Microbial sulphate reduction at a low pH

It is now well established that microbial sulphate-reduction can proceed in environments with a pH<5. This review summarizes existing reports on sulphate reduction at low pH and discusses possible pH effects on sulphate-reducing bacteria. Microbial sulphate reduction has been observed in acidic lakes, wetlands, mesocosms, acidic sulphate soils and bioreactors. Possible inhibitory factors include the metabolites H(2)S and organic acids, which can be toxic depending on pH. Metal sulphide precipitation and competition with other bacteria, namely iron-reducing bacteria, can inhibit sulphate reduction. Theoretical considerations show that normal sulphate reduction rates are too low to maintain a neutral micro niche in an acidic environment. The first acidotolerant sulphate-reducing bacteria have been isolated recently.     

Cadmium Resistance Mechanism in Acidophilic and Alkalophilic Bacterial Isolates and their Application in Bioremediation of Metal-Contaminated Soil

Acidophilic and alkalophilic cadmium-resistant bacterial strains were isolated from the metal-contaminated soil of Panki thermal power plant, Kanpur, India, and two strains, namely SB21 (acidophilic) and SB20 (alkalophilic), were selected as representative strains on the basis of their high resistance to cadmium as compared to other strains. The phylogenetic analysis of the strains using 16s rDNA sequence revealed the strains to be Pseudomonas putida and Comamonas species strain, respectively. Furthermore, to determine the mechanism involved in cadmium resistance, a czc gene was amplified from the strains and sequenced. Homology of the sequences of the two strains, when compared with the available database using a BLASTn search, showed that the 650bp amplicons possess a partial czcA gene sequence. Moreover, the mechanism was confirmed by the results of atomic absorption spectroscopy and transmission electron microscopy studies, which supported the efflux mechanism in the alkalophilic strain SB20, but the acidophilic strain SB21 showed intracellular and periplasmic accumulation of metal in the cells in spite of the presence of the czcA gene, indicating the presence of multiple mechanisms of metal resistance in strain SB21.

Further, the strains were characterized functionally for their bioremediation potential in cadmium-contaminated soil under acidic and alkaline conditions by performing an in-situ greenhouse experiment using mungbean plants. A marked increase in agronomical parameters was observed under acidic conditions in the presence of strain SB21. Moreover, the concentration of metal decreased in both plants and soil in the presence of these bioinoculants; however, strain SB21 was found to be a better decontaminator than SB20 and thus can further be employed in bioremediation applications.     

Sulfate reduction in methanogenic bioreactors

Abstract: In the anaerobic treatment of sulfate-containing wastewater, sulfate reduction interferes with methanogenesis. Both mutualistic and competitive interactions between sulfate-reducing bacteria and methanogenic bacteria have been observed. Sulfate reducers will compete with methanogens for the common substrates hydrogen, formate and acetate. In general, sulfate reducers have better growth kinetic properties than methanogens, but additional factors which may be of importance in the competition are adherence properties, mixed substrate utilization, affinity for sulfate of sulfate reducers, relative numbers of bacteria, and reactor conditions such as pH, temperature and sulfide concentration. Sulfate reducers also compete with syntrophic methanogenic consortia involved in the degradation of substrates like propionate and butyrate. In the absence of sulfate these methanogenic consortia are very important, but in the presence of sulfate they are thought to be easily outcompeted by sulfate reducers. However, at relatively low sulfate concentrations, syntrophic degradation of propionate and butyrate coupled to HZ removal via sulfate reduction rather than via methanogenesis may become important. A remarkable feature of some sulfate reducers is their ability to grow fermentatively or to grow in syntrophic association with methanogens in the absence of sulfate.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Sulfate reduction by a syntrophic propionate-oxidizing bacterium

The syntrophic propionate-oxidizing bacterium MPOB was able to grow in the absence of methanogens by coupling the oxidation of propionate to the reduction of sulfate. Growth on propionate plus sulfate was very slow (=0.024 day^–1). An average growth yield was found of 1.5 g (dry weight) per mol of propionate. MPOB grew even slower than other sulfate-reducing syntrophic propionate-oxidizing bacteria. The growth rates and yields of strict sulfate-reducing bacteria (Desulfobulbus sp.) grown on propionate plus sulfate are considerably higher.     

