Role of APS reductase in biogeochemical sulfur isotope fractionation

Abstract

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved.

Phylogenomic analysis of novel Diaforarchaea is consistent with sulfite but not sulfate reduction in volcanic environments on early Earth

“…Further, localized evaporation of these waters could generate still higher SO 4 2− concentrations [14]. Intriguingly, it was recently shown using purified DsrAB that the primary enzymatic influence on sulfur isotope fractionation during SO 4 2− reduction is at the level of reduction of SO 3 2− to sulfide (via DsrAB) [68], although a fractionation effect by enzymatic SO 4 2activation and reduction (i.e., by APS reductase; AprAB) may also occur [69]. Thus, it is unclear whether SO 3 2− or SO 4 2− reducing organisms (or both) were responsible for the measured sulfur isotope fractionations between sulfides produced from such activities and barites in the~3.5 Ga Dresser Formation hydrothermal deposits.…”

Sulfur Chemistry May Have Paved the Way for Evolution of Antioxidants

“…Not only is the geological rock record suggestive of an early sulfur cycling metabolism (see Section 1.4; Johnston, 2011; Paris et al, 2013;Knoll, 2016;Montinaro and Strauss, 2016;Ranjan et al, 2018;Sim et al, 2019) but it also provides information about the phylogenetic diversity of the sulfur redox enzyme DsrAB-type dissimilatory (bi)sulfite reductase, which produces sulfide as an end product (Müller et al, 2015). This group of enzymes has prosthetic siroheme* (e.g., nonprotein helper groups attached to proteins) as an active site for redox reactions of sulfur and nitrogen (Murphy et al, 1974), and it has been phylogenetically determined to be very ancient, present in microorganisms even before the separation between bacteria and archaea.…”

Siberian Permafrost Thawing Accelerated at the Bølling/Allerød and Preboreal Warm Periods During the Last Deglaciation

“…The resulting hydrogen sulfide ions (HS -) diffuse back to the water column to be reoxidized. In the terrestrial environment, sulfur isotope fractionation between 30‰ and 60‰ is more common (Sim et al, 2019). The produced HSis depleted in 34 S relative to 32 S during their metabolism.…”

“…Its isotopic fractionation ranged from 16‰ to 42‰ (28‰ on average; Habicht & Canfield, 1997), when the SO 4 2concentration in water was high (>2 mM, Canfield, 2001). In the terrestrial environment, sulfur isotope fractionation between 30‰ and 60‰ is more common (Sim et al, 2019). Because of the isotopic fractionation, the original δ 34 S values of SO 4 2from watershed would have been more than 16.1-34.0‰.…”