Biological soil crusts (BSCs) are topsoil biosedimentary structures built by photosynthetic microbes commonly found today on arid soils. They play a role in soil stabilization and the fertility of arid lands, and are considered modern analogues of ancient terrestrial microbial communities. We determined the concentrations of four biogenic and 21 other elements, mostly metals, in surface soils that hosted BSCs, in the soils underneath those crusts, and in proximate but non-crusted surface soils. The samples were from six sites in the Colorado Plateau highlands and the Sonoran Desert lowlands. In spite of the variability in climate and geologic setting, we found statistically significant overall trends of enrichment in biogenic elements and depletion in non-biogenic elements when BSCs were compared with non-crusted soils. The differences between crusted and non-crusted soils were statistically significant at approximately 95% confidence for C, N (enrichments) and for Ca, Cr, Mn, Cu, Zn, As, and Zr (depletions). These trends are best explained by the activity of microbes. As expected, no differences in the concentrations of C, N, P, and S were detected between the soils underneath the crusts and the non-crusted soils, but the former showed depletion of non-biogenic elements, indicating that the leaching effect of crust microbes extends downward in the soil. These patterns speak to the need for a sustained input of allochthonous material, possibly dust, to maintain BSC fertility. These elemental patterns can be considered a biosignature that may be preserved in the rock record and might help identify ancient microbial communities on land.

Terrestrial ecosystems have been largely regarded as plant-dominated land surfaces, with the earliest records appearing in the early Phanerozoic (<550 Ma). Yet the presence of biological components in pre-Phanerozoic rocks, in habitats as different as soils, peats, ponds, lakes, streams, and dune fields, implies a much earlier type of terrestrial ecosystems. Microbes were abundant by~3,500 Ma ago and surely adapted to live in subaerial conditions in coastal and inland environments, as they do today. This implies enormous capacities for rapid adaptations to changing conditions, which is supported by a suggestive fossil record. Yet, evidence of "terrestrial" microbes is rare and indirect in comparison with fossils from shallow or deeper marine environments, and its record has been largely overlooked. Consequently, the notion that microbial communities may have formed the earliest land ecosystems has not been widely accepted nor integrated into our general knowledge. Currently, an ample record of shallow marine and lacustrine biota in~3,500 Ma-old deposits, together with evidence of microbial colonization of coastal environments~3,450 Ma ago and indirect geochemical evidence that suggests biological activity in >3,400 Ma-old paleosols endorses the idea that life on land perhaps occurred in parallel with aquatic life back in the Paleoarchean. The rapid adaptations seen in modern terrestrial microbes, their outstanding tolerance to extreme and fluctuating conditions, their early and rapid diversification, and their old fossil record collectively suggest that they constituted the earliest terrestrial ecosystems, at least since the Neoarchean, further succeeding on land and forming a biomass-rich cover with mature soils where plant-dominated ecosystems later evolved. Understanding how life diversified and adapted to non-aquatic conditions from the actualistic and paleontological perspective is critical to understanding the impact of life on the Earth's systems over thousands of millions of years.

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