Methanobacterium enables high rate electricity-driven autotrophic sulfate reduction

Abstract

The autotrophic reduction of sulfate can be sustained with a cathode as the only electron donor in bioelectrochemical systems (BES). This work studies the effect of inoculum source on autotrophic sulfate reduction start-up and performance of autotrophic sulfide production rates using a biocathode in a fed-batch operation mode. After 180 days, low electron and sulfate consumption was observed using BES controlled at −0.9 V vs. SHE and inoculated with mixed microbial consortia from sewer biofilm reactors, anaerobic sludge and mangrove sediments. However, when an enriched electroactive consortium capable of cathodic CO₂ reduction to acetate was used as biocatalyst in combination with the above inocula, the maximal cathodic current increased to −3.4 A m⁻² within 55 days at the same applied potential. High-throughput microbial community sequencing revealed that enhanced performance was likely caused by the enrichment of hydrogen-producing Methanobacterium (26% relative abundance). The biofilm and planktonic cells also contained the autotrophic hydrogen and sulfate consumer Desulfovibrio at 2.8% relative abundance. The resulting microbial community demonstrated sulfate and electron consumption rates of 0.115 ± 0.009 mol SO₄²⁻–S per m² per d and 1.5 ± 0.7 mol m⁻² d⁻¹ (39 times higher sulfate reduction rate and 186-fold cathodic electron consumption rate than control reactors with the same configuration but lacking the enriched electroactive consortia). Cyclic voltammetry furthermore revealed a positive shift of the cathodic onset current by ∼0.2 V, which points to the electrocatalytic role of the biocatalyst.