A common pathway of sulfide oxidation by sulfate-reducing bacteria

Sulfate-reducing bacteria unearthed: ecological functions of the diverse prokaryotic group in terrestrial environments

Sulfate-reducing prokaryotes (SRPs) are essential microorganisms that play crucial roles in various ecological processes. Even though SRPs have been studied for over a century, there are still gaps in our understanding of their biology. In the past two decades, a significant amount of data on SRP ecology has been accumulated. This review aims to consolidate that information, focusing on SRPs in soils, their relation to the rare biosphere, uncultured sulfate reducers, and their interactions with other organisms in terrestrial ecosystems. SRPs in soils form part of the rare biosphere and contribute to various processes as a low-density population. The data reveal a diverse range of sulfate-reducing taxa intricately involved in terrestrial carbon and sulfur cycles. While some taxa like Desulfitobacterium and Desulfosporosinus are well studied, others are more enigmatic. For example, members of the Acidobacteriota phylum appear to hold significant importance for the terrestrial sulfur cycle. Many aspects of SRP ecology remain mysterious, including sulfate reduction in different bacterial phyla, interactions with bacteria and fungi in soils, and the existence of soil sulfate-reducing archaea. Utilizing metagenomic, metatranscriptomic, and culture-dependent approaches will help uncover the diversity, functional potential, and adaptations of SRPs in the global environment.

Stochastic Processes Dominate in the Water Mass-Based Segregation of Diazotrophs in a High Arctic Fjord (Svalbard)

Nitrogen-fixing or diazotrophic microbes fix atmospheric nitrogen (N2) to ammonia (NH3+) using nitrogenase enzyme and play a crucial role in regulating marine primary productivity and carbon dioxide sequestration. However, there is a lack of information about the diversity, structure, and environmental regulations of the diazotrophic communities in the high Arctic fjords, such as Kongsfjorden. Here, we employed nifH gene sequencing to clarify variations in composition, community structure, and assembly mechanism among the diazotrophs of the salinity-driven stratified waters of Kongsfjorden. The principal environmental and ecological drivers of the observed variations were identified. The majority of the nifH gene sequences obtained in the present study belonged to cluster I and cluster III nifH phylotypes, accounting for 65% and 25% of the total nifH gene sequences. The nifH gene diversity and composition, irrespective of the size fractions (free-living and particle attached), showed a clear separation among water mass types, i.e., Atlantic-influenced versus glacier-influenced water mass. Higher nifH gene diversity and relative abundances of non-cyanobacterial nifH OTUs, affiliated with uncultured Rhizobiales, Burkholderiales, Alteromonadaceae, Gallionellaceae (cluster I) and uncultured Deltaproteobacteria including Desulfuromonadaceae (cluster III), were prevalent in GIW while uncultured Gammaproteobacteria and Desulfobulbaceae were abundant in AIW. The diazotrophic community assembly was dominated by stochastic processes, principally ecological drift, and to lesser degrees dispersal limitation and homogeneous dispersal. Differences in the salinity and dissolved oxygen content lead to the vertical segregation of diazotrophs among water mass types. These findings suggest that water column stratification affects the composition and assembly mechanism of diazotrophic communities and thus could affect nitrogen fixation in the Arctic fjord.

Stepwise pathway for early evolutionary assembly of dissimilatory sulfite and sulfate reduction

Microbial dissimilatory sulfur metabolism utilizing dissimilatory sulfite reductases (Dsr) influenced the biochemical sulfur cycle during Earth's history and the Dsr pathway is thought to be an ancient metabolic process. Here we performed comparative genomics, phylogenetic, and synteny analyses of several Dsr proteins involved in or associated with the Dsr pathway across over 195,000 prokaryotic metagenomes. The results point to an archaeal origin of the minimal DsrABCMK(N) protein set, having as primordial function sulfite reduction. The acquisition of additional Dsr proteins (DsrJOPT) increased the Dsr pathway complexity. Archaeoglobus would originally possess the archaeal-type Dsr pathway and the archaeal DsrAB proteins were replaced with the bacterial reductive-type version, possibly at the same time as the acquisition of the QmoABC and DsrD proteins. Further inventions of two Qmo complex types, which are more spread than previously thought, allowed microorganisms to use sulfate as electron acceptor. The ability to use the Dsr pathway for sulfur oxidation evolved at least twice, with Chlorobi and Proteobacteria being extant descendants of these two independent adaptations.

