Transcriptional response of the obligate anaerobe Desulfuribacillus stibiiarsenatis MLFW-2T to growth on antimonate and other terminal electron acceptors.

Enzymes of the dimethyl sulfoxide reductase (DMSOR) family catalyse two-electron redox reactions pivotal to the dissimilatory metabolism of a variety of organic and inorganic compounds. The draft genome of the obligately anaerobic bacterium Desulfuribacillus stibiiarsenatis MLFW-2T contains 14 genes that are predicted to encode catalytic subunits of DMSOR family enzymes. We quantified transcription of these genes during growth on antimonate, arsenate, nitrate and selenate, with the goal of identifying the respiratory antimonate reductase. Transcription of BHU72_10330, BHU72_03635 and BHU72_07355 was enhanced during growth on arsenate, nitrate and selenate, respectively, implicating these genes as encoding the catalytic subunits of a respiratory arsenate reductase (arrA), periplasmic nitrate reductase (napA) and membrane-bound selenate reductase (srdA) respectively. Transcription of BHU72_07145 increased markedly when MLFW-2T was grown on antimonate, suggesting that this gene encodes the catalytic subunit of a respiratory antimonate reductase, designated anrA. We also compared the transcriptomes of MLFW-2T during growth on antimonate and arsenate to examine the broader physiological response of the organism to growth on these substrates. Relative to arsenate, antimonate was found to induce transcription of genes involved in pathways for dealing with oxidative stress, including those involved in repairing damaged cellular biomolecules and scavenging reactive oxygen species.

Ecotype diversification of an abundant Roseobacter lineage.

The Roseobacter DC5-80-3 cluster (also known as the RCA clade) is among the most abundant bacterial lineages in temperate and polar oceans. Previous studies revealed two phylotypes within this cluster that are distinctly distributed in the Antarctic and other ocean provinces. Here, we report a nearly complete genome co-assembly of three closely related single cells co-occurring in the Antarctic, and compare it to the available genomes of the other phylotype from ocean regions where iron is more accessible but phosphorus and nitrogen are less. The Antarctic phylotype exclusively contains an operon structure consisting of a dicitrate transporter fecBCDE and an upstream regulator likely for iron uptake, whereas the other phylotype consistently carry a high-affinity phosphate pst transporter and the phoB-phoR regulatory system, a high-affinity ammonium amtB transporter, urea and taurine utilization systems. Moreover, the Antarctic phylotype uses proteorhodopsin to acquire light, whereas the other uses bacteriochlorophyll-a and the sulfur-oxidizing sox cluster for energy acquisition. This is potentially an iron-saving strategy for the Antarctic phylotype because only the latter two pathways have iron-requiring cytochromes. Therefore, the two DC5-80-3 phylotypes, while diverging by only 1.1% in their 16S rRNA genes, have evolved systematic differences in metabolism to support their distinct ecologies.

Oxidation of urea-derived nitrogen by thaumarchaeota-dominated marine nitrifying communities.

Urea nitrogen has been proposed to contribute significantly to nitrification by marine thaumarchaeotes. These inferences are based on distributions of thaumarchaeote urease genes rather than activity measurements. We found that ammonia oxidation rates were always higher than oxidation rates of urea-derived N in samples from coastal Georgia, USA (means ± SEM: 382 ± 35 versus 73 ± 24 nmol L-1  d-1 , Mann-Whitney U-test p < 0.0001), and the South Atlantic Bight (20 ± 8.8 versus 2.2 ± 1.7 nmol L-1  d-1 , p = 0.026) but not the Gulf of Alaska (8.8 ± 4.0 versus 1.5 ± 0.6, p > 0.05). Urea-derived N was relatively more important in samples from Antarctic continental shelf waters, though the difference was not statistically significant (19.4 ± 4.8 versus 12.0 ± 2.7 nmol L-1  d-1 , p > 0.05). We found only weak correlations between oxidation rates of urea-derived N and the abundance or transcription of putative Thaumarchaeota ureC genes. Dependence on urea-derived N does not appear to be directly related to pH or ammonium concentrations. Competition experiments and release of 15 NH3 suggest that urea is hydrolyzed to ammonia intracellularly, then a portion is lost to the dissolved pool. The contribution of urea-derived N to nitrification appears to be minor in temperate coastal waters, but may represent a significant portion of the nitrification flux in Antarctic coastal waters.

