Evolution: The Age of Anaerobes

“…In summary, the geochemists apparently cannot say when there was no oxygen. They can say when there was life (Arndt and Nisbet 2012) with the 3.3-billion-year-old hydrothermal vent microbial community recently reported by Westall et al (2015) being a dramatic case in point.…”