The DsrD functional marker protein is an allosteric activator of the DsrAB dissimilatory sulfite reductase

Metagenomic data have recently transformed our view of the role played by sulfur metabolism in anoxic environments by showing that this trait is much more widespread than previously believed. A key enzyme in sulfur metabolism is the dissimilatory sulfite reductase DsrAB that is ubiquitous in organisms with a reductive, oxidative, or disproportionating activity. However, the function of some dsr genes, such as dsrD, has so far been unknown despite its use as a functional marker to genomically assign the type of sulfur energy metabolism, sometimes with unclear results. Here, we disclose the function of DsrD as an activator of DsrAB that significantly increases its activity, providing important insights into the mechanism of this enzyme in different types of sulfur metabolism.

Dissimilatory sulfur metabolism was recently shown to be much more widespread among bacteria and archaea than previously believed. One of the key pathways involved is the dsr pathway that is responsible for sulfite reduction in sulfate-, sulfur-, thiosulfate-, and sulfite-reducing organisms, sulfur disproportionators and organosulfonate degraders, or for the production of sulfite in many photo- and chemotrophic sulfur-oxidizing prokaryotes. The key enzyme is DsrAB, the dissimilatory sulfite reductase, but a range of other Dsr proteins is involved, with different gene sets being present in organisms with a reductive or oxidative metabolism. The dsrD gene codes for a small protein of unknown function and has been widely used as a functional marker for reductive or disproportionating sulfur metabolism, although in some cases this has been disputed. Here, we present in vivo and in vitro studies showing that DsrD is a physiological partner of DsrAB and acts as an activator of its sulfite reduction activity. DsrD is expressed in respiratory but not in fermentative conditions and a ΔdsrD deletion strain could be obtained, indicating that its function is not essential. This strain grew less efficiently during sulfate and sulfite reduction. Organisms with the earliest forms of dsrAB lack the dsrD gene, revealing that its activating role arose later in evolution relative to dsrAB.

Electrochemical mitigation of hydrogen sulfide in deep-pit swine manure storage

Hydrogen sulfide (H₂S) is a toxic and hazardous gas and is commonly present in livestock operations, which occasionally causes associated exposure accidents. This study evaluated the effectiveness of electrochemical control of H₂S in lab-scale swine manure storage using different electrode materials, and further selected suitable materials to demonstrate the performance of a pilot-scale field test in the deep-pit manure storage of a 200-head swine barn. In the lab-scale test, electrochemical sulfide oxidation mainly contributed to the H₂S mitigation, resulting in high H₂S removal efficiencies when using low carbon steel (LCS) and stainless steel 304 (SS304) as electrodes. Based on their better H₂S treatment performance and lower material costs, LCS and SS304 were selected for the pilot-scale test. In a 92-day operation, the pilot-scale demonstration showed H₂S removal efficiencies of 84.0% and 63.5% for LCS and SS304, respectively. A techno-economic assessment indicated that the installation and operation of the electrochemical system accounted for 16% of barn construction cost using LCS as electrodes. Further optimization may substantially decrease the electrode material consumption and the overall cost.

Influence of environmental factors on benthic nitrogen fixation and role of sulfur reducing diazotrophs in a eutrophic tropical estuary

Benthic nitrogen fixation in the tropical estuaries plays a major role in marine nitrogen cycle, its contribution to nitrogen budget and players behind process is not well understood. The present study was estimated the benthic nitrogen fixation rate in a tropical estuary (Cochin) and also evaluated the contribution of various diazotrophic bacterial communities. Nitrogen fixation was detected throughout year (0.1-1.11 nmol N g^-1 h^-1); higher activity was observed in post-monsoon. The nifH gene abundance was varied from 0.8 × 10⁴ to 0.6 × 10⁸ copies g^-1dry sediment; highest was detected in post-monsoon. The Cluster I and Cluster III were the dominant diazotrophs. Sulfur reducing bacterial phylotypes (Deltaproteobacteria) contributed up to 2-72% of total nitrogen fixation. These bacteria may provide new nitrogen to these systems, counteracting nitrogen loss via denitrification and anammox. Overall, the study explained the importance of benthic nitrogen fixation and role of diazotrophs in a monsoon influenced tropical estuarine environments.

