Anaerobic oxidation of acetylene by estuarine sediments and enrichment cultures

Acetylene disappeared from the gas phase of anaerobically incubated estuarine sediment slurries, and loss was accompanied by increased levels of carbon dioxide. Acetylene loss was inhibited by chloramphenicol, air, and autoclaving. Addition of C(2)H(2) to slurries resulted in the formation of CO(2) and the transient appearance of C-soluble intermediates, of which acetate was a major component. Acetylene oxidation stimulated sulfate reduction; however, sulfate reduction was not required for the loss of C(2)H(2) to occur. Enrichment cultures were obtained which grew anaerobically at the expense of C(2)H(2).

Hydrogen metabolism by decomposing cyanobacterial aggregates in big soda lake, nevada

Hydrogen production by incubated cyanobacterial epiphytes occurred only in the dark, was stimulated by C(2)H(2), and was inhibited by O(2). Addition of NO(3) inhibited dark, anaerobic H(2) production, whereas the addition of NH(4) inhibited N(2) fixation (C(2)H(2) reduction) but not dark H(2) production. Aerobically incubated cyanobacterial aggregates consumed H(2), but light-incubated rates (3.6 mumol of H(2) g h) were statistically equivalent to dark uptake rates (4.8 mumol of H(2) g h), which were statistically equivalent to dark, anaerobic production rates (2.5 to 10 mumol of H(2) g h). Production rates of H(2) were fourfold higher for aggregates in a more advanced stage of decomposition. Enrichment cultures of H(2)-producing fermentative bacteria were recovered from freshly harvested, H(2)-producing cyanobacterial aggregates. Hydrogen production in these cyanobacterial communities appears to be caused by the resident bacterial flora and not by the cyanobacteria. In situ areal estimates of dark H(2) production by submerged epiphytes (6.8 mumol of H(2) m h) were much lower than rates of light-driven N(2) fixation by the epiphytic cyanobacteria (310 mumol of C(2)H(4) m h).

Bacterial Dissimilatory Reduction of Arsenic(V) to Arsenic(III) in Anoxic Sediments

Incubation of anoxic salt marsh sediment slurries with 10 mM As(V) resulted in the disappearance over time of the As(V) in conjunction with its recovery as As(III). No As(V) reduction to As(III) occurred in heat-sterilized or formalin-killed controls or in live sediments incubated in air. The rate of As(V) reduction in slurries was enhanced by addition of the electron donor lactate, H(inf2), or glucose, whereas the respiratory inhibitor/uncoupler dinitrophenol, rotenone, or 2-heptyl-4-hydroxyquinoline N-oxide blocked As(V) reduction. As(V) reduction was also inhibited by tungstate but not by molybdate, sulfate, or phosphate. Nitrate inhibited As(V) reduction by its action as a preferred respiratory electron acceptor rather than as a structural analog of As(V). Nitrate-respiring sediments could reduce As(V) to As(III) once all the nitrate was removed. Chloramphenicol blocked the reduction of As(V) to As(III) in nitrate-respiring sediments, suggesting that nitrate and arsenate were reduced by separate enzyme systems. Oxidation of [2-(sup14)C]acetate to (sup14)CO(inf2) by salt marsh and freshwater sediments was coupled to As(V). Collectively, these results show that reduction of As(V) in sediments proceeds by a dissimilatory process. Bacterial sulfate reduction was completely inhibited by As(V) as well as by As(III).

Selenate reduction to elemental selenium by anaerobic bacteria in sediments and culture: biogeochemical significance of a novel, sulfate-independent respiration

Interstitial water profiles of SeO(4), SeO(3), SO(4), and Cl in anoxic sediments indicated removal of the seleno-oxyanions by a near-surface process unrelated to sulfate reduction. In sediment slurry experiments, a complete reductive removal of SeO(4) occurred under anaerobic conditions, was more rapid with H(2) or acetate, and was inhibited by O(2), NO(3), MnO(2), or autoclaving but not by SO(4) or FeOOH. Oxidation of acetate in sediments could be coupled to selenate but not to molybdate. Reduction of selenate to elemental selenium was determined to be the mechanism for loss from solution. Selenate reduction was inhibited by tungstate and chromate but not by molybdate. A small quantity of the elemental selenium precipitated into sediments from solution could be resolublized by oxidation with either nitrate or FeOOH, but not with MnO(2). A bacterium isolated from estuarine sediments demonstrated selenate-dependent growth on acetate, forming elemental selenium and carbon dioxide as respiratory end products. These results indicate that dissimilatory selenate reduction to elemental selenium is the major sink for selenium oxyanions in anoxic sediments. In addition, they suggest application as a treatment process for removing selenium oxyanions from wastewaters and also offer an explanation for the presence of selenite in oxic waters.

Anaerobic oxalate degradation: widespread natural occurrence in aquatic sediments

Significant concentrations of oxalate (dissolved plus particulate) were present in sediments taken from a diversity of aquatic environments, ranging from 0.1 to 0.7 mmol/liter of sediment. These included pelagic and littoral sediments from two freshwater lakes (Searsville Lake, Calif., and Lake Tahoe, Calif.), a hypersaline, meromictic, alkaline lake (Big Soda Lake, Nev.), and a South San Francisco Bay mud flat and salt marsh. The oxalate concentration of several plant species which are potential detrital inputs to these aquatic sediments ranged from 0.1 to 5.0% (wt/wt). In experiments with litter bags, the oxalate content of Myriophyllum sp. samples buried in freshwater littoral sediments decreased to 7% of the original value in 175 days. This suggests that plant detritus is a potential source of the oxalate within these sediments. [C]oxalic acid was anaerobically degraded to CO(2) in all sediment types tested, with higher rates evident in littoral sediments than in the pelagic sediments of the lakes studied. The turnover time of the added [C]oxalate was less than 1 day in Searsville Lake littoral sediments. The total sediment oxalate concentration did not vary significantly between littoral and pelagic sediments and therefore did not appear to be controlling the rate of oxalate degradation. However, depth profiles of [C]oxalate mineralization and dissolved oxalate concentration were closely correlated in freshwater littoral sediments; both were greatest in the surface sediments (0 to 5 cm) and decreased with depth. The dissolved oxalate concentration (9.1 mumol/liter of sediment) was only 3% of the total extractable oxalate (277 mumol/liter of sediment) at the sediment surface. These results suggest that anaerobic oxalate degradation is a widespread phenomenon in aquatic sediments and may be limited by the dissolved oxalate concentration within these sediments.

