Mechanism of H2S Oxidation by the Dissimilatory Perchlorate-Reducing Microorganism Azospira suillum PS.

Misha G Mehta-Kolte,Dana Loutey,Mark E Conrad,Christopher G Hubbard,Ouwei Wang,John D Coates,Kelly M Wetmore,Matthew D Youngblut,Stephen J Giovannoni

doi:10.1128/mbio.02023-16

Misha G Mehta-Kolte, Dana Loutey + Show 7 more

Open Access

https://doi.org/10.1128/mbio.02023-16

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Abstract

ABSTRACTThe genetic and biochemical basis of perchlorate-dependent H2S 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 H2S, producing elemental sulfur (So). Although the process involving PSOX is thermodynamically favorable (ΔG°′ = −206 kJ ⋅ mol−1 H2S), the underlying biochemical and genetic mechanisms are currently unknown. Interestingly, H2S 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 H2S oxidation is mediated by electron transport chain short-circuiting in the periplasmic space where the PcrAB directly oxidizes H2S to So. The biogenically formed reactive intermediates (ClO2− and O2) subsequently react with additional H2S, producing polysulfide and So as end products.

Highlights

The genetic and biochemical basis of perchlorate-dependent H2S oxidation (PSOX) was investigated in the dissimilatory perchlorate-reducing microorganism (DPRM) Azospira suillum PS (PS)
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
While many DPRMs can alternatively utilize nitrate for heterotrophic metabolism, H2S oxidation is a perchlorate-dependent form of metabolism in these organisms, suggesting that it involves some unique components of the perchlorate respiratory pathway [8, 9]

Summary

Introduction

The genetic and biochemical basis of perchlorate-dependent H2S oxidation (PSOX) was investigated in the dissimilatory perchlorate-reducing microorganism (DPRM) Azospira suillum PS (PS). The inferred importance of this metabolism is that it is a novel and previously unrecognized component of the global sulfur redox cycle and is because of the recent demonstrated applicability of perchlorate and DPRMs in the control of biogenic sulfide production in engineered environments such as oil and gas reservoirs and wastewater treatment facilities, where excess H2S represents a significant environmental, process, and health risk [9, 10, 13]. While many DPRMs can alternatively utilize nitrate for heterotrophic metabolism, H2S oxidation is a perchlorate-dependent form of metabolism in these organisms, suggesting that it involves some unique components of the perchlorate respiratory pathway [8, 9] This perchlorate-specific H2S oxidation could be due to a combination of enzymatic and abiotic interactions with the reactive chlorine species (RCS) or molecular oxygen generated as intermediates of canonical perchlorate respiration [12].

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