Abundance and diversity of diazotrophs in the surface sediments of Kongsfjorden, an Arctic fjord

Diazotrophy in the Arctic environment is poorly understood compared to tropical and subtropical regions. Hence in this study, we report the abundance and diversity of diazotrophs in Arctic fjord sediments and elucidate the role of environmental factors on the distribution of diazotrophs. The study was conducted during the boreal summer in the Kongsfjorden, an Arctic fjord situated in the western coast of Spitsbergen. The abundance of nifH gene was measured through quantitative real-time PCR and the diversity of diazotrophs was assessed by nifH targeted clone library and next generation sequence analysis. Results revealed that the abundance of nifH gene in the surface sediments ranged from 2.3 × 10⁶ to 3.7 × 10⁷ copies g^- 1. The δ-proteobacterial diazotrophs (71% of total sequence) were the dominant class observed in this study. Major genera retrieved from the sequence analysis were Desulfovibrionaceae (25% of total sequence), Desulfuromonadaceae (18% of total sequence) and Desulfobacteriaceae (10% of total sequence); these are important diazotrophic iron and sulfur-reducing bacterial clade in the Kongsfjorden sediments. The abundance of nifH gene showed a significant positive correlation TOC/TN ratio (r² = 0.96, p ≤ 0.05) and total organic carbon (p ≤ 0.05) content in the fjord sediments. The higher TOC/TN ratio (4.24-14.5) indicated low nitrogen content organic matter in the fjord sediments through glacier runoff, which enhances the abundance and diversity of nitrogen fixing microorganisms.

On the evolution and physiology of cable bacteria

Cable bacteria of the family Desulfobulbaceae form centimeter-long filaments comprising thousands of cells. They occur worldwide in the surface of aquatic sediments, where they connect sulfide oxidation with oxygen or nitrate reduction via long-distance electron transport. In the absence of pure cultures, we used single-filament genomics and metagenomics to retrieve draft genomes of 3 marine Candidatus Electrothrix and 1 freshwater Ca. Electronema species. These genomes contain >50% unknown genes but still share their core genomic makeup with sulfate-reducing and sulfur-disproportionating Desulfobulbaceae, with few core genes lost and 212 unique genes (from 197 gene families) conserved among cable bacteria. Last common ancestor analysis indicates gene divergence and lateral gene transfer as equally important origins of these unique genes. With support from metaproteomics of a Ca. Electronema enrichment, the genomes suggest that cable bacteria oxidize sulfide by reversing the canonical sulfate reduction pathway and fix CO₂ using the Wood-Ljungdahl pathway. Cable bacteria show limited organotrophic potential, may assimilate smaller organic acids and alcohols, fix N₂, and synthesize polyphosphates and polyglucose as storage compounds; several of these traits were confirmed by cell-level experimental analyses. We propose a model for electron flow from sulfide to oxygen that involves periplasmic cytochromes, yet-unidentified conductive periplasmic fibers, and periplasmic oxygen reduction. This model proposes that an active cable bacterium gains energy in the anodic, sulfide-oxidizing cells, whereas cells in the oxic zone flare off electrons through intense cathodic oxygen respiration without energy conservation; this peculiar form of multicellularity seems unparalleled in the microbial world.

Complex Microbial Communities Drive Iron and Sulfur Cycling in Arctic Fjord Sediments

Glacial retreat is changing biogeochemical cycling in the Arctic, where glacial runoff contributes iron for oceanic shelf primary production. We hypothesize that in Svalbard fjords, microbes catalyze intense iron and sulfur cycling in low-organic-matter sediments. This is because low organic matter limits sulfide generation, allowing iron mobility to the water column instead of precipitation as iron monosulfides. In this study, we tested this with high-depth-resolution 16S rRNA gene libraries in the upper 20 cm at two sites in Van Keulenfjorden, Svalbard. At the site closer to the glaciers, iron-reducing Desulfuromonadales, iron-oxidizing Gallionella and Mariprofundus, and sulfur-oxidizing Thiotrichales and Epsilonproteobacteria were abundant above a 12-cm depth. Below this depth, the relative abundances of sequences for sulfate-reducing Desulfobacteraceae and Desulfobulbaceae increased. At the outer station, the switch from iron-cycling clades to sulfate reducers occurred at shallower depths (∼5 cm), corresponding to higher sulfate reduction rates. Relatively labile organic matter (shown by δ¹³C and C/N ratios) was more abundant at this outer site, and ordination analysis suggested that this affected microbial community structure in surface sediments. Network analysis revealed more correlations between predicted iron- and sulfur-cycling taxa and with uncultured clades proximal to the glacier. Together, these results suggest that complex microbial communities catalyze redox cycling of iron and sulfur, especially closer to the glacier, where sulfate reduction is limited due to low availability of organic matter. Diminished sulfate reduction in upper sediments enables iron to flux into the overlying water, where it may be transported to the shelf.IMPORTANCE Glacial runoff is a key source of iron for primary production in the Arctic. In the fjords of the Svalbard archipelago, glacial retreat is predicted to stimulate phytoplankton blooms that were previously restricted to outer margins. Decreased sediment delivery and enhanced primary production have been hypothesized to alter sediment biogeochemistry, wherein any free reduced iron that could potentially be delivered to the shelf will instead become buried with sulfide generated through microbial sulfate reduction. We support this hypothesis with sequencing data that showed increases in the relative abundance of sulfate reducing taxa and sulfate reduction rates with increasing distance from the glaciers in Van Keulenfjorden, Svalbard. Community structure was driven by organic geochemistry, suggesting that enhanced input of organic material will stimulate sulfate reduction in interior fjord sediments as glaciers continue to recede.