Methanogenesis and sulfate reduction: competitive and noncompetitive substrates in estuarine sediments

Sulfate ions did not inhibit methanogenesis in estuarine sediments supplemented with methanol, trimethylamine, or methionine. However, sulfate greatly retarded methanogenesis when hydrogen or acetate was the substrate. Sulfate reduction was stimulated by acetate, hydrogen, and acetate plus hydrogen, but not by methanol or trimethylamine. These results indicate that sulfate-reducing bacteria will outcompete methanogens for hydrogen, acetate, or both, but will not compete with methanogens for compounds like methanol, trimethylamine, or methionine, thereby allowing methanogenesis and sulfate reduction to operate simultaneously within anoxic, sulfate-containing sediments.

Nitrogen fixation dynamics of two diazotrophic communities in mono lake, california

Two types of diazotrophic microbial communities were found in the littoral zone of alkaline hypersaline Mono Lake, California. One consisted of anaerobic bacteria inhabiting the flocculent surface layers of sediments. Nitrogen fixation (acetylene reduction) by flocculent surface layers occurred under anaerobic conditions, was not stimulated by light or by additions of organic substrates, and was inhibited by O(2), nitrate, and ammonia. The second community consisted of a ball-shaped association of a filamentous chlorophyte (Ctenocladus circinnatus) with diazotrophic, nonheterocystous cyanobacteria, as well as anaerobic bacteria (Ctenocladus balls). Nitrogen fixation by Ctenocladus balls was usually, but not always, stimulated by light. Rates of anaerobic dark fixation equaled those in the light under air. Fixation in the light was stimulated by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea and by propanil [N-(3,4-dichlorophenyl)propanamide]. 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea-elicited nitrogenase activity was inhibited by ammonia (96%) and nitrate (65%). Fixation was greatest when Ctenocladus balls were incubated anaerobically in the light with sulfide. Dark anaerobic fixation was not stimulated by organic substrates in short-term (4-h) incubations, but was in long-term (67-h) ones. Areal estimates of benthic N(2) fixation were measured seasonally, using chambers. Highest rates ( approximately 29.3 mumol of C(2)H(4) m h) occurred under normal diel regimens of light and dark. These estimates indicate that benthic N(2) fixation has the potential to be a significant nitrogen source in Mono Lake.

Methylmercury decomposition in sediments and bacterial cultures: involvement of methanogens and sulfate reducers in oxidative demethylation

Demethylation of monomethylmercury in freshwater and estuarine sediments and in bacterial cultures was investigated with CH(3)HgI. Under anaerobiosis, results with inhibitors indicated partial involvement of both sulfate reducers and methanogens, the former dominating estuarine sediments, while both were active in freshwaters. Aerobes were the most significant demethylators in estuarine sediments, but were unimportant in freshwater sediments. Products of anaerobic demethylation were mainly CO(2) as well as lesser amounts of CH(4). Acetogenic activity resulted in fixation of some CO(2) produced from CH(3)HgI into acetate. Aerobic demethylation in estuarine sediments produced only CH(4), while aerobic demethylation in freshwater sediments produced small amounts of both CH(4) and CO(2). Two species of Desulfovibrio produced only traces of CH(4) from CH(3)HgI, while a culture of a methylotrophic methanogen formed traces of CO(2) and CH(4) when grown on trimethylamine in the presence of the CH(3)HgI. These results indicate that both aerobes and anaerobes demethylate mercury in sediments, but that either group may dominate in a particular sediment type. Aerobic demethylation in the estuarine sediments appeared to proceed by the previously characterized organomercurial-lyase pathway, because methane was the sole product. However, aerobic demethylation in freshwater sediments as well as anaerobic demethylation in all sediments studied produced primarily carbon dioxide. This indicates the presence of an oxidative pathway, possibly one in which methylmercury serves as an analog of one-carbon substrates.

Microbial formation of ethane in anoxic estuarine sediments

Estuarine sediment slurries produced methane and traces of ethane when incubated under hydrogen. Formation of methane occurred over a broad temperature range with an optimum above 65 degrees C. Ethane formation had a temperature optimum at 40 degrees C. Formation of these two gases was inhibited by air, autoclaving, incubation at 4 and 80 degrees C, and by the methanogenic inhibitor, 2-bromoethanesulfonic acid. Ethane production was stimulated by addition of ethylthioethanesulfonic acid, and production from ethylthioethanesulfonic acid was blocked by 2-bromoethanesulfonic acid. A highly purified enrichment culture of a methanogenic bacterium obtained from sediments produced traces of ethane from ethylthioethanesulfonic acid. These results indicate that the small quantities of ethane found in anaerobic sediments can be formed by certain methanogenic bacteria.

Growth of Strain SES-3 with Arsenate and Other Diverse Electron Acceptors

The selenate-respiring bacterial strain SES-3 was able to use a variety of inorganic electron acceptors to sustain growth. SES-3 grew with the reduction of arsenate to arsenite, Fe(III) to Fe(II), or thiosulfate to sulfide. It also grew in medium in which elemental sulfur, Mn(IV), nitrite, trimethylamine N-oxide, or fumarate was provided as an electron acceptor. Growth on oxygen was microaerophilic. There was no growth with arsenite or chromate. Washed suspensions of cells grown on selenate or nitrate had a constitutive ability to reduce arsenate but were unable to reduce arsenite. These results suggest that strain SES-3 may occupy a niche as an environmental opportunist by being able to take advantage of a diversity of electron acceptors.

Effects of imposed salinity gradients on dissimilatory arsenate reduction, sulfate reduction, and other microbial processes in sediments from two California soda lakes

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono Lake and Searles Lake sediments over a wide range of salt concentrations (25 to 346 g liter(-1)). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes amplified from As(V) reduction slurries revealed that distinct microbial populations grew at low (25 to 50 g liter(-1)), intermediate (100 to 200 g liter(-1)), and high (>300 g liter(-1)) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate reduction in Mono Lake slurries was undetectable at salt saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal) concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate reducers from that ecosystem.