Sulfate reduction and sediment metabolism in Tomales Bay,California

Sulfate reduction rates (SRR) in subtidal sediments of Tomales Bay, California, were variable by sediment type, season and depth. Higher rates were measured in near-surface muds during summer (up to 45 nmol cm^-3 h^-1), with lower rates in sandy sediments, in winter and deeper in the sediment. Calculations of annual, average SRR throughout the upper 20 cm of muddy subtidal sediments (about 30 mmol S m^-2 d^-1) were much larger than previously reported net estimates of SRR derived from both benthic alkalinity flux measurements and bay wide, budget stoichiometry (3.5 and 2.6 mmol m^-2 d^-1, respectively), indicating that most reduced sulfur in these upper, well-mixed sediments is re-oxidized. A portion of the net alkalinity flux across the sediment surface may be derived from sulfate reduction in deeper sediments, estimated from sulfate depletion profiles at 1.5 mmol m^-2 d^-1. A small net flux of CO₂ measured in benthic chambers despite a large SRR suggests that sediment sinks for CO₂ must also exist (e.g., benthic microalgae).     

Seasonality in coastal benthic ecosystems

For historical reasons, knowledge about seasonality in the dynamics of marine benthic suspension feeders from temperate areas comes mainly from studies of cold temperate seas. Recent surveys of Mediterranean taxa show different patterns from those observed in cold temperate seas, which are characterized by winter dormancy. In the Mediterranean, summer dormancy predominates among taxa and appears to be related to energetic constraints. Temperature and food availability are crucial to the dynamics of benthic suspension feeders. However, because these factors tend to be positively correlated in cold temperate seas, it is difficult to distinguish between their effects. Such correlation does not occur in Mediterranean ecosystems. The contrast between recent studies in the Mediterranean and in other areas can help to disentangle confounded environmental controls.     

Microbial reduction of Cr(VI) in the presence of chromate conversion coating constituents

The reduction of Cr(VI) by the metal-reducing bacterium Shewanella oneidensis MR-1 was evaluated, to determine the potential for exploiting Cr(VI) bioreduction as a means of treating chromate conversion coating (CCC) waste streams. Inclusion of Cr(VI) at concentrations ≥1 mM inhibited aerobic growth of S. oneidensis, but that organism was able to reduce Cr(VI) at a concentration of up to 1 mM under anaerobic, nongrowth conditions. S. oneidensis reduced Cr(VI) in the presence of common CCC constituents, with the exception of ferricyanide, when these CCC constituents were included at concentrations typical of CCC waste streams. Ferricyanide inhibited neither aerobic growth nor metabolism under aerobic, nitrate- or iron-reducing conditions, suggesting that the ferricyanide-depended inhibition of Cr(VI) reduction is not due to broad metabolic inhibition, but is specific to Cr(VI) reduction. Results indicate that under some conditions, the activities of metal-reducing bacteria, such as S. oneidensis, could be exploited for the removal of Cr(VI) from CCC waste streams under appropriate conditions.     

Sulfate reduction in fine-grained sediments in the Eastern Scheldt,southwest Netherlands

Sulfate reduction and sulfide accumulation were examined in fine-grained sediments from rapidly accreting abandoned channels and mussel culture areas in the Eastern Scheldt, which covered 4 and 5% of the total surface area, respectively.Reduction rates were measured in batch experiments in which the SO₄ ^2– depletion was measured during anoxic incubation. The reduction rates in summer varied between 14–68 mmol SO₄ ^2– m^–2 day^–1 and were related to the sedimentation rate. In the most rapidly accreting channels, SO₄ ^2– was exhausted below 15–50 cm and methanogenesis became the terminal process of organic carbon oxidationOne-dimensional modelling of sulfate profiles in mussel banks indicated that the subsurface influx of SO₄ ^2– was almost of the same order as the diffusive flux at the sediment-seawater interface, during the initial stages of the mussel bank accretion. The energy dissipation of waves and tidal currents on the mussel bank surface increased the apparent sediment diffusivity up to 3-fold, especially in the winterThe results indicate that acid volatile sulfide (AVS) was the major, in-situ reduced, sulfur compound in the sediment. The sulfidation of easily extractable iron was nearly complete. Pyrite concentrations (40–80 M S cm^–3) were as high as the AVS concentrations, but there was apparently no in-situ transformation of AVS into pyrite. The detrital pyrite originated from eroding marine sediments elsewhere     