Large sulfur isotope fractionation by bacterial sulfide oxidation

A sulfide-oxidizing microorganism, Desulfurivibrio alkaliphilus (DA), generates a consistent enrichment of sulfur-34 (³⁴ S) in the produced sulfate of +12.5 per mil or greater. This observation challenges the general consensus that the microbial oxidation of sulfide does not result in large ³⁴ S enrichments and suggests that sedimentary sulfides and sulfates may be influenced by metabolic activity associated with sulfide oxidation. Since the DA-type sulfide oxidation pathway is ubiquitous in sediments, in the modern environment, and throughout Earth history, the enrichments and depletions in ³⁴ S in sediments may be the combined result of three microbial metabolisms: microbial sulfate reduction, the disproportionation of external sulfur intermediates, and microbial sulfide oxidation.

Growth of an anaerobic sulfate-reducing bacterium sustained by oxygen respiratory energy conservation after O2 -driven experimental evolution

Desulfovibrio species are representatives of microorganisms at the boundary between anaerobic and aerobic lifestyles, since they contain the enzymatic systems required for both sulfate and oxygen reduction. However, the latter has been shown to be solely a protective mechanism. By implementing the oxygen-driven experimental evolution of Desulfovibrio vulgaris Hildenborough, we have obtained strains that have evolved to grow with energy derived from oxidative phosphorylation linked to oxygen reduction. We show that a few mutations are sufficient for the emergence of this phenotype and reveal two routes of evolution primarily involving either inactivation or overexpression of the gene encoding heterodisulfide reductase. We propose that the oxygen respiration for energy conservation that sustains the growth of the O₂ -evolved strains is associated with a rearrangement of metabolite fluxes, especially NAD⁺ /NADH, leading to an optimized O₂ reduction. These evolved strains are the first sulfate-reducing bacteria that exhibit a demonstrated oxygen respiratory process that enables growth.

Peatland Acidobacteria with a dissimilatory sulfur metabolism

Sulfur-cycling microorganisms impact organic matter decomposition in wetlands and consequently greenhouse gas emissions from these globally relevant environments. However, their identities and physiological properties are largely unknown. By applying a functional metagenomics approach to an acidic peatland, we recovered draft genomes of seven novel Acidobacteria species with the potential for dissimilatory sulfite (dsrAB, dsrC, dsrD, dsrN, dsrT, dsrMKJOP) or sulfate respiration (sat, aprBA, qmoABC plus dsr genes). Surprisingly, the genomes also encoded DsrL, which so far was only found in sulfur-oxidizing microorganisms. Metatranscriptome analysis demonstrated expression of acidobacterial sulfur-metabolism genes in native peat soil and their upregulation in diverse anoxic microcosms. This indicated an active sulfate respiration pathway, which, however, might also operate in reverse for dissimilatory sulfur oxidation or disproportionation as proposed for the sulfur-oxidizing Desulfurivibrio alkaliphilus. Acidobacteria that only harbored genes for sulfite reduction additionally encoded enzymes that liberate sulfite from organosulfonates, which suggested organic sulfur compounds as complementary energy sources. Further metabolic potentials included polysaccharide hydrolysis and sugar utilization, aerobic respiration, several fermentative capabilities, and hydrogen oxidation. Our findings extend both, the known physiological and genetic properties of Acidobacteria and the known taxonomic diversity of microorganisms with a DsrAB-based sulfur metabolism, and highlight new fundamental niches for facultative anaerobic Acidobacteria in wetlands based on exploitation of inorganic and organic sulfur molecules for energy conservation.