Dissimilatory arsenate and sulfate reduction in sediments of two hypersaline, arsenic-rich soda lakes: Mono and Searles Lakes, California

A radioisotope method was devised to study bacterial respiratory reduction of arsenate in sediments. The following two arsenic-rich soda lakes in California were chosen for comparison on the basis of their different salinities: Mono Lake (approximately 90 g/liter) and Searles Lake (approximately 340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [73As]arsenate occurred at all depth intervals in the cores from Mono Lake (rate constant [k] = 0.103 to 0.04 h(-1)) and Searles Lake (k = 0.012 to 0.002 h(-1)), and the highest activities occurred in the top sections of each core. In contrast, [35S]sulfate reduction was measurable in Mono Lake (k = 7.6 x10(4) to 3.2 x 10(-6) h(-1)) but not in Searles Lake. Sediment DNA was extracted, PCR amplified, and separated by denaturing gradient gel electrophoresis (DGGE) to obtain phylogenetic markers (i.e., 16S rRNA genes) and a partial functional gene for dissimilatory arsenate reduction (arrA). The amplified arrA gene product showed a similar trend in both lakes; the signal was strongest in surface sediments and decreased to undetectable levels deeper in the sediments. More arrA gene signal was observed in Mono Lake and was detectable at a greater depth, despite the higher arsenate reduction activity observed in Searles Lake. A partial sequence (about 900 bp) was obtained for a clone (SLAS-3) that matched the dominant DGGE band found in deeper parts of the Searles Lake sample (below 3 cm), and this clone was found to be closely related to SLAS-1, a novel extremophilic arsenate respirer previously cultivated from Searles Lake.

Microbial transformation of elements: the case of arsenic and selenium

Microbial activity is responsible for the transformation of at least one third of the elements in the periodic table. These transformations are the result of assimilatory, dissimilatory, or detoxification processes and form the cornerstones of many biogeochemical cycles. Arsenic and selenium are two elements whose roles in microbial ecology have only recently been recognized. Known as "essential toxins", they are required in trace amounts for growth and metabolism but are toxic at elevated concentrations. Arsenic is used as an osmolite in some marine organisms while selenium is required as selenocysteine (i.e. the twenty-first amino acid) or as a ligand to metal in some enzymes (e.g. FeNiSe hydrogenase). Arsenic resistance involves a small-molecular-weight arsenate reductase (ArsC). The use of arsenic and selenium oxyanions for energy is widespread in prokaryotes with representative organisms from the Crenarchaeota, thermophilic bacteria, low and high G+C gram-positive bacteria, and Proteobacteria. Recent studies have shown that both elements are actively cycled and play a significant role in carbon mineralization in certain environments. The occurrence of multiple mechanisms involving different enzymes for arsenic and selenium transformation indicates several different evolutionary pathways (e.g. convergence and lateral gene transfer) and underscores the environmental significance and selective impact in microbial evolution of these two elements.

A review of bacterial methyl halide degradation: biochemistry, genetics and molecular ecology

Methyl halide-degrading bacteria are a diverse group of organisms that are found in both terrestrial and marine environments. They potentially play an important role in mitigating ozone depletion resulting from methyl chloride and methyl bromide emissions. The first step in the pathway(s) of methyl halide degradation involves a methyltransferase and, recently, the presence of this pathway has been studied in a number of bacteria. This paper reviews the biochemistry and genetics of methyl halide utilization in the aerobic bacteria Methylobacterium chloromethanicum CM4T, Hyphomicrobium chloromethanicum CM2T, Aminobacter strain IMB-1 and Aminobacter strain CC495. These bacteria are able to use methyl halides as a sole source of carbon and energy, are all members of the alpha-Proteobacteria and were isolated from a variety of polluted and pristine terrestrial environments. An understanding of the genetics of these bacteria identified a unique gene (cmuA) involved in the degradation of methyl halides, which codes for a protein (CmuA) with unique methyltransferase and corrinoid functions. This unique functional gene, cmuA, is being used to develop molecular ecology techniques to examine the diversity and distribution of methyl halide-utilizing bacteria in the environment and hopefully to understand their role in methyl halide degradation in different environments. These techniques will also enable the detection of potentially novel methyl halide-degrading bacteria.

Consumption of tropospheric levels of methyl bromide by C(1) compound-utilizing bacteria and comparison to saturation kinetics

Pure cultures of methylotrophs and methanotrophs are known to oxidize methyl bromide (MeBr); however, their ability to oxidize tropospheric concentrations (parts per trillion by volume [pptv]) has not been tested. Methylotrophs and methanotrophs were able to consume MeBr provided at levels that mimicked the tropospheric mixing ratio of MeBr (12 pptv) at equilibrium with surface waters ( approximately 2 pM). Kinetic investigations using picomolar concentrations of MeBr in a continuously stirred tank reactor (CSTR) were performed using strain IMB-1 and Leisingeria methylohalidivorans strain MB2(T) - terrestrial and marine methylotrophs capable of halorespiration. First-order uptake of MeBr with no indication of threshold was observed for both strains. Strain MB2(T) displayed saturation kinetics in batch experiments using micromolar MeBr concentrations, with an apparent K(s) of 2.4 microM MeBr and a V(max) of 1.6 nmol h(-1) (10(6) cells)(-1). Apparent first-order degradation rate constants measured with the CSTR were consistent with kinetic parameters determined in batch experiments, which used 35- to 1 x 10(7)-fold-higher MeBr concentrations. Ruegeria algicola (a phylogenetic relative of strain MB2(T)), the common heterotrophs Escherichia coli and Bacillus pumilus, and a toluene oxidizer, Pseudomonas mendocina KR1, were also tested. These bacteria showed no significant consumption of 12 pptv MeBr; thus, the ability to consume ambient mixing ratios of MeBr was limited to C(1) compound-oxidizing bacteria in this study. Aerobic C(1) bacteria may provide model organisms for the biological oxidation of tropospheric MeBr in soils and waters.

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria

The largest biological fractionations of stable carbon isotopes observed in nature occur during production of methane by methanogenic archaea. These fractionations result in substantial (as much as approximately 70 per thousand) shifts in delta(13)C relative to the initial substrate. We now report that a stable carbon isotopic fractionation of comparable magnitude (up to 70 per thousand) occurs during oxidation of methyl halides by methylotrophic bacteria. We have demonstrated biological fractionation with whole cells of three methylotrophs (strain IMB-1, strain CC495, and strain MB2) and, to a lesser extent, with the purified cobalamin-dependent methyltransferase enzyme obtained from strain CC495. Thus, the genetic similarities recently reported between methylotrophs, and methanogens with respect to their pathways for C(1)-unit metabolism are also reflected in the carbon isotopic fractionations achieved by these organisms. We found that only part of the observed fractionation of carbon isotopes could be accounted for by the activity of the corrinoid methyltransferase enzyme, suggesting fractionation by enzymes further along the degradation pathway. These observations are of potential biogeochemical significance in the application of stable carbon isotope ratios to constrain the tropospheric budgets for the ozone-depleting halocarbons, methyl bromide and methyl chloride.

