Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5-3.3 Ga) microbial mats

Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining the nature of early life. Here, we apply Fourier Transform Infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific molecules and functional groups, to characterise fossilised biopolymers in 3.5-3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Using multiple statistical methods, we show that microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite uniformly high aromaticity. This diversity can only be explained by precursor biological heterogeneity since all horizons underwent similar grades of metamorphism. Low methylene to end-methyl (CH2/CH3) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH2/CH3 ratios suggesting longer, unbranched fatty acids from bacterial membrane lipid precursors. Absorbance ratios of end-methyl to methylene (CH3/CH2) in Josefsdal Chert and Footbridge Chert mats yield a range of values (0.20-0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Low R3/2 values (< 0.5) in Hooggenoeg mats denote dominantly Bacteria, whereas high (0.78-1.25) R3/2 ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from the precursor biomass. Since silicification commenced during the lifetime of the microbial mat, the reported FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Taken together, these results show statistically significant distinctions and similarities demonstrating that Bacteria and Archaea flourished together in Earth’s earliest ecosystems.