Microbiological Leaching; an Environmentally Friendly and Cost Effective Method for Extraction of Metals

Finding a cleaner, environmentally friendly and cost-effective way of metal and mineral extraction has a great importance in today’s world. Using microorganisms in bio-leaching and bio-oxidation process is of great value. From Archaea to bacteria and fungi, microorganisms can play an important role in extraction of metals from mine drainage and un-accessible sources, both in aquatic and terrestrial environments. Optimization of environmental factors such as the temperature, pH and substrate concentration is crucially important to access the optimum extraction of selected metals from an ore or mine drainage. The present paper will review the bio-leaching and bio-oxidation process of minerals with emphasis on the most well-known species of bacterial communities of such ability, through the literature.

Turnover Rates of Intermediate Sulfur Species (Sx2-, S0, S2O32-, S4O62-, SO32-) in Anoxic Freshwater and Sediments

The microbial reduction of sulfate to sulfide coupled to organic matter oxidation followed by the transformation of sulfide back to sulfate drives a dynamic sulfur cycle in a variety of environments. The oxidative part of the sulfur cycle in particular is difficult to constrain because the eight electron oxidation of sulfide to sulfate occurs stepwise via a suite of biological and chemical pathways and produces a wide variety of intermediates (S x 2 - , S0, S2O 3 2 - , S4O 6 2 - , and SO 3 2 - ), which may in turn be oxidized, reduced or disproportionated. Although the potential processes affecting these intermediates are well-known from microbial culture and geochemical studies, their significance and rates in the environment are not well constrained. In the study presented here, time-course concentration measurements of intermediate sulfur species were made in amended freshwater water column and sediment incubation experiments in order to constrain consumption rates and processes. In sediment incubations, consumption rates were S colloidal 0 > S x 2 - > SO 3 2 - ≈ S4O 6 2 - > S2O 3 2 - , which is consistent with previous measurements of SO 3 2 - , S4O 6 2 - , and S2O 3 2 - consumption rates in marine sediments. In water column incubations, however, the relative reactivity was S colloidal 0 > SO 3 2 - > S x 2 - > S2O 3 2 - > S4O 6 2 - . Consumption of thiosulfate, tetrathionate and sulfite was primarily biological, whereas it was not possible to distinguish between abiotic and biological polysulfide consumption in either aqueous or sediment incubations. S colloidal 0 consumption in water column experiments was biologically mediated, however, rapid sedimentary consumption was likely due to reactions with iron minerals. These experiments provide important constraints on the biogeochemical reactivity of intermediate sulfur species and give further insight into the diversity of biological and geochemical processes that comprise (cryptic) environmental sulfur cycling.

Disguised as a Sulfate Reducer: Growth of the Deltaproteobacterium Desulfurivibrio alkaliphilus by Sulfide Oxidation with Nitrate

This study demonstrates that the deltaproteobacterium Desulfurivibrio alkaliphilus can grow chemolithotrophically by coupling sulfide oxidation to the dissimilatory reduction of nitrate and nitrite to ammonium. Key genes of known sulfide oxidation pathways are absent from the genome of D. alkaliphilus. Instead, the genome contains all of the genes necessary for sulfate reduction, including a gene for a reductive-type dissimilatory bisulfite reductase (DSR). Despite this, growth by sulfate reduction was not observed. Transcriptomic analysis revealed a very high expression level of sulfate-reduction genes during growth by sulfide oxidation, while inhibition experiments with molybdate pointed to elemental sulfur/polysulfides as intermediates. Consequently, we propose that D. alkaliphilus initially oxidizes sulfide to elemental sulfur, which is then either disproportionated, or oxidized by a reversal of the sulfate reduction pathway. This is the first study providing evidence that a reductive-type DSR is involved in a sulfide oxidation pathway. Transcriptome sequencing further suggests that nitrate reduction to ammonium is performed by a novel type of periplasmic nitrate reductase and an unusual membrane-anchored nitrite reductase.

Sulfide oxidation and sulfate reduction, the two major branches of the sulfur cycle, are usually ascribed to distinct sets of microbes with distinct diagnostic genes. Here we show a more complex picture, as D. alkaliphilus, with the genomic setup of a sulfate reducer, grows by sulfide oxidation. The high expression of genes typically involved in the sulfate reduction pathway suggests that these genes, including the reductive-type dissimilatory bisulfite reductases, are also involved in as-yet-unresolved sulfide oxidation pathways. Finally, D. alkaliphilus is closely related to cable bacteria, which grow by electrogenic sulfide oxidation. Since there are no pure cultures of cable bacteria, D. alkaliphilus may represent an exciting model organism in which to study the physiology of this process.

