Environmental control on microbial diversification and methane production in the Mesoarchean

Multiple lines of evidence have revealed a thriving marine biosphere capable of diverse metabolic strategies back to at least 3.5 billion years ago (Ga). However, little is known about microbial ecosystems in lakes and rivers during the Mesoarchean and their role in the evolution of the biosphere. Here we report new carbon and nitrogen isotopic data from the fluvio-lacustrine Lalla Rookh Sandstone in Western Australia (∼3.0 Ga) – one of the oldest known non-marine sedimentary deposits. Organic δ¹³C values (-30‰ to -38‰) are best interpreted as recording carbon fixation by methanogens using the acetyl CoA pathway mixed with organisms using the Calvin cycle, while δ¹⁵N data (0‰ to -1‰) likely reflect biological N₂ fixation using FeMo-nitrogenase. When compared with data from the literature, we show that lacustrine habitats of Mesoarchean age (3.2-2.8 Ga) are systematically depleted in δ¹³C (-37 ± 5‰) relative to marginal marine (-32 ± 7‰) and open marine settings (-27 ± 3‰), suggesting that methanogenesis was relatively more important in lacustrine communities. Our findings highlight: (a) the widespread use of biological N₂ fixation by the Mesoarchean biosphere, (b) the potential importance of continental habitats for methane production and perhaps for the formation of hydrocarbon haze, and (c) the possible role of land masses in driving microbial diversification on the early Earth.