Desulfuribacillus stibiiarsenatis sp. nov., an obligately anaerobic, dissimilatory antimonate- and arsenate-reducing bacterium isolated from anoxic sediments, and emended description of the genus Desulfuribacillus.

A novel anaerobic, Gram-stain-negative, endospore-forming bacterium, designated strain MLFW-2T, was isolated from anoxic sediments collected from the drainage area of a geothermal spring near Mono Lake, CA, USA. Optimal growth was achieved at 34 °C and pH 8.25-8.50 in medium containing 0.75 % (w/v) NaCl. Catalase, but not oxidase, was produced. Strain MLFW-2T was an obligate anaerobe capable of respiring with nitrate, nitrite, DMSO, arsenate, antimonate, selenate and selenite as terminal electron acceptors. Lactate, pyruvate, formate and H2 could serve as electron donors to support growth. The isolate was incapable of fermentation. The predominant fatty acids were C16 : 0, C16 : 1ω9c, C16 : 1ω7c, C18 : 1ω9c and C18 : 1ω7c. The major polar lipids were phosphatidylglycerol and phosphatidylethanolamine. The only isoprenoid quinone detected was menaquinone 7 (MK-7). The DNA G+C content was 38.2 mol%. Phylogenetic analysis based on the 16S rRNA gene sequence demonstrated that strain MLFW-2T was a member of the order Bacillales and was most closely related to Desulfuribacillus alkaliarsenatis AHT28T (93.9 % similarity). On the basis of phenotypic and phylogenomic evidence, strain MLFW-2T represents a novel species of the genus Desulfuribacillus, for which the name Desulfuribacillus stibiiarsenatis sp. nov. is proposed. The type strain is MLFW-2T (=DSM 28709T=JCM 30866T). An emended description of the genus Desulfuribacillus is also provided.

Temperature Decouples Ammonium and Nitrite Oxidation in Coastal Waters.

Nitrification is a two-step process linking the reduced and oxidized sides of the nitrogen cycle. These steps are typically tightly coupled with the primary intermediate, nitrite, rarely accumulating in coastal environments. Nitrite concentrations can exceed 10 µM during summer in estuarine waters adjacent to Sapelo Island, Georgia, U.S.A. Similar peaks at other locations have been attributed to decoupling of the two steps of nitrification by hypoxia; however, the waters around Sapelo Island are aerobic and well-mixed. Experiments examining the response to temperature shifts of a nitrifying assemblage composed of the same organisms found in the field indicate that ammonia- and nitrite-oxidation become uncoupled between 20 and 30 °C, leading to nitrite accumulation. This suggests that nitrite peaks in coastal waters might be explained by differences in the responses of ammonia- and nitrite-oxidizers to increased summer temperatures. Analysis of field data from 270 stations in 29 temperate and subtropical estuaries and lagoons show transient accumulation of nitrite driven primarily by water temperatures, rather than by hypoxia. Increased climate variability and warming coastal waters may therefore increase the frequency of these nitrite peaks, with potential ecosystem consequences that include increased N2O production, NO2- toxicity, and shifts in phytoplankton community composition.

Temporal and vertical distributions of bacterioplankton at the Gray's Reef National Marine Sanctuary.

Large spatial scales and long-term shifts of bacterial community composition (BCC) in the open ocean can often be reliably predicted based on the dynamics of physical-chemical variables. The power of abiotic factors in shaping BCC on shorter time scales in shallow estuarine mixing zones is less clear. We examined the diurnal variation in BCC at different water depths in the spring and fall of 2011 at a station in the Gray's Reef National Marine Sanctuary (GRNMS). This site is located in the transition zone between the estuarine plume and continental shelf waters of the South Atlantic Bight. A total of 234,516 pyrotag sequences of bacterial 16S rRNA genes were recovered; they were taxonomically affiliated with >200 families of 23 bacterial phyla. Nonmetric multidimensional scaling analysis revealed significant differences in BCC between spring and fall samples, likely due to seasonality in the concentrations of dissolved organic carbon and nitrate plus nitrite. Within each diurnal sampling, BCC differed significantly by depth only in the spring and differed significantly between day and night only in the fall. The former variation largely tracked changes in light availability, while the latter was most correlated with concentrations of polyamines and chlorophyll a. Our results suggest that at the GRNMS, a coastal mixing zone, diurnal variation in BCC is attributable to the mixing of local and imported bacterioplankton rather than to bacterial growth in response to environmental changes. Our results also indicate that, like members of the Roseobacter clade, SAR11 bacteria may play an important role in processing dissolved organic material in coastal oceans.