Cable Bacteria and the Bioelectrochemical Snorkel: The Natural and Engineered Facets Playing a Role in Hydrocarbons Degradation in Marine Sediments

The composition and metabolic traits of the microbial communities acting in an innovative bioelectrochemical system were here investigated. The system, known as Oil Spill Snorkel, was recently developed to stimulate the oxidative biodegradation of petroleum hydrocarbons in anoxic marine sediments. Next Generation Sequencing was used to describe the microbiome of the bulk sediment and of the biofilm growing attached to the surface of the electrode. The analysis revealed that sulfur cycling primarily drives the microbial metabolic activities occurring in the bioelectrochemical system. In the anoxic zone of the contaminated marine sediment, petroleum hydrocarbon degradation occurred under sulfate-reducing conditions and was lead by different families of Desulfobacterales (46% of total OTUs). Remarkably, the occurrence of filamentous Desulfubulbaceae, known to be capable to vehicle electrons deriving from sulfide oxidation to oxygen serving as a spatially distant electron acceptor, was demonstrated. Differently from the sediment, which was mostly colonized by Deltaproteobacteria, the biofilm at the anode hosted, at high extent, members of Alphaproteobacteria (59%) mostly affiliated to Rhodospirillaceae family (33%) and including several known sulfur- and sulfide-oxidizing genera. Overall, we showed the occurrence in the system of a variety of electroactive microorganisms able to sustain the contaminant biodegradation alone or by means of an external conductive support through the establishment of a bioelectrochemical connection between two spatially separated redox zones and the preservation of an efficient sulfur cycling.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

Effect of variation of liquid condition on transformation of sulfur and carbon in the sediment of sanitary sewer

This study aims to estimate the influence of the typical variation in liquid conditions on the biochemical reaction characteristics of sulfur and carbon in the sediment of gravity sanitary sewers. Thus, a series of experimental tests were conducted with real wastewater and sewage sediment to investigate the potential biochemical process of carbon and sulfur in sediment. Results indicated that the sulfur and carbon biochemical process in sediment with neutral pH is significant in the gravity sewage system. The changes in concentration and the ratios of wastewater component substrates are the key factors in chemical oxygen demand and sulfate reduction rates. Furthermore, the condition of dissolved oxygen in liquid significantly affected the biochemical reaction processes of sulfur and carbon. Finally, the frequent alternation of anaerobic and anoxic with low dissolved oxygen effectively inhibits sulfide accumulation and simultaneously reduces carbon loss in the sewage system.

Influence of nitrate and sulfate reduction in the bioelectrochemically assisted dechlorination of cis-DCE

This paper investigated the reductive dechlorination (RD) of cis-dichloroethylene (cis-DCE) (average influent 14.2±0.7 μM) by a bioelectrochemical system (BES), in the presence of real contaminated groundwater containing high levels of nitrate and sulfate. The BES enhanced both the RD and competing reactions, such as nitrate and sulfate reductions, which occurred with neither an external organic carbon source nor any inoculum other than the indigenous microbial consortia in the real groundwater. In preliminary batch tests, RD and full nitrate removal occurred after a short lag phase, whereas sulfate reduction occurred slowly and alongside the RD. Under continuous flow conditions (hydraulic retention time, HRT, 1.4 d), the competition of different electron acceptors was strongly affected by the cathodic potential in the range -550 to -750 mV vs. standard hydrogen electrode (SHE). Nitrate reduction was driven to completion at all tested cathodic potentials, whereas sulfate reduction and the RD rate increased as the cathodic potential became more negative. At -750 mV vs. SHE, strong methanogenesis was also observed and became the most important sink of electrons. The overall coulombic efficiency decreased while the potential became more negative. The RD contribution was always less than 1%. Hence, greater energy consumption was required to obtain higher RD rate and better conversion. Anodic oxidation was only observed at -750 mV vs. SHE where almost 39% of residual vinyl chloride (VC) was oxidized and the sulfate was formed back from sulfide (further contributing to electric waste).

