Mechanism of H ₂S Oxidation by the Dissimilatory Perchlorate-Reducing Microorganism Azospira suillum PS

ABSTRACT

The genetic and biochemical basis of perchlorate-dependent H ₂S oxidation (PSOX) was investigated in the dissimilatory perchlorate-reducing microorganism (DPRM) Azospira suillum PS (PS). Previously, it was shown that all known DPRMs innately oxidize H ₂S, producing elemental sulfur (S ^o). Although the process involving PSOX is thermodynamically favorable ( ΔG°′ = −206 kJ ⋅ mol ⁻¹ H ₂S), the underlying biochemical and genetic mechanisms are currently unknown. Interestingly, H ₂S is preferentially utilized over physiological electron donors such as lactate or acetate although no growth benefit is obtained from the metabolism. Here, we determined that PSOX is due to a combination of enzymatic and abiotic interactions involving reactive intermediates of perchlorate respiration. Using various approaches, including barcode analysis by sequencing (Bar-seq), transcriptome sequencing (RNA-seq), and proteomics, along with targeted mutagenesis and biochemical characterization, we identified all facets of PSOX in PS. In support of our proposed model, deletion of identified upregulated PS genes traditionally known to be involved in sulfur redox cycling (e.g., Sox, sulfide:quinone reductase [SQR]) showed no defect in PSOX activity. Proteomic analysis revealed differential abundances of a variety of stress response metal efflux pumps and divalent heavy-metal transporter proteins, suggesting a general toxicity response. Furthermore, in vitro biochemical studies demonstrated direct PSOX mediated by purified perchlorate reductase (PcrAB) in the absence of other electron transfer proteins. The results of these studies support a model in which H ₂S oxidation is mediated by electron transport chain short-circuiting in the periplasmic space where the PcrAB directly oxidizes H ₂S to S ^o. The biogenically formed reactive intermediates (ClO ₂ ⁻ and O ₂) subsequently react with additional H ₂S, producing polysulfide and S ^o as end products.

IMPORTANCE

Inorganic sulfur compounds are widespread in nature, and microorganisms are central to their transformation, thereby playing a key role in the global sulfur cycle. Sulfur oxidation is mediated by a broad phylogenetic diversity of microorganisms, including anoxygenic phototrophs and either aerobic or anaerobic chemotrophs coupled to oxygen or nitrate respiration, respectively. Recently, perchlorate-respiring microorganisms were demonstrated to be innately capable of sulfur oxidation regardless of their phylogenetic affiliation. As recognition of the prevalence of these organisms intensifies, their role in global geochemical cycles is being queried. This is further highlighted by the recently recognized environmental pervasiveness of perchlorate not only across Earth but also throughout our solar system. The inferred importance of this metabolism not only is that it is a novel and previously unrecognized component of the global sulfur redox cycle but also is because of the recently demonstrated applicability of perchlorate respiration in the control of biogenic sulfide production in engineered environments such as oil reservoirs and wastewater treatment facilities, where excess H ₂S represents a significant environmental, process, and health risk, with associated costs approximating $90 billion annually.