Tracing The Early Emergence of Microbial Sulfur Metabolisms

Morrison, Patrick

Graduate Thesis Or Dissertation

Tracing The Early Emergence of Microbial Sulfur Metabolisms Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/t435gf09p

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 ultraviolet light. A principal photolysis pathway at that time produced elemental sulfur aerosols with mass-independently fractionated (MIF) isotopic values carrying variable minor isotope compositions (³³S, ³⁶S). These rained into the sulfate-deficient Archean (ca. 3.85-2.5 giga-annum [Ga], or billions of years ago) 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-dependent ³⁴S/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, I trace probable pathways of elemental sulfur aerosols into Archean sediments, and couple my study with analysis of the evolutionary relationships of enzymes involved in sulfur metabolism to explain the observed trends. The narrative proposed is that while an immediateprecursor to the enzyme required to metabolize S⁰ was present prior to 3.5 Ga, at this point, sulfur reduction was e- donor limited due to high ratios of oxidized sulfur compounds to molecular hydrogen in volcanic vents. What distinguished the biosphere after 3.5 Ga was the advent of (and if not the rapid expansion of brought on by P fertilization of the oceans) of photosynthesis which given the high ratio of sulfides to sulfate in hydrothermal vents, would have produced an excess of electron donors in the form of organic carbon. The removal of sulfate as barite in the periphery of the vent communities would induce a situation where sulfur reduction was electron acceptor limited thus driving contemporary organisms to utilize another form of sulfur (in this case, photolytically produced elemental sulfur aerosols). This 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.

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