Growth and chemosensory behavior of sulfate-reducing bacteria in oxygen-sulfide gradients

Growth and chemotactic behavior in oxic-anoxic gradients were studied with two freshwater and four marine strains of sulfate-reducing bacteria related to the genera Desulfovibrio, Desulfomicrobium or Desulfobulbus. Cells were grown in oxygen-sulfide counter-gradients within tubes filled with agar-solidified medium. The immobilized cells grew mainly in the anoxic zone, revealing a peak below the oxic-anoxic interface. All tested strains survived exposure to air for 8 h and all were capable of oxygen reduction with lactate. Most strains also oxidized sulfide with oxygen. Desulfovibrio desulfuricans responded chemotactically to lactate, nitrate, sulfate and thiosulfate, and even sulfide functioned as an attractant. In oxic-anoxic gradients the bacteria moved away from high oxygen concentrations and formed bands at the outer edge of the oxic zone at low oxygen concentration (<5% O2 saturation). They were able to actively change the extension and slope of the gradients by oxygen reduction with lactate or even sulfide as electron donor. Generally, the chemotactic behavior was in agreement with a defense strategy that re-establishes anoxic conditions, thus promoting anaerobic growth and, in a natural community, fermentative production of the preferred electron donors of the sulfate-reducing bacteria.

Real-time molecular monitoring of chemical environment in obligate anaerobes during oxygen adaptive response

Determining the transient chemical properties of the intracellular environment can elucidate the paths through which a biological system adapts to changes in its environment, for example, the mechanisms that enable some obligate anaerobic bacteria to survive a sudden exposure to oxygen. Here we used high-resolution Fourier transform infrared (FTIR) spectromicroscopy to continuously follow cellular chemistry within living obligate anaerobes by monitoring hydrogen bond structures in their cellular water. We observed a sequence of well orchestrated molecular events that correspond to changes in cellular processes in those cells that survive, but only accumulation of radicals in those that do not. We thereby can interpret the adaptive response in terms of transient intracellular chemistry and link it to oxygen stress and survival. This ability to monitor chemical changes at the molecular level can yield important insights into a wide range of adaptive responses.

Evaluation of oxygen injection as a means of controlling sulfide production in a sewer system

Oxygen injection is often used to control biogenic production of hydrogen sulfide in sewers. Experiments were carried out on a laboratory system mimicking a rising main to investigate the impact of oxygen injection on anaerobic sewer biofilm activities. Oxygen injection (15-25mg O(2)/L per pump event) to the inlet of the system decreased the overall sulfide discharge levels by 65%. Oxygen was an effective chemical and biological oxidant of sulfide but did not cause a cessation in sulfide production, which continued in the deeper layers of the biofilm irrespective of the oxygen concentration in the bulk. Sulfide accumulation resumed instantaneously on depletion of the oxygen. Oxygen did not exhibit any toxic effect on sulfate reducing bacteria (SRB) in the biofilm. It further stimulated SRB growth and increased SRB activity in downstream biofilms due to increased availability of sulfate at these locations as the result of oxic conditions upstream. The oxygen uptake rate of the system increased with repeated exposure to oxygen, with concomitant consumption of organic carbon in the wastewater. These results suggest that optimization of oxygen injection is necessary for maximum effectiveness in controlling sulfide concentrations in sewers.

Linked redox precipitation of sulfur and selenium under anaerobic conditions by sulfate-reducing bacterial biofilms

A biofilm-forming strain of sulfate-reducing bacteria (SRB), isolated from a naturally occurring mixed biofilm and identified by 16S rDNA analysis as a strain of Desulfomicrobium norvegicum, rapidly removed 200 micro M selenite from solution during growth on lactate and sulfate. Elemental selenium and elemental sulfur were precipitated outside SRB cells. Precipitation occurred by an abiotic reaction with bacterially generated sulfide. This appears to be a generalized ability among SRB, arising from dissimilatory sulfide biogenesis, and can take place under low redox conditions and in the dark. The reaction represents a new means for the deposition of elemental sulfur by SRB under such conditions. A combination of transmission electron microscopy, environmental scanning electron microscopy, and cryostage field emission scanning electron microscopy were used to reveal the hydrated nature of SRB biofilms and to investigate the location of deposited sulfur-selenium in relation to biofilm elements. When pregrown SRB biofilms were exposed to a selenite-containing medium, nanometer-sized selenium-sulfur granules were precipitated within the biofilm matrix. Selenite was therefore shown to pass through the biofilm matrix before reacting with bacterially generated sulfide. This constitutes an efficient method for the removal of toxic concentrations of selenite from solution. Implications for environmental cycling and the fate of sulfur and selenium are discussed, and a general model for the potential action of SRB in selenium transformations is presented.