Nitrogen cycling in coastal marine ecosystems

It is generally considered that nitrogen availability is one of the major factors regulating primary production in temperate coastal marine environments. Coastal regions often receive large anthropogenic inputs of nitrogen that cause eutrophication. The impact of these nitrogen additions has a profound effect in estuaries and coastal lagoons where water exchange is limited. Such increased nutrient loading promotes the growth of phytoplankton and fast growing pelagic macroalgae while rooted plants (sea-grasses) and benthic are suppressed due to reduced light availability. This shift from benthic to pelagic primary production introduces large diurnal variations in oxygen concentrations in the water column. In addition oxygen consumption in the surface sediments increases due to the deposition of readily degradable biomass. In this review the physico-chemical and biological factors regulating nitrogen cycling in coastal marine ecosystems are considered in relation to developing effective management programmes to rehabilitate seagrass communities in lagoons currently dominated by pelagic macroalgae and/or cyanobacteria.     

Bivalve carrying capacity in coastal ecosystems

The carrying capacity of suspension feeding bivalvesin 11 coastal and estuarine ecosystems is examined. Bivalve carrying capacity is defined in terms of watermass residence time, primary production time andbivalve clearance time. Turnover times for the 11ecosystems are compared both two and threedimensionally. Fast systems, e.g., Sylt and NorthInlet, have turnover times of days or less, while,slow systems, e.g., Delaware Bay, have turnover timesof months and years. Some systems,Marennes-Oléron, South San Francisco Bay and NorthInlet, require a net influx of phytoplankton in orderto support their bivalve populations. Three systems,Carlingford Lough, Chesapeake Bay and Delaware Bay,have very long bivalve clearance times due to small orreduced bivalve filter feeder populations. Carlingford Lough stands out because it is a naturallyplanktonic system now being converted to bivalveculture with an adherently stronger benthic-pelagiccoupling. Existing models of bivalve carrying capacity arereviewed. The Herman model is utilized as anappropriate ecosystem level model to examine carryingcapacity because it includes the three major turnovertime elements of water mass residence time, primaryproduction time and bivalve filter feeder clearancetime. The graphical analysis suggests that massive andsuccessful bivalve filter feeder populations are foundin systems with relatively short residence times(<40 days) and short primary production times (<4days) in order to sustain a high bivalve biomass withits associated rapid clearance times. Outliersystems are constrained by long water mass residencetimes, extended primary production times, and longclearance times. This revised version was published online in August 2006 with corrections to the Cover Date.     

Progress in management of coastal ecosystems

The process of coastal zone management in the United States has progressed to the point that it is now possible to embrace whole coastal ecosystems in management programs. The structural and dynamical features of the ecosystems have to be known before management can succeed. Furthermore, it is necessary to disaggregate the systems into subsystems. The following six subsystems have been used as the best compromise between scientific and administrative needs: the watershed terrain, the land drainage system, the coastal basin, the basin floor, the coastal waters, and the ocean. The Apalachicola National Estuarine Sanctuary in Florida is used as a case history of a managed ecosystem.     

Sulfate reduction from phosphogypsum using a mixed culture of sulfate-reducing bacteria

Phosphogypsum (CaSO₄), a primary by-product of phosphoric acid production, is accumulated in large stockpiles and occupies vast areas of land. It poses a severe threat to the quality of water and land in countries producing phosphoric acid. In this study, the potential of sulfate-reducing bacteria for biodegradation of this sulfur-rich industrial solid waste was assessed. The effect of phosphogypsum concentration, carbon and nitrogen sources, temperature, pH and stirring on the growth of sulfate-reducing bacteria was investigated. Growth of sulfate-reducing bacteria was monitored by measuring sulfide production. Phosphogypsum was shown to be a good source of sulfate, albeit that the addition of organic carbon was necessary for bacterial growth. Biogenic sulfide production occurred with phosphogypsum up to a concentration of 40 g L⁻¹, above which no growth of sulfate-reducing bacteria was observed. Optimal growth was obtained at 10 g L⁻¹ phosphogypsum. Both the gas mixture H₂/CO₂ and lactate supported high amounts of H₂S formation (19 and 11 mM, respectively). The best source of nitrogen for sulfate-reducing bacteria was yeast extract, followed by ammonium chloride. The presence of nitrate had an inhibitory effect on the process of sulfate reduction. Stirring the culture at 150 rpm slightly stimulated H₂S formation, probably by improving sulfate solubility.     

