Isotopic evidence for microbial sulphate reduction in the early Archaean era

Abstract

Sulphate-reducing microbes affect the modern sulphur cycle, and may be quite ancient, though when they evolved is uncertain. These organisms produce sulphide while oxidizing organic matter or hydrogen with sulphate. At sulphate concentrations greater than 1 mM, the sulphides are isotopically fractionated (depleted in 34S) by 10-40/1000 compared to the sulphate, with fractionations decreasing to near 0/1000 at lower concentrations. The isotope record of sedimentary sulphides shows large fractionations relative to seawater sulphate by 2.7 Gyr ago, indicating microbial sulphate reduction. In older rocks, however, much smaller fractionations are of equivocal origin, possibly biogenic but also possibly volcanogenic. Here we report microscopic sulphides in approximately 3.47-Gyr-old barites from North Pole, Australia, with maximum fractionations of 21.1/1000, about a mean of 11.6/1000, clearly indicating microbial sulphate reduction. Our results extend the geological record of microbial sulphate reduction back more than 750 million years, and represent direct evidence of an early specific metabolic pathway--allowing time calibration of a deep node on the tree of life.

Paleoarchean sulfur cycling: Multiple sulfur isotope constraints from the Barberton Greenstone Belt, South Africa

“…Considering respective observations and underlying systematics of sulfur isotopic fractionation (reviewed, e.g., in Canfield and Raiswell, 1999;Johnston, 2011), the sedimentary records of Archean ı 34 S sulfate and ı 34 S sulfide have been regarded as generally reflecting (i) a low-sulfate ocean (Habicht et al, 2002;Crowe et al, 2014), whereas (ii) controversial views exist with respect to an early activity of biological sulfur cycling. Notably, the absence of consistently sizeable fractionations in 34 S has been regarded as evidence for a limited importance of biological sulfur cycling in Archean sedimentary surface environments (e.g., Strauss et al, 2003), despite individual reports of highly 34 S-depleted pyrite in Paleoarchean sedimentary rocks (e.g., Ohmoto et al, 1993;Shen et al, 2001Shen et al, , 2009Philippot et al, 2007;Wu and Farquhar, 2013) and the notion from molecular biology that bacterial sulfate reduction represents an ancient metabolic pathway (Shen and Buick, 2004;Blumenberg et al, 2006;Philippot et al, 2007;Ueno et al, 2008;Shen et al, 2009;Johnston, 2011). Several recent reports of highly 34 S-depleted pyrite occurrences of early Archean age (e.g., Philippot et al, 2007;Ueno et al, 2008;Shen et al, 2009;Wacey et al, 2011a,b;Roerdink et al, 2013) strengthen the case for early biological sulfur cycling, even utilizing diverse metabolic pathways such as bacterial sulfate reduction, elemental sulfur reduction, elemental sulfur disproportionation, and sulfide oxidation.…”

“…The possibility of a microbial origin of Paleoarchean (barite-hosted) microscopic pyrite was previously suggested, e.g., by Shen et al (2001) and Philippot et al (2007, but alternative views have been proposed (e.g., Bao et al, 2008;Philippot et al, 2012;see below).…”

“…Three different pathways of microbial sulfur metabolism have been proposed previously for the Paleoarchean: elemental sulfur disproportionation (Philippot et al, 2007;Wacey et al, 2010Wacey et al, , 2011a, sulfate reduction (Shen et al, 2001(Shen et al, , 2009Ueno et al, 2008;Wacey et al, 2010Wacey et al, , 2011a and sulfide oxidation (Wacey et al, 2011b).…”

Barite, BIFs and bugs: evidence for the evolution of the Earth’s early hydrosphere

“…1) with average values for all deposits in the range of 3.8^5.4x (all N 34 S values are reported relative to the CDT standard), bar the Big Stubby and Geco volcanic-hosted massive sul¢de (VHMS) deposits. Moreover, with the exception of the Dresser deposits [15], coexisting sul¢de minerals have a narrow range of 0 þ 4x [16]. Sedimentary sul¢de elsewhere has N 34 S in the range of 0 þ 10x [13,17].…”

E

“…Parts of the North Pole barite have been interpreted as pseudomorphosed gypsum, based on interfacial angles of individual crystals (Buick and Dunlop 1990;Jewell 2000;Shen et al 2001). ▶ Sulfur isotope variations within these sulfates in microcrystalline ▶ pyrite suggest the existence of sulfate-reducing bacteria (Shen et al 2001;Shen and Buick 2004).…”