Bioenergetics of the alkaliphilic sulfate-reducing bacterium Desulfonatronovibrio hydrogenovorans

Energy metabolism of the alkaliphilic sulfate-reducing bacterium Desulfonatronovibrio hydrogenovorans strain Z-7935 was investigated in continuous culture and in physiological experiments on washed cells. When grown in chemostats with H2 as electron donor, the cells had extrapolated growth yields [Y(max), g dry cell mass (mol electron acceptor)(-1)] of 5.5 with sulfate and 12.8 with thiosulfate. The maintenance energy coefficients were 1.9 and 1.3 mmol (g dry mass)(-1) x h(-1), and the minimum doubling times were 27 and 20 h with sulfate and thiosulfate, respectively. Cell suspensions reduced sulfate, thiosulfate, sulfite, elemental sulfur and molecular oxygen in the presence of H2. In the absence of H2, sulfite, thiosulfate and sulfur were dismutated to sulfide and sulfate. Sulfate and sulfite were only reduced in the presence of sodium ions, whereas sulfur was reduced also in the absence of Na+. Plasmolysis experiments showed that sulfate entered the cells via an electroneutral symport with Na+ ions. The presence of an electrogenic Na+-H+ antiporter was demonstrated in experiments applying monensin (an artificial electroneutral Na+-H+ antiporter) and propylbenzylylcholine mustard.HCl (a specific inhibitor of Na+-H+ antiporters). Sulfate reduction was sensitive to uncouplers (protonophores), whereas sulfite reduction was not affected. Changes in pH upon lysis of washed cells with butanol indicated that the intracellular pH was lower than the optimum pH for growth (pH 9.5). Pulses of NaCl (0.52 M) to cells incubated in the absence of Na+ did not result in ATP formation, whereas HCl pulses (shifting the pH from 9.2 to 7.0) did. Small oxygen pulses, which were reduced within a few seconds, caused a transient alkalinization. The results of preliminary experiments with chemiosmotic inhibitors provided further evidence that the alkalinization was caused by sodium--proton antiport following a primary electron-transport-driven sodium ion translocation. It is concluded that energy conservation in D. hydrogenovorans depends on a proton-translocating ATPase, whereas electron transport appears to be coupled to sodium ion translocation.

Oxygen respiration by desulfovibrio species

Throughout the first 90 years after their discovery, sulfate-reducing bacteria were thought to be strict anaerobes. During the last 15 years, however, it has turned out that they have manifold properties that enable them to cope with oxygen. Sulfate-reducing bacteria not only survive oxygen exposure for at least days, but many of them even reduce oxygen to water. This process can be a true respiration process when it is coupled to energy conservation. Various oxygen-reducing systems are present in Desulfovibrio species. In Desulfovibrio vulgaris and Desulfovibrio desulfuricans, oxygen reduction was coupled to proton translocation and ATP conservation. In these species, the periplasmic fraction, which contains hydrogenase and cytochrome c3, was found to catalyze oxygen reduction with high rates. In Desulfovibrio gigas, a cytoplasmic rubredoxin oxidase was identified as an oxygen-reducing terminal oxidase. Generally, the same substrates as with sulfate are oxidized with oxygen. As additional electron donors, reduced sulfur compounds can be oxidized to sulfate. Sulfate-reducing bacteria are thus able to catalyze all reactions of a complete sulfur cycle. Despite a high respiration rate and energy coupling, aerobic growth of pure cultures is poor or absent. Instead, the respiration capacity appears to have a protective function. High numbers of sulfate-reducing bacteria are present in the oxic zones and near the oxic-anoxic boundaries of sediments and in stratified water bodies, microbial mats and termite guts. Community structure analyses and microbiological studies have shown that the populations in those zones are especially adapted to oxygen. How dissimilatory sulfate reduction can occur in the presence of oxygen is still enigmatic, because in pure culture oxygen blocks sulfate reduction. Behavioral responses to oxygen include aggregation, migration to anoxic zones, and aerotaxis. The latter leads to band formation in oxygen-containing zones at concentrations of </=20% air saturation.

Oxygen-dependent growth of the obligate anaerobe Desulfovibrio vulgaris Hildenborough

Desulfovibrio vulgaris Hildenborough, a sulfate-reducing bacterium classified as an obligate anaerobe, swam to a preferred oxygen concentration of 0.02 to 0.04% (0.24 to 0.48 microM), a level which also supported growth. Oxygen concentrations of 0.08% and higher arrested growth. We propose that in zones of transition from an oxic to an anoxic environment, D. vulgaris protects anoxic microenvironments from intrusion of oxygen.