Identification of methyl halide-utilizing genes in the methyl bromide-utilizing bacterial strain IMB-1 suggests a high degree of conservation of methyl halide-specific genes in gram-negative bacteria

Strain IMB-1, an aerobic methylotrophic member of the alpha subgroup of the Proteobacteria, can grow with methyl bromide as a sole carbon and energy source. A single cmu gene cluster was identified in IMB-1 that contained six open reading frames: cmuC, cmuA, orf146, paaE, hutI, and partial metF. CmuA from IMB-1 has high sequence homology to the methyltransferase CmuA from Methylobacterium chloromethanicum and Hyphomicrobium chloromethanicum and contains a C-terminal corrinoid-binding motif and an N-terminal methyltransferase motif. However, cmuB, identified in M. chloromethanicum and H. chloromethanicum, was not detected in IMB-1.

Selenihalanaerobacter shriftii gen. nov., sp. nov., a halophilic anaerobe from Dead Sea sediments that respires selenate

We isolated an obligately anaerobic halophilic bacterium from the Dead Sea that grew by respiration of selenate. The isolate, designated strain DSSe-1, was a gram-negative, non-motile rod. It oxidized glycerol or glucose to acetate + CO2 with concomitant reduction of selenate to selenite plus elemental selenium. Other electron acceptors that supported anaerobic growth on glycerol were nitrate and trimethylamine-N-oxide; nitrite, arsenate, fumarate, dimethylsulfoxide, thiosulfate, elemental sulfur, sulfite or sulfate could not serve as electron acceptors. Growth on glycerol in the presence of nitrate occurred over a salinity range from 100 to 240 g/l, with an optimum at 210 g/l. Analysis of the 16S rRNA gene sequence suggests that strain DSSe-1 belongs to the order Halanaerobiales, an order of halophilic anaerobes with a fermentative or homoacetogenic metabolism, in which anaerobic respiratory metabolism has never been documented. The highest 16S rRNA sequence similarity (90%) was found with Acetohalobium arabaticum (X89077). On the basis of physiological properties as well as the relatively low homology of 16S rRNA from strain DSSe-1 with known genera, classification in a new genus within the order Halanaerobiales, family Halobacteroidaceae is warranted. We propose the name Selenihalanaerobacter shriftii. Type strain is strain DSSe-1 (ATCC accession number BAA-73).

Distribution and diversity of archaea corresponding to the limnological cycle of a hypersaline stratified lake (Solar lake, Sinai, Egypt)

The vertical and seasonal distribution and diversity of archaeal sequences was investigated in a hypersaline, stratified, monomictic lake, Solar Lake, Sinai, Egypt, during the limnological development of stratification and mixing. Archaeal sequences were studied via phylogenetic analysis of 16S rDNA sequences as well as denaturing gradient gel electrophoresis analysis. The 165 clones studied were grouped into four phylogenetically different clusters. Most of the clones isolated from both the aerobic epilimnion and the sulfide-rich hypolimnion were defined as cluster I, belonging to the Halobacteriaceae family. The three additional clusters were all isolated from the anaerobic hypolimnion. Cluster II is phylogenetically located between the genera Methanobacterium and Methanococcus. Clusters III and IV relate to two previously documented groups of uncultured euryarchaeota, remotely related to the genus Thermoplasma. No crenarchaeota were found in the water column of the Solar Lake. The archaeal community in the Solar Lake under both stratified and mixed conditions was dominated by halobacteria in salinities higher than 10%. During stratification, additional clusters, some of which may possibly relate to uncultured halophilic methanogens, were found in the sulfide- and methane-rich hypolimnion.

Oxidation of methyl halides by the facultative methylotroph strain IMB-1

Washed cell suspensions of the facultative methylotroph strain IMB-1 grown on methyl bromide (MeBr) were able to consume methyl chloride (MeCl) and methyl iodide (MeI) as well as MeBr. Consumption of >100 microM MeBr by cells grown on glucose, acetate, or monomethylamine required induction. Induction was inhibited by chloramphenicol. However, cells had a constitutive ability to consume low concentrations (<20 nM) of MeBr. Glucose-grown cells were able to readily oxidize [(14)C]formaldehyde to (14)CO(2) but had only a small capacity for oxidation of [(14)C]methanol. Preincubation of cells with MeBr did not affect either activity, but MeBr-induced cells had a greater capacity for [(14)C]MeBr oxidation than did cells without preincubation. Consumption of MeBr was inhibited by MeI, and MeCl consumption was inhibited by MeBr. No inhibition of MeBr consumption occurred with methyl fluoride, propyl iodide, dibromomethane, dichloromethane, or difluoromethane, and in addition cells did not oxidize any of these compounds. Cells displayed Michaelis-Menten kinetics for the various methyl halides, with apparent K(s) values of 190, 280, and 6,100 nM for MeBr, MeI, and MeCl, respectively. These results suggest the presence of a single oxidation enzyme system specific for methyl halides (other than methyl fluoride) which runs through formaldehyde to CO(2). The ease of induction of methyl halide oxidation in strain IMB-1 should facilitate its mass culture for the purpose of reducing MeBr emissions to the atmosphere from fumigated soils.

Bacterial respiration of arsenic and selenium

Oxyanions of arsenic and selenium can be used in microbial anaerobic respiration as terminal electron acceptors. The detection of arsenate and selenate respiring bacteria in numerous pristine and contaminated environments and their rapid appearance in enrichment culture suggest that they are widespread and metabolically active in nature. Although the bacterial species that have been isolated and characterized are still few in number, they are scattered throughout the bacterial domain and include Gram-positive bacteria, beta, gamma and epsilon Proteobacteria and the sole member of a deeply branching lineage of the bacteria, Chrysiogenes arsenatus. The oxidation of a number of organic substrates (i.e. acetate, lactate, pyruvate, glycerol, ethanol) or hydrogen can be coupled to the reduction of arsenate and selenate, but the actual donor used varies from species to species. Both periplasmic and membrane-associated arsenate and selenate reductases have been characterized. Although the number of subunits and molecular masses differs, they all contain molybdenum. The extent of the environmental impact on the transformation and mobilization of arsenic and selenium by microbial dissimilatory processes is only now being fully appreciated.