Sulfate reduction in Catharanthus roseus (L.)

Cell homogenates from Catharanthus roseus (L.) G. Don. grown S-autotrophically on sulfate in the dark are capable of reducing adenylysulfate (APS) to cysteine. This reduction required a particulate protein fraction from the cell extract and reduced ferredoxin as the electron donor. The protein fraction (MW 700,000±50,000) was found to contain Fd:NADP⁺ reductase, glutathione reductase and an unspecific dithiol reductase, and APS-sulfotransferase and thiosulfonate reductase activity. Resolution into these individual enzyme activities led to a non-restorable loss of the APS reducing activity. It was observed that a slow gradual decay of the APS reducing activity was accompanied by a likewise slow generation of a ferredoxin-dependent sulfite reductase.Enzymes and abbreviations APS Adenosine 5-phosphosulfate - APS-kinase E.C.2.7.1.25 - ATP-sulfurylase E.C.2.7.7.4 - Fd ferredoxin - Fd-NADP⁺-reductase E.C.1.6.7.1. - Glutathione reductase E.C.1.6.4.2. - G6P Glucose 6-phosphate - G6PDH glucose 6-phosphate dehydrogenase, E.C.1.1.49 - GSSG oxidized glutathione - GSSO₃H S-sulfoglutathione - MVH reduced methylviologen - OASS O-acetylserine sulfhydrylase-E.C. 4.2.99.8 - Sulfite reductase E.C.1.8.1.2     

Sulfate reduction at pH 4.0 for treatment of process and wastewaters

Acidic industrial process and wastewaters often contain high sulfate and metal concentrations and their direct biological treatment is thus far not possible as biological processes at pH < 5 have been neglected. Sulfate‐reducing bacteria convert sulfate to sulfide that can subsequently be used to recover metals as metal‐sulfides precipitate. This study reports on high‐rate sulfate reduction with a mixed microbial community at pH 4.0 and 4.5 with hydrogen and/or formate as electron donors. The maximum sulfate reducing activity at pH 4.0 was sustained for over 40 days with a specific activity 500‐fold greater than previously reported values: 151 mmol sulfate reduced/L reactor liquid per day with a maximum specific activity of 84 mmol sulfate per gram of volatile suspended solids per day. The biomass yield gradually decreased from 38 to 0.4 g volatile suspended solids per kilogram of sulfate when decreasing the reactor pH from pH 6 to 4. The microorganisms had a high maintenance requirement probably due maintaining pH homeostasis and the toxicity of sulfide at low pH. The microbial community diversity in the pH 4.0 membrane bioreactor decreased over time, while the diversity of the sulfate reducing community increased. Thus, a specialized microbial community containing a lower proportion of microorganisms capable of activity at pH 4 developed in the reactor compared with those present at the start of the experiment. The 16S rRNA genes identified from the pH 4.0 grown mixed culture were most similar to those of Desulfovibrio species and Desulfosporosinus sp. M1. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Potential rates and pathways of microbial nitrate reduction in coastal sediments

Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal sediments. Potential rates of nitrate reduction were measured on undisturbed sediment slices from two eutrophic coastal environments using flow-through reactors (FTR). Maximum potential nitrate reduction rates ranged over an order of magnitude, with values of up to 933 nmol cm(-3) h(-1), whereas affinity constants for NO(3) (-) fell mostly between 200 and 600 microM. Homogenized sediment slurries systematically yielded higher rates of nitrate reduction than the FTR experiments. Dentrification was the major nitrate removal pathway in the sediments, although excess ammonium production indicated a contribution of dissimilatory nitrate reduction to ammonium under nitrate-limiting conditions.