Dissimilatory antimonate reduction and production of antimony trioxide microcrystals by a novel microorganism.

Antimony (Sb) is a metalloid that has been exploited by humans since the beginning of modern civilization. The importance of Sb to such diverse industries as nanotechnology and health is underscored by the fact that it is currently the ninth-most mined metal worldwide. Although its toxicity mirrors that of its Group 15 neighbor arsenic, its environmental chemistry is very different, and, unlike arsenic, relatively little is known about the fate and transport of Sb, especially with regard to biologically mediated redox reactions. To further our understanding of the interactions between microorganisms and Sb, we have isolated a bacterium that is capable of using antimonate [Sb(V)] as a terminal electron acceptor for anaerobic respiration, resulting in the precipitation of antimonite [Sb(III)] as microcrystals of antimony trioxide. The bacterium, designated strain MLFW-2, is a sporulating member of a deeply branching lineage within the order Bacillales (phylum Firmicutes). This report provides the first unequivocal evidence that a bacterium is capable of conserving energy for growth and reproduction from the reduction of antimonate. Moreover, microbiological antimonate reduction may serve as a novel route for the production of antimony trioxide microcrystals of commercial significance to the nanotechnology industry.

Succession of hydrocarbon-degrading bacteria in the aftermath of the deepwater horizon oil spill in the gulf of Mexico.

The Deepwater Horizon oil spill produced large subsurface plumes of dispersed oil and gas in the Gulf of Mexico that stimulated growth of psychrophilic, hydrocarbon degrading bacteria. We tracked succession of plume bacteria before, during and after the 83-day spill to determine the microbial response and biodegradation potential throughout the incident. Dominant bacteria shifted substantially over time and were dependent on relative quantities of different hydrocarbon fractions. Unmitigated flow from the wellhead early in the spill resulted in the highest proportions of n-alkanes and cycloalkanes at depth and corresponded with dominance by Oceanospirillaceae and Pseudomonas. Once partial capture of oil and gas began 43 days into the spill, petroleum hydrocarbons decreased, the fraction of aromatic hydrocarbons increased, and Colwellia, Cycloclasticus, and Pseudoalteromonas increased in dominance. Enrichment of Methylomonas coincided with positive shifts in the δ(13)C values of methane in the plume and indicated significant methane oxidation occurred earlier than previously reported. Anomalous oxygen depressions persisted at plume depths for over six weeks after well shut-in and were likely caused by common marine heterotrophs associated with degradation of high-molecular-weight organic matter, including Methylophaga. Multiple hydrocarbon-degrading bacteria operated simultaneously throughout the spill, but their relative importance was controlled by changes in hydrocarbon supply.

Ciliate ingestion and digestion: flow cytometric measurements and regrowth of a digestion-resistant Campylobacter jejuni.

We measured ingestion and digestion rates of the pathogenic bacterium Campylobacter jejuni by a freshwater ciliate Colpoda sp. to determine whether Campylobacter is able to resist protist digestion. Campylobacter and the nonpathogenic bacterium Pseudomonas putida LH1 were labeled with a 5-chloromethylfluorescein diacetate, which fluoresces in intact and active cells but fades when exposed to low pH environments, such as protistan food vacuoles. Ingestion and digestion rates were measured via flow cytometry as the change in ciliate fluorescence over time, which corresponded to the quantity of intracellular bacteria. The rate of Campylobacter ingestion exceeded the digestion rate. Ciliates retained labeled Campylobacter 5 h after ingestion was stopped. In contrast, ciliates grazing upon P. putida returned to baseline fluorescence within 5 h, indicating that P. putida were completely digested. The ability of intracellular Campylobacter to remain viable after ingestion was tested by sorting individual ciliates and bacterial cells into Campylobacter-selective media. Campylobacter growth occurred in 15% (± 5 SE) of wells seeded with highly fluorescent ciliates, whereas only 4% (± 1) of wells seeded with free-living Campylobacter exhibited growth. A key advantage of this approach is that it is rapid and should be applicable to other phagocytotis studies.

Ascomycete fungal communities associated with early decaying leaves of Spartina spp. from central California estuaries.