Simultaneous reduction of nitrate and selenate by cell suspensions of selenium-respiring bacteria

Washed-cell suspensions of Sulfurospirillum barnesii reduced selenate [Se(VI)] when cells were cultured with nitrate, thiosulfate, arsenate, or fumarate as the electron acceptor. When the concentration of the electron donor was limiting, Se(VI) reduction in whole cells was approximately fourfold greater in Se(VI)-grown cells than was observed in nitrate-grown cells; correspondingly, nitrate reduction was approximately 11-fold higher in nitrate-grown cells than in Se(VI)-grown cells. However, a simultaneous reduction of nitrate and Se(VI) was observed in both cases. At nonlimiting electron donor concentrations, nitrate-grown cells suspended with equimolar nitrate and selenate achieved a complete reductive removal of nitrogen and selenium oxyanions, with the bulk of nitrate reduction preceding that of selenate reduction. Chloramphenicol did not inhibit these reductions. The Se(VI)-respiring haloalkaliphile Bacillus arsenicoselenatis gave similar results, but its Se(VI) reductase was not constitutive in nitrate-grown cells. No reduction of Se(VI) was noted for Bacillus selenitireducens, which respires selenite. The results of kinetic experiments with cell membrane preparations of S. barnesii suggest the presence of constitutive selenate and nitrate reduction, as well as an inducible, high-affinity nitrate reductase in nitrate-grown cells which also has a low affinity for selenate. The simultaneous reduction of micromolar Se(VI) in the presence of millimolar nitrate indicates that these organisms may have a functional use in bioremediating nitrate-rich, seleniferous agricultural wastewaters. Results with (75)Se-selenate tracer show that these organisms can lower ambient Se(VI) concentrations to levels in compliance with new regulations proposed for release of selenium oxyanions into the environment.

Sulfurospirillum barnesii sp. nov. and Sulfurospirillum arsenophilum sp. nov., new members of the Sulfurospirillum clade of the epsilon Proteobacteria

Two strains of dissimilatory arsenate-reducing vibrio-shaped bacteria are assigned to the genus Sulfurospirillum. These two new species, Sulfurospirillum barnesii strain SES-3T and Sulfurospirillum arsenophilum strain MIT-13T, in addition to Sulfurospirillum sp. SM-5, two strains of Sulfurospirillum deleyianum, and Sulfurospirillum arcachonense, form a distinct clade within the epsilon subclass of the Proteobacteria based on 16S rRNA analysis.

Bacillus arsenicoselenatis, sp. nov., and Bacillus selenitireducens, sp. nov.: two haloalkaliphiles from Mono Lake, California that respire oxyanions of selenium and arsenic

Two gram-positive anaerobic bacteria (strains E1H and MLS10) were isolated from the anoxic muds of Mono Lake, California, an alkaline, hypersaline, arsenic-rich water body. Both grew by dissimilatory reduction of As(V) to As(III) with the concomitant oxidation of lactate to acetate plus CO2. Bacillus arsenicoselenatis (strain E1H) is a spore-forming rod that also grew by dissimilatory reduction of Se(VI) to Se(IV). Bacillus selenitireducens (strain MLS10) is a short, non-spore-forming rod that grew by dissimilatory reduction of Se(IV) to Se(0). When the two isolates were cocultured, a complete reduction of Se(VI) to Se(0) was achieved. Both isolates are alkaliphiles and had optimal specific growth rates in the pH range of 8.5-10. Strain E1H had a salinity optimum at 60 g l-1 NaCl, while strain MLS10 had optimal growth at lower salinities (24-60 g l-1 NaCl). Both strains have limited abilities to grow with electron donors and acceptors other than those given above. Strain MLS10 demonstrated weak growth as a microaerophile and was also capable of fermentative growth on glucose, while strain E1H is a strict anaerobe. Comparative 16S rRNA gene sequence analysis placed the two isolates with other Bacillus spp. in the low G+C gram-positive group of bacteria.

Strain IMB-1, a novel bacterium for the removal of methyl bromide in fumigated agricultural soils

A facultatively methylotrophic bacterium, strain IMB-1, that has been isolated from agricultural soil grows on methyl bromide (MeBr), methyl iodide, methyl chloride, and methylated amines, as well as on glucose, pyruvate, or acetate. Phylogenetic analysis of its 16S rRNA gene sequence indicates that strain IMB-1 classes in the alpha subgroup of the class Proteobacteria and is closely related to members of the genus Rhizobium. The ability of strain IMB-1 to oxidize MeBr to CO2 is constitutive in cells regardless of the growth substrate. Addition of cell suspensions of strain IMB-1 to soils greatly accelerates the oxidation of MeBr, as does pretreatment of soils with low concentrations of methyl iodide. These results suggest that soil treatment strategies can be devised whereby bacteria can effectively consume MeBr during field fumigations, which would diminish or eliminate the outward flux of MeBr to the atmosphere.

Bacterial oxidation of methyl bromide in fumigated agricultural soils

The oxidation of [(sup14)C]methyl bromide ([(sup14)C]MeBr) to (sup14)CO(inf2) was measured in field experiments with soils collected from two strawberry plots fumigated with mixtures of MeBr and chloropicrin (CCl(inf3)NO(inf2)). Although these fumigants are considered potent biocides, we found that the highest rates of MeBr oxidation occurred 1 to 2 days after injection when the fields were tarped, rather than before or several days after injection. No oxidation of MeBr occurred in heat-killed soils, indicating that microbes were the causative agents of the oxidation. Degradation of MeBr by chemical and/or biological processes accounted for 20 to 50% of the loss of MeBr during fumigation, with evasion to the atmosphere inferred to comprise the remainder. In laboratory incubations, complete removal of [(sup14)C]MeBr occurred within a few days, with 47 to 67% of the added MeBr oxidized to (sup14)CO(inf2) and the remainder of counts associated with the solid phase. Chloropicrin inhibited the oxidation of MeBr, implying that use of this substance constrains the extent of microbial degradation of MeBr during fumigation. Oxidation was by direct bacterial attack of MeBr and not of methanol, a product of the chemical hydrolysis of MeBr. Neither nitrifying nor methane-oxidizing bacteria were sufficiently active in these soils to account for the observed oxidation of MeBr, nor could the microbial degradation of MeBr be linked to cooxidation with exogenously supplied electron donors. However, repeated addition of MeBr to live soils resulted in higher rates of its removal, suggesting that soil bacteria used MeBr as an electron donor for growth. To support this interpretation, we isolated a gram-negative, aerobic bacterium from these soils which grew with MeBr as a sole source of carbon and energy.

Inhibition of Methane Oxidation by Methylococcus capsulatus with Hydrochlorofluorocarbons and Fluorinated Methanes

The inhibition of methane oxidation by cell suspensions of Methylococcus capsulatus (Bath) exposed to hydrochlorofluorocarbon 21 (HCFC-21; difluorochloromethane [CHF(inf2)Cl]), HCFC-22 (fluorodichloromethane [CHFCl(inf2)]), and various fluorinated methanes was investigated. HCFC-21 inhibited methane oxidation to a greater extent than HCFC-22, for both the particulate and soluble methane monooxygenases. Among the fluorinated methanes, both methyl fluoride (CH(inf3)F) and difluoromethane (CH(inf2)F(inf2)) were inhibitory while fluoroform (CHF(inf3)) and carbon tetrafluoride (CF(inf4)) were not. The inhibition of methane oxidation by HCFC-21 and HCFC-22 was irreversible, while that by methyl fluoride was reversible. The HCFCs also proved inhibitory to methanol dehydrogenase, which suggests that they disrupt other aspects of C(inf1) catabolism in addition to methane monooxygenase activity.

