Independent of direct interspecies electron transfer: Magnetite-mediated sulphur cycle for anaerobic degradation of benzoate under low-concentration sulphate conditions

Abstract

The aim of this study was to investigate the primary mechanism of magnetite promoting anaerobic degradation of aromatic compounds under the low-concentration sulphate conditions. Under influent conditions of benzoate at 50 mM-chemical oxygen demand (COD) and sulphate at 15 mM, magnetite promoted benzoate degradation (77.1% vs 56.3%), while the effluent sulphate concentration was slightly higher than that without magnetite (1.6 mM vs 0.7 mM), inconsistent with functional gene prediction that both sulphate respiration and sulphur compound respiration were relatively more active in the presence of magnetite. Remarkably, X-ray diffraction showed that, signal related to Fe₃O₄ faded away and finally was replaced by FeSO₄ and FeS, indicating that magnetite participated in benzoate degradation coupled to sulphate reduction via dissimilatory Fe(III) reduction. Further X-ray photoelectron spectroscopy showed that, signal related to S⁰ was only detected with magnetite, suggesting the possibility of re-oxidation of sulphide to elemental sulphur coupled to Fe(III) reduction. This was further supported by the increase in abundance of Desulfuromonas acetexigens capable of growing on Fe(III). In addition, magnetite specially enriched the chemolithotrophic sulphur-disproportionating microbes, Desulfovibrio aminophilus, which might proceed the disproportionation of elemental sulphur to sulphate and sulphide to achieve a sulphur cycle for benzoate degradation.

Keywords: Benzoate Degradation; Magnetite; Sulphate Reduction; Sulphur Cycle; Sulphur Disproportionation.