Ascomycetous fungi play an important role in the early stages of decomposition of Spartina alterniflora, but their role in the decomposition of other Spartina species has not been investigated. Here we use fingerprint (terminal restriction fragment length polymorphism) and phylogenetic analyses of the 18S to 28S internal transcribed spacer region to compare the composition of the ascomycete fungal communities on early decay blades of Spartina species (Spartina alterniflora, Spartina densiflora, Spartina foliosa, and a hybrid (S. alterniflora x S. foliosa)) collected from three salt marshes in San Francisco Bay and one in Tomales Bay, California, USA. Phaeosphaeria spartinicola was found on all samples collected and was often dominant. Two other ascomycetes, Phaeosphaeria halima and Mycosphaerella sp. strain 2, were also common. These three species are the same ascomycetes previously identified as the dominant fungal decomposers on S. alterniflora on the east coast. Ascomycetes appeared to exhibit varying degrees of host specificity, demonstrated by grouping patterns on phylogenetic trees. Neither the exotic S. alterniflora nor the hybrid supported fungal flora different from that of the native S. foliosa. However, S. densiflora had a significantly different fungal community than the other species, and hosted at least two unique ascomycetes. Significant differences in the fungal decomposer communities were also detected within species (two clones of S. foliosa), but these were minor and may be due to morphological differences among the plants.

Correction: Single-cell genomics shedding light on marine Thaumarchaeota diversification.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Roadmap for naming uncultivated Archaea and Bacteria.

The assembly of single-amplified genomes (SAGs) and metagenome-assembled genomes (MAGs) has led to a surge in genome-based discoveries of members affiliated with Archaea and Bacteria, bringing with it a need to develop guidelines for nomenclature of uncultivated microorganisms. The International Code of Nomenclature of Prokaryotes (ICNP) only recognizes cultures as 'type material', thereby preventing the naming of uncultivated organisms. In this Consensus Statement, we propose two potential paths to solve this nomenclatural conundrum. One option is the adoption of previously proposed modifications to the ICNP to recognize DNA sequences as acceptable type material; the other option creates a nomenclatural code for uncultivated Archaea and Bacteria that could eventually be merged with the ICNP in the future. Regardless of the path taken, we believe that action is needed now within the scientific community to develop consistent rules for nomenclature of uncultivated taxa in order to provide clarity and stability, and to effectively communicate microbial diversity.

Ammonia-oxidizing Archaea in the Arctic Ocean and Antarctic coastal waters.

We compared abundance, distributions and phylogenetic composition of Crenarchaeota and ammonia-oxidizing Archaea (AOA) in samples collected from coastal waters west of the Antarctic Peninsula during the summers of 2005 and 2006, with samples from the central Arctic Ocean collected during the summer of 1997. Ammonia-oxidizing Archaea and Crenarchaeota abundances were estimated from quantitative PCR measurements of amoA and 16S rRNA gene abundances. Crenarchaeota and AOA were approximately fivefold more abundant at comparable depths in the Antarctic versus the Arctic Ocean. Crenarchaeota and AOA were essentially absent from the Antarctic Summer Surface Water (SSW) water mass (0-45 m depth). The ratio of Crenarchaeota 16S rRNA to archaeal amoA gene abundance in the Winter Water (WW) water mass (45-105 m depth) of the Southern Ocean was much lower (0.15) than expected and in sharp contrast to the ratio (2.0) in the Circumpolar Deep Water (CDW) water mass (105-3500 m depth) immediately below it. We did not observe comparable segregation of this ratio by depth or water mass in Arctic Ocean samples. A ubiquitous, abundant and polar-specific crenarchaeote was the dominant ribotype in the WW and important in the upper halocline of the Arctic Ocean. Our data suggest that this organism does not contain an ammonia monooxygenase gene. In contrast to other studies where Crenarchaeota populations apparently lacking amoA genes are found in bathypelagic waters, this organism appears to dominate in well-defined, ammonium-rich, near-surface water masses in polar oceans.

Population growth away from the coastal zone: thirty years of land use change and nutrient export in the Altamaha River, GA.

We used more than thirty years of water quality monitoring data collected by the United States Geological Survey at several stations in the Altamaha River and its tributaries to examine the relationship between population density, agricultural land use, and nutrient export from the watershed. Population densities in the Altamaha River watershed increased during the study period, most notably in the upper watershed near metropolitan Atlanta, while agricultural land use declined throughout the watershed. NO(x), TN and P in rivers were related to human population densities, while OC and NH(4)(+) concentrations in rivers were apparently related to agricultural land use. A general pattern of increasing NO(x) and TN and decreasing NH(4)(+), P and OC over time throughout the watershed reflected changing population and land use. The overall average load from the Altamaha River to the coastal zone during the study period was 1.1, 5.6, 16.9, 0.9 and 262 kmol km(-2) yr(-1), delivering 40, 197, 596, 30, and 9213.10(6) mol yr(-1) of NH(4)(+), NO(x), TN, P and OC, respectively, to the coastal zone. The nutrient export patterns suggest that N and P loading to rivers in the Altamaha River watershed was greatest in the upper watershed where high population densities were found, and in-stream processing, dilution, and only moderate inputs during transit through the lower watershed resulted in relatively low export from the watershed to coastal waters.

