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

Area of Science:

• Soil Microbiology and Biogeochemistry
• Environmental Science
• Biotechnology

Background:

• Sulfur is an essential macronutrient for soil fertility and crop development.
• The bioavailability of soil sulfur is influenced by microbial electron transfer processes.
• Understanding the sulfur cycle is crucial for sustainable agriculture.

Purpose of the Study:

• To investigate the impact of biological electron transfer on the soil sulfur cycle.
• To enhance extracellular electron transfer efficiency using a microbial electrochemical system (MES).
• To elucidate pathways of organic sulfur conversion mediated by electron transfer.

Main Methods:

• Establishment of a microbial electrochemical system (MES) to amplify extracellular electron transfer.
• Analysis of sulfur compounds (sulfate, sulfide, fulvic acid sulfur, humic acid sulfur, cysteine) and their isotopic signatures (δ³⁴S).
• Quantification of redox cofactors (NAD⁺/NADH) and secreted proteins (cytochrome c).
• Screening of sulfur-transforming genes and identification of microbial genera.

Main Results:

• Biocurrent significantly promoted the reduction of sulfate (SO₄²⁻) to sulfide (S²⁻) and subsequent reoxidation to fulvic acid sulfur (FAS), with a 32% increase.
• Humic acid sulfur (HAS) decreased by 77%, and isotopic analysis confirmed dissimilatory sulfate reduction for FAS and assimilatory sulfate reduction for HAS.
• Assimilatory sulfate reduction to cysteine increased by 30%-33%, supported by enhanced NAD⁺/NADH ratios and cytochrome c secretion; cysteine formation accounted for 55% of electron transport flux.
• Significant proliferation (31%-84%) of Desulfatitalea genera was observed among the 609 identified genera.

Conclusions:

• Biological electron transfer, amplified by MES, effectively drives organic sulfur conversion in soil.
• This process enhances the bioavailability of sulfur, primarily through the formation of fulvic acid sulfur and cysteine.
• The findings provide insights into microbial mechanisms for sulfur cycling and suggest potential for biological amendments to improve soil sulfur availability.