Microbial degradation of hydrochlorofluorocarbons (CHCl2F and CHCl2CF3) in soils and sediments

The ability of microorganisms to degrade trace levels of the hydrochlorofluorocarbons HCFC-21 and HCFC-123 was investigated. Methanotroph-linked oxidation of HCFC-21 was observed in aerobic soils, and anaerobic degradation of HCFC-21 occurred in freshwater and salt marsh sediments. Microbial degradation of HCFC-123 was observed in anoxic freshwater and salt marsh sediments, and the recovery of 1,1,1-trifluoro-2-chloroethane indicated the involvement of reductive dechlorination. No degradation of HCFC-123 was observed in aerobic soils. In some experiments, HCFCs were degraded at low (parts per billion) concentrations, raising the possibility that bacteria in nature remove HCFCs from the atmosphere.

Methylmercury oxidative degradation potentials in contaminated and pristine sediments of the carson river, nevada

Sediments from mercury-contaminated and uncontaminated reaches of the Carson River, Nevada, were assayed for sulfate reduction, methanogenesis, denitrification, and monomethylmercury (MeHg) degradation. Demethylation of [(sup14)C]MeHg was detected at all sites as indicated by the formation of (sup14)CO(inf2) and (sup14)CH(inf4). Oxidative demethylation was indicated by the formation of (sup14)CO(inf2) and was present at significant levels in all samples. Oxidized/reduced demethylation product ratios (i.e., (sup14)CO(inf2)/(sup14)CH(inf4) ratios) generally ranged from 4.0 in surface layers to as low as 0.5 at depth. Production of (sup14)CO(inf2) was most pronounced at sediment surfaces which were zones of active denitrification and sulfate reduction but was also significant within zones of methanogenesis. In a core taken from an uncontaminated site having a high proportion of oxidized, coarse-grain sediments, sulfate reduction and methanogenic activity levels were very low and (sup14)CO(inf2) accounted for 98% of the product formed from [(sup14)C]MeHg. There was no apparent relationship between the degree of mercury contamination of the sediments and the occurrence of oxidative demethylation. However, sediments from Fort Churchill, the most contaminated site, were most active in terms of demethylation potentials. Inhibition of sulfate reduction with molybdate resulted in significantly depressed oxidized/reduced demethylation product ratios, but overall demethylation rates of inhibited and uninhibited samples were comparable. Addition of sulfate to sediment slurries stimulated production of (sup14)CO(inf2) from [(sup14)C]MeHg, while 2-bromoethanesulfonic acid blocked production of (sup14)CH(inf4). These results reveal the importance of sulfate-reducing and methanogenic bacteria in oxidative demethylation of MeHg in anoxic environments.

Degradation of methyl bromide by methanotrophic bacteria in cell suspensions and soils

Cell suspensions of Methylococcus capsulatus mineralized methyl bromide (MeBr), as evidence by its removal from the gas phase, the quantitative recovery of Br- in the spent medium, and the production of 14CO2 from [14C]MeBr. Methyl fluoride fluoride (MeF) inhibited oxidation of methane as well as that of [14C]MeBr. The rate of MeBr consumption by cells varied inversely with the supply of methane, which suggested a competitive relationship between these two substrates. However, MeBr did not support growth of the methanotroph. In soils exposed to high levels (10,000 ppm) of MeBr, methane oxidation was completely inhibited. At this concentration, MeBr removal rates were equivalent in killed and live controls, which indicated a chemical rather than biological removal reaction. At lower concentration (1,000 ppm) of MeBr, methanotrophs were active and MeBr consumption rates were 10-fold higher in live controls than in killed controls. Soils exposed to trace levels (10 ppm) of MeBr demonstrated complete consumption within 5 h of incubation, while controls inhibited with MeF or incubated without O2 had 50% lower removal rates. Aerobic soils oxidized [14C]MeBr to 14CO2, and MeF inhibited oxidation by 72%. Field experiments demonstrated slightly lower MeBr removal rates in chambers containing MeF than in chambers lacking MeF. Collectively, these results show that soil methanotrophic bacteria, as well as other microbes, can degrade MeBr present in the environment.

Isolation, Growth, and Metabolism of an Obligately Anaerobic, Selenate-Respiring Bacterium, Strain SES-3

A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO 2 with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Se 0 ). Hence, SES-3 can carry out a complete reduction of selenate to Se 0 . Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [ 14 C]lactate to 14 CO 2 coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m -chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Se 0 by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [ 75 Se]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se 0 . This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Se 0 may proceed, in part, by some of the components of a dissimilatory system for sulfur oxyanions.

Selective Inhibition of Ammonium Oxidation and Nitrification-Linked N 2 O Formation by Methyl Fluoride and Dimethyl Ether

Methyl fluoride (CH 3 F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH 3 F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO 2 - and N 2 O production from NH 4 + in incubations of N. europaea , while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH 2 OH) by N. europaea and oxidation of NO 2 - by a Nitrobacter sp. were unaffected by CH 3 F or DME. In nitrifying soils, CH 3 F and DME inhibited N 2 O production. In field experiments with surface flux chambers and intact cores, CH 3 F reduced the release of N 2 O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH 3 F also resulted in decreased NO 3 - + NO 2 - levels and increased NH 4 + levels in soils. CH 3 F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N 2 O metabolism in denitrifying soils. CH 3 F and DME will be useful in discriminating N 2 O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO 2 - and NO 3 - production.

Evaluation of Methyl Fluoride and Dimethyl Ether as Inhibitors of Aerobic Methane Oxidation

Methyl fluoride (MF) and dimethyl ether (DME) were effective inhibitors of aerobic methanotrophy in a variety of soils. MF and DME blocked consumption of CH 4 as well as the oxidation of 14 CH 4 to 14 CO 2 , but neither MF nor DME affected the oxidation of [ 14 C]methanol or [ 14 C]formate to 14 CO 2 . Cooxidation of ethane and propane by methane-oxidizing soils was also inhibited by MF. Nitrification (ammonia oxidation) in soils was inhibited by both MF and DME. Production of N 2 O via nitrification was inhibited by MF; however, MF did not affect N 2 O production associated with denitrification. Methanogenesis was partially inhibited by MF but not by DME. Methane oxidation was ∼100-fold more sensitive to MF than was methanogenesis, indicating that an optimum concentration could be employed to selectively block methanotrophy. MF inhibited methane oxidation by cell suspensions of Methylococcus capsulatus ; however, DME was a much less effective inhibitor.