Coastal Ocean Metagenomes and Curated Metagenome-Assembled Genomes from Marsh Landing, Sapelo Island (Georgia, USA).

Microbes play a dominant role in the biogeochemistry of coastal waters, which receive organic matter from diverse sources. We present metagenomes and 45 metagenome-assembled genomes (MAGs) from Sapelo Island, Georgia, to further understand coastal microbial populations. Notably, four MAGs are archaea, with two Thaumarchaeota and two marine group II Euryarchaeota.

Phylogenomics suggests oxygen availability as a driving force in Thaumarchaeota evolution.

Ammonia-oxidizing archaea (AOA) of the phylum Thaumarchaeota are widespread in marine and terrestrial habitats, playing a major role in the global nitrogen cycle. However, their evolutionary history remains unexplored, which limits our understanding of their adaptation mechanisms. Here, our comprehensive phylogenomic tree of Thaumarchaeota supports three sequential events: origin of AOA from terrestrial non-AOA ancestors, colonization of the shallow ocean, and expansion to the deep ocean. Careful molecular dating suggests that these events coincided with the Great Oxygenation Event around 2300 million years ago (Mya), and oxygenation of the shallow and deep ocean around 800 and 635-560 Mya, respectively. The first transition was likely enabled by the gain of an aerobic pathway for energy production by ammonia oxidation and biosynthetic pathways for cobalamin and biotin that act as cofactors in aerobic metabolism. The first transition was also accompanied by the loss of dissimilatory nitrate and sulfate reduction, loss of oxygen-sensitive pyruvate oxidoreductase, which reduces pyruvate to acetyl-CoA, and loss of the Wood-Ljungdahl pathway for anaerobic carbon fixation. The second transition involved gain of a K+ transporter and of the biosynthetic pathway for ectoine, which may function as an osmoprotectant. The third transition was accompanied by the loss of the uvr system for repairing ultraviolet light-induced DNA lesions. We conclude that oxygen availability drove the terrestrial origin of AOA and their expansion to the photic and dark oceans, and that the stressors encountered during these events were partially overcome by gene acquisitions from Euryarchaeota and Bacteria, among other sources.

Selenate-dependent anaerobic arsenite oxidation by a bacterium from Mono Lake, California.

Arsenate was produced when anoxic Mono Lake water samples were amended with arsenite and either selenate or nitrate. Arsenite oxidation did not occur in killed control samples or live samples with no added terminal electron acceptor. Potential rates of anaerobic arsenite oxidation with selenate were comparable to those with nitrate ( approximately 12 to 15 mumol.liter(-1) h(-1)). A pure culture capable of selenate-dependent anaerobic arsenite oxidation (strain ML-SRAO) was isolated from Mono Lake water into a defined salts medium with selenate, arsenite, and yeast extract. This strain does not grow chemoautotrophically, but it catalyzes the oxidation of arsenite during growth on an organic carbon source with selenate. No arsenate was produced in pure cultures amended with arsenite and nitrate or oxygen, indicating that the process is selenate dependent. Experiments with washed cells in mineral medium demonstrated that the oxidation of arsenite is tightly coupled to the reduction of selenate. Strain ML-SRAO grows optimally on lactate with selenate or arsenate as the electron acceptor. The amino acid sequences deduced from the respiratory arsenate reductase gene (arrA) from strain ML-SRAO are highly similar (89 to 94%) to those from two previously isolated Mono Lake arsenate reducers. The 16S rRNA gene sequence of strain ML-SRAO places it within the Bacillus RNA group 6 of gram-positive bacteria having low G+C content.

Composition and Activity of Microbial Communities along the Redox Gradient of an Alkaline, Hypersaline, Lake.

