The evolution and spread of sulfur cycling enzymes reflect the redox state of the early Earth.

The biogeochemical sulfur cycle plays a central role in fueling microbial metabolisms, regulating the Earth's redox state, and affecting climate. However, geochemical reconstructions of the ancient sulfur cycle are confounded by ambiguous isotopic signals. We use phylogenetic reconciliation to ascertain the timing of ancient sulfur cycling gene events across the tree of life. Our results suggest that metabolisms using sulfide oxidation emerged in the Archean, but those involving thiosulfate emerged only after the Great Oxidation Event. Our data reveal that observed geochemical signatures resulted not from the expansion of a single type of organism but were instead associated with genomic innovation across the biosphere. Moreover, our results provide the first indication of organic sulfur cycling from the Mid-Proterozoic onwards, with implications for climate regulation and atmospheric biosignatures. Overall, our results provide insights into how the biological sulfur cycle evolved in tandem with the redox state of the early Earth.

Isotopes illustrate vertical transport of anthropogenic Pb by reversible scavenging within Pacific Ocean particle veils.

Reversible scavenging, the oceanographic process by which dissolved metals exchange onto and off sinking particles and are thereby transported to deeper depths, has been well established for the metal thorium for decades. Reversible scavenging both deepens the elemental distribution of adsorptive elements and shortens their oceanic residence times in the ocean compared to nonadsorptive metals, and scavenging ultimately removes elements from the ocean via sedimentation. Thus, it is important to understand which metals undergo reversible scavenging and under what conditions. Recently, reversible scavenging has been invoked in global biogeochemical models of a range of metals including lead, iron, copper, and zinc to fit modeled data to observations of oceanic dissolved metal distributions. Nonetheless, the effects of reversible scavenging remain difficult to visualize in ocean sections of dissolved metals and to distinguish from other processes such as biological regeneration. Here, we show that particle-rich "veils" descending from high-productivity zones in the equatorial and North Pacific provide idealized illustrations of reversible scavenging of dissolved lead (Pb). A meridional section of dissolved Pb isotope ratios across the central Pacific shows that where particle concentrations are sufficiently high, such as within particle veils, vertical transport of anthropogenic surface-dissolved Pb isotope ratios toward the deep ocean is manifested as columnar isotope anomalies. Modeling of this effect shows that reversible scavenging within particle-rich waters allows anthropogenic Pb isotope ratios from the surface to penetrate ancient deep waters on timescales sufficiently rapid to overcome horizontal mixing of deep water Pb isotope ratios along abyssal isopycnals.

Radiation of nitrogen-metabolizing enzymes across the tree of life tracks environmental transitions in Earth history.

Nitrogen is an essential element to life and exerts a strong control on global biological productivity. The rise and spread of nitrogen-utilizing microbial metabolisms profoundly shaped the biosphere on the early Earth. Here, we reconciled gene and species trees to identify birth and horizontal gene transfer events for key nitrogen-cycling genes, dated with a time-calibrated tree of life, in order to examine the timing of the proliferation of these metabolisms across the tree of life. Our results provide new insights into the evolution of the early nitrogen cycle that expand on geochemical reconstructions. We observed widespread horizontal gene transfer of molybdenum-based nitrogenase back to the Archean, minor horizontal transfer of genes for nitrate reduction in the Archean, and an increase in the proliferation of genes metabolizing nitrite around the time of the Mesoproterozoic (~1.5 Ga). The latter coincides with recent geochemical evidence for a mid-Proterozoic rise in oxygen levels. Geochemical evidence of biological nitrate utilization in the Archean and early Proterozoic may reflect at least some contribution of dissimilatory nitrate reduction to ammonium (DNRA) rather than pure denitrification to N2 . Our results thus help unravel the relative dominance of two metabolic pathways that are not distinguishable with current geochemical tools. Overall, our findings thus provide novel constraints for understanding the evolution of the nitrogen cycle over time and provide insights into the bioavailability of various nitrogen sources in the early Earth with possible implications for the emergence of eukaryotic life.