Nitrate Is a Preferred Electron Acceptor for Growth of Freshwater Selenate-Respiring Bacteria

An anaerobic, freshwater enrichment grew with either nitrate or selenate as an electron acceptor. With both ions present, nitrate reduction preceded selenate reduction. An isolate from the enrichment grew on either ion, but the presence of nitrate precluded the reduction of selenate. Stock cultures of denitrifiers grew anaerobically on nitrate but not on selenate.

In situ bacterial selenate reduction in the agricultural drainage systems of western Nevada

Dissimilatory in situ selenate reduction to elemental selenium in sediments from irrigated agricultural drainage regions of western Nevada was measured at ambient Se oxyanion concentrations. Selenate reduction was rapid, with turnover rate constants ranging from 0.04 to 1.8 h-1 at total Se concentrations in pore water of 13 to 455 nM. Estimates of removal rates of selenium oxyanions were 14.38, and 155 mumol m-2 day-1 for South Lead Lake, Massie Slough, and Hunter Drain, respectively.

Dissimilatory Selenate Reduction Potentials in a Diversity of Sediment Types

We measured potential rates of bacterial dissimilatory reduction of 75 SeO 4 2− to 75 Se 0 in a diversity of sediment types, with salinities ranging from freshwater (salinity = 1 g/liter) to hypersaline (salinity = 320 g/liter and with pH values ranging from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/liter but not in samples with a salinity of 320 g/liter. Potential selenate reduction rates (25 nmol of SeO 4 2− per ml of sediment added with isotope) ranged from 0.07 to 22 μmol of SeO 4 2− reduced liter −1 h −1 . Activity followed Michaelis-Menten kinetics in relation to SeO 4 2− concentration ( K m of selenate = 7.9 to 720 μM). There was no linear correlation between potential rates of SeO 4 2− reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent K m for selenate and with potential rates of denitrification ( r = 0.92 and 0.81, respectively). NO 3 − , NO 2 − , MoO 4 2− , and WO 4 2− inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nev. Sulfate partially inhibited activity in sediment from freshwater (salinity = 1 g/liter) Massie Slough samples but not from the saline (salinity = 60 g/liter) Hunter Drain samples. We conclude that dissimilatory selenate reduction in sediments is widespread in nature. In addition, in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their K m s.

Description of an Estuarine Methylotrophic Methanogen Which Grows on Dimethyl Sulfide

Characteristics of an obligately methylotrophic coccoid methanogen (strain GS-16) previously isolated from estuarine sediment are described. Growth was demonstrated on dimethyl sulfide (DMS) or trimethylamine (TMA), but not on methane thiol, methane thiol plus hydrogen, dimethyl disulfide, or methionine. DMS-grown cells were able to metabolize DMS and TMA simultaneously when inoculated into media containing substrate levels of these compounds. However, TMA-grown cells could not metabolize [ 14 C]DMS to 14 CH 4 , although they could convert [ 14 C]methanol to 14 CH 4 . These results suggest that metabolism of DMS proceeds along a somewhat different route than that of TMA and perhaps also that of methanol. The organism exhibited doubling times of 23 and 32 h for growth (25°C) in mineral media on TMA and DMS, respectively. Doubling times were more rapid (∼6 h) when the organisms were grown on TMA in complex broth. In mineral media, the fastest growth on DMS occurred between pH levels of 7.0 and 8.7, at 29°C, and with 0.2 to 0.4 M Na + and 0.04 M Mg 2+ . Somewhat different results occurred for growth on TMA in complex broth. Cells had a moles percent G+C value of 44.5% for their DNA. Growth on DMS, TMA, and methanol yielded stable carbon isotope fractionation factors of 1.044, 1.037, and 1.063, respectively. Fractionation factors for hydrogen were 1.203 (DMS) and 1.183 (TMA).

Acetylene as a substrate in the development of primordial bacterial communities

The fermentation of atmospheric acetylene by anaerobic bacteria is proposed as the basis of a primordial heterotrophic food chain. The accumulation of fermentation products (acetaldehyde, ethanol, acetate and hydrogen) would create niches for sulfate-respiring bacteria as well as methanogens. Formation of acetylene-free environments in soils and sediments would also alter the function of nitrogenase from detoxification to nitrogen-fixation. The possibility of an acetylene-based anaerobic food chain in Jovian-type atmospheres is discussed.

Reduction of Selenate to Selenide by Sulfate-Respiring Bacteria: Experiments with Cell Suspensions and Estuarine Sediments

Washed cell suspensions of Desulfovibrio desulfuricans subsp. aestuarii were capable of reducing nanomolar levels of selenate to selenide as well as sulfate to sulfide. Reduction of these species was inhibited by 1 mM selenate or tungstate. The addition of 1 mM sulfate decreased the reduction of selenate and enhanced the reduction of sulfate. Increasing concentrations of sulfate inhibited rates of selenate reduction but enhanced sulfate reduction rates. Cell suspensions kept in 1 mM selenate were incapable of reducing either selenate or sulfate when the selenate/sulfate ratio was ≥0.02, indicating that irreversible inhibition occurs at high selenate concentrations. Anoxic estuarine sediments having an active flora of sulfate-respiring bacteria were capable of a small amount of selenate reduction when ambient sulfate concentrations were low (<4 mM). These results indicate that sulfate is an inhibitor of the reduction of trace quantities of selenate. Therefore, direct reduction of traces of selenate to selenide by sulfate-respiring bacteria in natural environments is constrained by the ambient concentration of sulfate ions. The significance of this observation with regard to the role sediments play in sequestering selenium is discussed.