We compared the composition of microbial communities obtained by sequencing 16S rRNA gene amplicons with taxonomy derived from metatranscriptomes from the same samples. Samples were collected from alkaline, hypersaline Mono Lake, California, USA at five depths that captured the major redox zones of the lake during the onset of meromixis. The prokaryotic community was dominated by bacteria from the phyla Proteobacteria, Firmicutes, and Bacteroidetes, while the picoeukaryotic chlorophyte Picocystis dominated the eukaryotes. Most (80%) of the abundant (>1% relative abundance) OTUs recovered as amplicons of 16S rRNA genes have been reported in previous surveys, indicating that Mono Lake's microbial community has remained stable over 12 years that have included periods of regular, annual overturn interspersed by episodes of prolonged meromixis that result in extremely reducing conditions in bottom water. Metatranscriptomic sequences binned predominately to the Gammaproteobacteria genera Thioalkalivibrio (4-13%) and Thioalkalimicrobium (0-14%); and to the Firmicutes genera Dethiobacter (0-5%) and Clostridium (1-4%), which were also abundant in the 16S rRNA gene amplicon libraries. This study provides insight into the taxonomic affiliations of transcriptionally active communities of the lake's water column under different redox conditions.

Light and temperature control the seasonal distribution of thaumarchaeota in the South Atlantic bight.

Mid-summer peaks in the abundance of Thaumarchaeota and nitrite concentration observed on the Georgia, USA, coast could result from in situ activity or advection of populations from another source. We collected data on the distribution of Thaumarchaeota, ammonia-oxidizing betaproteobacteria (AOB), Nitrospina, environmental variables and rates of ammonia oxidation during six cruises in the South Atlantic Bight (SAB) from April to November 2014. These data were used to examine seasonality of nitrification in offshore waters and to test the hypothesis that the bloom was localized to inshore waters. The abundance of Thaumarchaeota marker genes (16S rRNA and amoA) increased at inshore and nearshore stations starting in July and peaked in August at >107 copies L-1. The bloom did not extend onto the mid-shelf, where Thaumarchaeota genes ranged from 103 to 105 copies L-1. Ammonia oxidation rates (AO) were highest at inshore stations during summer (to 840 nmol L-1 d-1) and were always at the limit of detection at mid-shelf stations. Nitrite concentrations were correlated with AO (R = 0.94) and were never elevated at mid-shelf stations. Gene sequences from samples collected at mid-shelf stations generated using Archaea 16S rRNA primers were dominated by Euryarchaeota; sequences from inshore and nearshore stations were dominated by Thaumarchaeota. Thaumarchaeota were also abundant at depth at the shelf-break; however, this population was phylogenetically distinct from the inshore/nearshore population. Our analysis shows that the bloom is confined to inshore waters during summer and suggests that Thaumarchaeota distributions in the SAB are controlled primarily by photoinhibition and secondarily by water temperature.

Metatranscriptomic analysis of prokaryotic communities active in sulfur and arsenic cycling in Mono Lake, California, USA.

This study evaluates the transcriptionally active, dissimilatory sulfur- and arsenic-cycling components of the microbial community in alkaline, hypersaline Mono Lake, CA, USA. We sampled five depths spanning the redox gradient (10, 15, 18, 25 and 31 m) during maximum thermal stratification. We used custom databases to identify transcripts of genes encoding complex iron-sulfur molybdoenzyme (CISM) proteins, with a focus on arsenic (arrA, aioA and arxA) and sulfur cycling (dsrA, aprA and soxB), and assigned them to taxonomic bins. We also report on the distribution of transcripts related to the ars arsenic detoxification pathway. Transcripts from detoxification pathways were not abundant in oxic surface waters (10 m). Arsenic cycling in the suboxic and microaerophilic zones of the water column (15 and 18 m) was dominated by arsenite-oxidizing members of the Gammaproteobacteria most closely affiliated with Thioalkalivibrio and Halomonas, transcribing arxA. We observed a transition to arsenate-reducing bacteria belonging to the Deltaproteobacteria and Firmicutes transcribing arsenate reductase (arrA) in anoxic bottom waters of the lake (25 and 31 m). Sulfur cycling at 15 and 18 m was dominated by Gammaproteobacteria (Thioalkalivibrio and Thioalkalimicrobium) oxidizing reduced S species, with a transition to sulfate-reducing Deltaproteobacteria at 25 and 31 m. Genes related to arsenic and sulfur oxidation from Thioalkalivibrio were more highly transcribed at 15 m relative to other depths. Our data highlight the importance of Thioalkalivibrio to arsenic and sulfur biogeochemistry in Mono Lake and identify new taxa that appear capable of transforming arsenic.