Sulfur disproportionation realizes an organic-free sulfidogenic process for sustainable treatment of acid mine drainage

Microbial enhanced manganese-autotrophic denitrification in reactor: performance, microbial diversity, potential functions

Autotrophic denitrification technology has gained increasing attention in recent years owing to its effectiveness, economical, and environmentally friendly nature. However, the sluggish reaction rate has emerged as the primary impediment to its widespread application. Herein, a bio-enhanced autotrophic denitrification reactor with modified loofah sponge (LS) immobilized microorganisms was established to achieve efficient denitrification. Under autotrophic conditions, a nitrate removal efficiency of 59.55 % (0.642 mg/L/h) and a manganese removal efficiency of 86.48 % were achieved after bio-enhance, which increased by 20.92 % and 36.34 %. The bioreactor achieved optimal performance with denitrification and manganese removal efficiencies of 99.84 % (1.09 mg/L/h) and 91.88 %. ETSA and 3D-EEM analysis reveled manganese promoting electron transfer and metabolic activity of microorganisms. High-throughput sequencing results revealed as the increase of Mn(II) concentration, Cupriavidus became one of the dominant strains in the reactor. Prediction of metabolic functions results proved the great potential for Mn(II)-autotrophic denitrification of LS bioreactor.

Groundwater chromate removal by autotrophic sulfur disproportionation

Chromate [Cr(VI)] contamination in groundwater is a global environmental challenge. Traditional elemental sulfur-based biotechnologies for Cr(VI) removal depend heavily on the synthesis of dissolved organic carbon to fuel heterotrophic Cr(VI) reduction, a bottleneck in the remediation process. Here we show an alternative approach by leveraging sulfur-disproportionating bacteria (SDB) inherent to groundwater ecosystems, offering a novel and efficient Cr(VI) removal strategy. We implemented SDB within a sulfur-packed bed reactor for treating Cr(VI)-contaminated groundwater, achieving a notable removal rate of 6.19 mg L-1 h-1 under oligotrophic conditions. We identified the chemical reduction of Cr(VI) via sulfide, produced through sulfur disproportionation, as a key mechanism, alongside microbial Cr(VI) reduction within the sulfur-based biosystem. Genome-centric metagenomic analysis revealed a symbiotic relationship among SDB, sulfur-oxidizing, and chromate-reducing bacteria within the reactor, suggesting that Cr(VI) detoxification by these microbial communities enhances the sulfur-disproportionation process. This research highlights the significance of sulfur disproportionation in the cryptic sulfur cycle in Cr(VI)-contaminated groundwater and proposes its practical application in groundwater remediation efforts.

Harnessing the potential of the microbial sulfur cycle for environmental biotechnology

The sulfur cycle is a complex biogeochemical cycle characterized by the high variability in the oxidation states of sulfur. While sulfur is essential for life processes, certain sulfur compounds, such as hydrogen sulfide, are toxic to all life forms. Micro-organisms facilitate the sulfur cycle, playing a prominent role even in extreme environments, such as soda lakes, acid mine drainage sites, hot springs, and other harsh habitats. The activity of these micro-organisms presents unique opportunities for mitigating sulfur-based pollution and enhancing the recovery of sulfur and metals. This review highlights the application of sulfur-oxidizing and -reducing micro-organisms in environmental biotechnology through three illustrative examples. Additionally, it discusses the challenges, recent trends, and prospects associated with these applications.

Adaptability of sulfur-disproportionating bacteria for mine water remediation under the pressures of heavy metal ions and high sulfate content

Biological sulfide production processes mediated by sulfate/sulfur reduction have gained attention for metal removal from industrial wastewater (e.g., mine water (MW) and metallurgical wastewater) via forming insoluble metal sulfides. However, these processes often necessitate the addition of external organic compounds as electron donors, which poses a constraint on the broad application of this technology. A recent proof of concept study reported that microbial sulfur disproportionation (SD) produced sulfide with no demand for organics, which could achieve more cost-benefit MW treatment against the above-mentioned processes. However, the resistance of SD bioprocess to different metals and high sulfate content in MW remains mysterious, which may substantially affect the practical applicability of such process. In this study, the sulfur-disproportionating bacteria (SDB)-dominated consortium was enriched from a previously established SD-driven bioreactor, in which Dissulfurimicrobium sp. with a relative abundance of 39.9 % was the predominated SDB. When exposed to the real pretreated acidic MW after the pretreatment process of pH amelioration, the sulfur-disproportionating activity remained active, and metals were effectively removed from the MW. Metal tolerance assays further demonstrated that the consortium had a good tolerance to different metal ions (i.e., Pb2+, Cu2+, Ni2+, Mn2+, Zn2+), especially for Mn2+ with a concentration of approximately 20 mg/L. It suggested the robustness of Dissulfurimicrobium sp. likely due to the presence of genes encoding for the enzymes associated with metal(loid) resistance/uptake. Additionally, although high sulfate content resulted in a slight inhibition on the sulfur-disproportionating activity, the consortium still achieved sulfide production rates of 27.3 mg S/g VSS-d on average under an environmentally relevant sulfate level (i.e., 1100 mg S/L), which is comparable to those reported in sulfate reduction. Taken together, these findings imply that SDB could ensure sustainable MW treatment in a more cost-effective and organic-free way.

Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Elemental sulfur (S0) plays a vital role in the coupled cycling of sulfur and iron, which in turn affects the transformation of carbon and various pollutants. These processes have been well characterized under static anoxic or oxic conditions, however, how the natural redox fluctuations affect the bio-mediated sulfur cycling and coupled iron cycling remain enigmatic. The present work examined S0 disproportionation as driven by natural microbial communities under fluctuating redox conditions and the contribution of S0 disproportionation to ferrihydrite transformation. Samples were incubated at either neutral or alkaline pH values, applying sequential anaerobic, aerobic and anaerobic conditions over 60 days. Under anaerobic conditions, S0 was found to undergo disproportionation to sulfate and sulfide, which subsequently reduced ferrihydrite at both pH 7.4 and 9.5. Ferrihydrite promoted S0 disproportionation by scavenging biogenic sulfide and maintaining a suitable degree of sulfate formation. After an oxic period, during the subsequent anoxic incubation, bioreduction of sulfate occurred and the biogenic sulfide reduced iron (hydr)oxides at a rate approximately 25 % lower than that observed during the former anoxic period. A 16S rDNA-based microbial community analysis revealed changes in the microbial community in response to the redox fluctuations, implying an intimate association with the coupled cycling of sulfur and iron. Microscopic and spectroscopic analyses confirmed the S0-mediated transformation of ferrihydrite to crystalline iron (hydr)oxide minerals such as lepidocrocite and magnetite and the formation of iron sulfides precipitated under fluctuating redox conditions. Finally, a reaction mechanism based on mass balance was proposed, demonstrating that bio-mediated sulfur transformation maintained a sustainable redox reaction with iron (hydr)oxides under fluctuating anaerobic-aerobic-anaerobic conditions tested in this study. Altogether, the finding of our study is critical for obtaining a more complete understanding of the dynamics of iron redox reactions and pollutant transformation in sulfur-rich aquatic environments.