Metabolism of Reduced Methylated Sulfur Compounds in Anaerobic Sediments and by a Pure Culture of an Estuarine Methanogen

Addition of dimethylsulfide (DMS), dimethyldisulfide (DMDS), or methane thiol (MSH) to a diversity of anoxic aquatic sediments (e.g., fresh water, estuarine, alkaline/hypersaline) stimulated methane production. The yield of methane recovered from DMS was often 52 to 63%, although high concentrations of DMS (as well as MSH and DMDS) inhibited methanogenesis in some types of sediments. Production of methane from these reduced methylated sulfur compounds was blocked by 2-bromoethanesulfonic acid. Sulfate did not influence the metabolism of millimolar levels of DMS, DMDS, or MSH added to sediments. However, when DMS was added at ∼2-μM levels as [ 14 C]DMS, metabolism by sediments resulted in a 14 CH 4 / 14 CO 2 ratio of only 0.06. Addition of molybdate increased the ratio to 1.8, while 2-bromoethanesulfonic acid decreased it to 0, but did not block 14 CO 2 production. These results indicate the methanogens and sulfate reducers compete for DMS when it is present at low concentrations; however, at high concentrations, DMS is a “noncompetitive” substrate for methanogens. Metabolism of DMS by sediments resulted in the appearance of MSH as a transient intermediate. A pure culture of an obligately methylotrophic estuarine methanogen was isolated which was capable of growth on DMS. Metabolism of DMS by the culture also resulted in the transient appearance of MSH, but the organism could grow on neither MSH nor DMDS. The culture metabolized [ 14 C]-DMS to yield a 14 CH 4 / 14 CO 2 ratio of ∼2.8. Reduced methylated sulfur compounds represent a new class of substrates for methanogens and may be potential precursors of methane in a variety of aquatic habitats.

Formation of Methane and Carbon Dioxide from Dimethylselenide in Anoxic Sediments and by a Methanogenic Bacterium

Anaerobic San Francisco Bay salt marsh sediments rapidly metabolized [ 14 C]dimethylselenide (DMSe) to 14 CH 4 and 14 CO 2 . Addition of selective inhibitors (2-bromoethanesulfonic acid or molybdate) to these sediments indicated that both methanogenic and sulfate-respiring bacteria could degrade DMSe to gaseous products. However, sediments taken from the selenium-contaminated Kesterson Wildlife Refuge produced only 14 CO 2 from [ 14 C]DMSe, implying that methanogens were not important in the Kesterson samples. A pure culture of a dimethylsulfide (DMS)-grown methylotrophic methanogen converted [ 14 C]DMSe to 14 CH 4 and 14 CO 2 . However, the organism could not grow on DMSe. Addition of DMS to either sediments or the pure culture retarded the metabolism of DMSe. This effect appeared to be caused by competitive inhibition, thereby indicating a common enzyme system for DMS and DMSe metabolism. DMSe appears to be degraded as part of the DMS pool present in anoxic environments. These results suggest that methylotrophic methanogens may demethylate methylated forms of other metals and metalloids found in nature.

Measurement of Nitrous Oxide Reductase Activity in Aquatic Sediments

Denitrification in aquatic sediments was measured by an N 2 O reductase assay. Sediments consumed small added quantities of N 2 O over short periods (a few hours). In experiments with sediment slurries, N 2 O reductase activity was inhibited by O 2 , C 2 H 2 , heat treatment, and by high levels of nitrate (1 mM) or sulfide (10 mM). However, ambient levels of nitrate (<100 μM) did not influence activity, and moderate levels (about 150 μM) induced only a short lag before reductase activity began. Moderate levels of sulfide (<1 mM) had no effect on N 2 O reductase activity. Nitrous oxide reductase displayed Michaelis-Menten kinetics in sediments from freshwater ( K m = 2.17 μM), estuarine ( K m = 14.5 μM), and alkaline-saline ( K m = 501 μM) environments. An in situ assay was devised in which a solution of N 2 O was injected into sealed glass cores containing intact sediment. Two estimates of net rates of denitrification in San Francisco Bay under approximated in situ conditions were 0.009 and 0.041 mmol of N 2 O per m 2 per h. Addition of chlorate to inhibit denitrification in these intact-core experiments (to estimate gross rates of N 2 O consumption) resulted in approximately a 14% upward revision of estimates of net rates. These results were comparable to an in situ estimate of 0.022 mmol of N 2 O per m 2 per h made with the acetylene block assay.

Denitrification in San Francisco Bay Intertidal Sediments

The acetylene block technique was employed to study denitrification in intertidal estuarine sediments. Addition of nitrate to sediment slurries stimulated denitrification. During the dry season, sediment-slurry denitrification rates displayed Michaelis-Menten kinetics, and ambient NO 3 − + NO 2 − concentrations (≤26 μM) were below the apparent K m (50 μM) for nitrate. During the rainy season, when ambient NO 3 − + NO 2 − concentrations were higher (37 to 89 μM), an accurate estimate of the K m could not be obtained. Endogenous denitrification activity was confined to the upper 3 cm of the sediment column. However, the addition of nitrate to deeper sediments demonstrated immediate N 2 O production, and potential activity existed at all depths sampled (the deepest was 15 cm). Loss of N 2 O in the presence of C 2 H 2 was sometimes observed during these short-term sediment incubations. Experiments with sediment slurries and washed cell suspensions of a marine pseudomonad confirmed that this N 2 O loss was caused by incomplete blockage of N 2 O reductase by C 2 H 2 at low nitrate concentrations. Areal estimates of denitrification (in the absence of added nitrate) ranged from 0.8 to 1.2 μmol of N 2 m −2 h −1 (for undisturbed sediments) to 17 to 280 μmol of N 2 m −2 h −1 (for shaken sediment slurries).

Methanogenesis in Big Soda Lake, Nevada: an Alkaline, Moderately Hypersaline Desert Lake

Incubated sediment slurries from Big Soda Lake, Nevada, produced significant levels of CH 4 , and production was inhibited by 2-bromoethanesulfonic acid and by autoclaving. Methane production was stimulated by methanol, trimethylamine, and, to a lesser extent, methionine. Surprisingly, hydrogen, acetate, and formate amendments provided only slight or no stimulation of methanogenesis. Methane production by sediment slurries had a pH optimum of 9.7. A methanol-grown enrichment culture containing a small, epifluorescent coccus as the predominant organism was recovered from sediments. The enrichment grew best when FeS or autoclaved sediment particles were included in the media, had a pH optimum of 9.7, and produced 14 CH 4 from 14 CH 3 OH. The methane formed by methanolgrown enrichment cultures was depleted in 13 C by 72 to 77‰ relative to the methanol.

Denitrification Associated with Periphyton Communities

Scrapings of decomposing Cladophora sp. mats (periphyton) covering stream bed rocks produced N 2 O when incubated under N 2 plus 15% C 2 H 2 . Denitrification (N 2 O formation) was enhanced by NO 3 − and was inhibited by autoclaving, Hg 2+ , and O 2 . No N 2 O was formed in the absence of C 2 H 2 (air or N 2 atmosphere). Chloramphenicol did not block N 2 O formation, indicating that the enzymes were constitutive. In field experiments, incubation of periphyton scrapings in the light inhibited denitrification because of algal photosynthetic O 2 production. The diurnal periphyton-associated denitrification rate was estimated to be 45.8 μmol of N 2 O·m −2 ·day −1 , as determined by averaging light, aerobic plus dark, and anaerobic rates over a 24-h period.