Tracing the Early Emergence of Microbial Sulfur Metabolisms

Abstract

Hydrogen sulfide (nH₂S) and sulfur oxide (SO _n ; n?=?1, 2, 3) gases in early Earth’s globally anoxic atmosphere were subjected to gas-phase chemical transformations by UV light. A principal photolysis pathway at that time produced elemental sulfur aerosols with mass-independently fractionated (MIF) isotopic values carrying variable minor isotope (³³S, ³⁶S) compositions. These rained into the sulfate-deficient Archean (ca. 3.85–2.5 Ga) oceans to react with Fe²⁺]_aq and form sedimentary sulfides. The MIF-bearing sulfides were incorporated into Archean sediments, including banded iron formations (BIF). Such aerosols may also have fueled microbial sulfur metabolisms, and thus are traceable by the MIF sulfur isotopes. Yet, data show that before ～3.5 Ga mass-dependent³⁴S/³²S values in Early Archean sediments tend to fall within a narrow (±0.1%) range even as they carry mass-independent values. By about 3.5 Ga, ³⁴S/³²S values show much greater changes (>1%) in range congruent with microbial metabolic processing. Here, we trace probable pathways of elemental sulfur aerosols into Archean sediments, and couple our study with analysis of the evolutionary relationships of enzymes involved in sulfur metabolism to explain the observed trends. Our model explains why elemental sulfur aerosols were apparently not utilized by the Eoarchean (pre-3.65 Ga) biosphere even though an immediate precursor to the required enzyme may have already been present.

• Highlights

• Evolution of microbial sulfur metabolisms is tracked by multiple sulfur isotopes

• Alkaline hydrothermal vents were an abode for early microbial life

• Sulfite detoxification prompted anaerobic respiration

• Reversal of respiratory electron transport chain (ETC) stimulated photothiotrophy

• Surplus e- acceptors permitted the emergence of elemental sulfur reduction