Electron Transfer Drives Preferentially Assimilatory Sulfate Reduction in Petroleum-Contaminated Soil

Sulfur is a vital macroelement for soil fertility and crop growth, and its bioavailability is converted with electron flux. To determine the effect of biological electron transfer on the sulfur cycle in soil, a microbial electrochemical system (MES) was established to amplify the efficiency of extracellular electron transfer. Results showed that biocurrent obviously promoted the reduction of water-soluble SO₄ ^2- to S^2-, and afterward reoxidised to fulvic acid sulfur (FAS, 32% increment) instead of humic acid sulfur (HAS, 77% decrement). The δ³⁴FAS values (12‰ to 24‰) were consistent with the typical isotopic signatures of dissimilatory sulfate reduction, while the δ³⁴HAS values (-40‰ to -60‰) matched the assimilatory sulfate reduction. Meanwhile, the assimilatory SO₄ ^2- reduction to cysteine improved with a 30%-33% increase for the NAD⁺/NADH and secretion of cytochrome c to support robust redox. Essentially, the precursor-product pairs of desulfonation were enhanced by 26%-93%. Compared to a 6% contribution of HAS, the formation of cysteine accounted for 55% of the electron transport flux. According to screening 32 sulfur-transforming genes, 609 genera were identified, especially 31%-84% proliferation for Desulfatitalea. This study reveals ways of organic sulfur conversion mediated by electron transfer, which can assist the biological amendment of sulfur bioavailability in soil.