Geology topics

S.E. Hoeft

Publications and source records attributed to S.E. Hoeft.

7 recordsLinked to original sources

USGS2011

A bacterium that can grow by using arsenic instead of phosphorus

Life is mostly composed of the elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. Although these six elements make up nucleic acids, proteins, and lipids and thus the bulk of living matter, it is theoretically possible that some other elements in the periodic table could serve the same functions. Here, we describe a bacterium, strain GFAJ-1 of the Halomonadaceae, isolated from Mono Lake, California, that is able to substitute arsenic for phosphorus to sustain its growth. Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins. Exchange of one of the major bio-elements may have profound evolutionary and geochemical importance.

Science↗

USGS2010

Characterization of microbial arsenate reduction in the anoxic bottom waters of Mono Lake, California

Dissimilatory reduction of arsenate (DAsR) occurs in the arsenic-rich, anoxic water column of Mono Lake, California, yet the microorganisms responsible for this observed in situ activity have not been identified. To gain insight as to which microorganisms mediate this phenomenon, as well as to some of the biogeochemical constraints on this activity, we conducted incubations of arsenate-enriched bottom water coupled with inhibition/amendment studies and Denaturing Gradient Gel Electrophoresis (DGGE) characterization techniques. DAsR was totally inhibited by filter-sterilization and by nitrate, partially inhibited (~50%) by selenate, but only slightly (~25%) inhibited by oxyanions that block sulfate-reduction (molybdate and tungstate). The apparent inhibition by nitrate, however, was not due to action as a preferred electron acceptor to arsenate. Rather, nitrate addition caused a rapid, microbial re-oxidation of arsenite to arsenate, which gave the overall appearance of no arsenate loss. A similar microbial oxidation of As(III) was also found with Fe(III), a fact that has implications for the recycling of As(V) in Mono Lake's anoxic bottom waters. DAsR could be slightly (10%) stimulated by substrate amendments of lactate, succinate, malate, or glucose, but not by acetate, suggesting that the DAsR microflora is not electron donor limited. DGGE analysis of amplified 16S rDNA gene fragments from incubated arsenate-enriched bottom waters revealed the presence of two bands that were not present in controls without added arsenate. The resolved sequences of these excised bands indicated the presence of members of the epsilon (Sulfurospirillum) and delta (Desulfovibrio) subgroups of the Proteobacteria, both of which have representative species that are capable of anaerobic growth using arsenate as their electron acceptor.

California↗

USGS2009

Respiratory arsenate reductase as a bidirectional enzyme

The haloalkaliphilic bacterium Alkalilimnicola ehrlichii is capable of anaerobic chemolithoautotrophic growth by coupling the oxidation of arsenite (As(III)) to the reduction of nitrate and carbon dioxide. Analysis of its complete genome indicates that it lacks a conventional arsenite oxidase (Aox), but instead possesses two operons that each encode a putative respiratory arsenate reductase (Arr). Here we show that one homolog is expressed under chemolithoautotrophic conditions and exhibits both arsenite oxidase and arsenate reductase activity. We also demonstrate that Arr from two arsenate respiring bacteria, Alkaliphilus oremlandii and Shewanella sp. strain ANA-3, is also biochemically reversible. Thus Arr can function as a reductase or oxidase. Its physiological role in a specific organism, however, may depend on the electron potentials of the molybdenum center and [Fe–S] clusters, additional subunits, or constitution of the electron transfer chain. This versatility further underscores the ubiquity and antiquity of microbial arsenic metabolism.

Biochemical and Biophysical Research Communication↗

USGS2008

Arsenic(III) fuels anoxygenic photosynthesis in hot spring biofilms from Mono Lake, California

Phylogenetic analysis indicates that microbial arsenic metabolism is ancient and probably extends back to the primordial Earth. In microbial biofilms growing on the rock surfaces of anoxic brine pools fed by hot springs containing arsenite and sulfide at high concentrations, we discovered light-dependent oxidation of arsenite [As(III)] to arsenate [As(V)] occurring under anoxic conditions. The communities were composed primarily of Ectothiorhodospira-like purple bacteria or Oscillatoria-like cyanobacteria. A pure culture of a photosynthetic bacterium grew as a photoautotroph when As(III) was used as the sole photosynthetic electron donor. The strain contained genes encoding a putative As(V) reductase but no detectable homologs of the As(III) oxidase genes of aerobic chemolithotrophs, suggesting a reverse functionality for the reductase. Production of As(V) by anoxygenic photosynthesis probably opened niches for primordial Earth's first As(V)-respiring prokaryotes.

Science↗

USGS2006

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 (∼90 g/liter) and Searles Lake (∼340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [ 73 As]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, [ 35 S]sulfate reduction was measurable in Mono Lake ( k = 7.6 ×10 4 to 3.2 × 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.

Applied and Environmental Microbiology↗

USGS2004

Redox transformations of arsenic oxyanions in periphyton communities

Periphyton ( Cladophora sp.) samples from a suburban stream lacking detectable dissolved As were able to reduce added As(V) to As(III) when incubated under anoxic conditions and, conversely, oxidized added As(III) to As(V) with aerobic incubation. Both types of activity were abolished in autoclaved controls, thereby demonstrating its biological nature. The reduction of As(V) was inhibited by chloramphenicol, indicating that it required the synthesis of new protein. Nitrate also inhibited As(V) reduction, primarily because it served as a preferred electron acceptor to which the periphyton community was already adapted. However, part of the inhibition was also caused by microbial reoxidation of As(III) linked to nitrate. Addition of [ 14 C]glucose to anoxic samples resulted in the production of 14 CO 2 , suggesting that the observed As(V) reduction was a respiratory process coupled to the oxidation of organic matter. The population density of As(V)-reducing bacteria within the periphyton increased with time and with the amount of As(V) added, reaching values as high as ∼10 6 cells ml −1 at the end of the incubation. This indicated that dissimilatory As(V) reduction in these populations was linked to growth. However, As(V)-respiring bacteria were found to be present, albeit at lower numbers (∼10 2 ml −1 ), in freshly sampled periphyton. These results demonstrate the presence of a bacterial population within the periphyton communities that is capable of two key arsenic redox transformations that were previously studied in As-contaminated environments, which suggests that these processes are widely distributed in nature. This assumption was reinforced by experiments with estuarine samples of Cladophora sericea in which we detected a similar capacity for anaerobic As(V) reduction and aerobic As(III) oxidation.

Applied and Environmental Microbiology↗

USGS2002

Dissimilatory arsenate reductase activity and arsenate-respiring bacteria in bovine rumen fluid, hamster feces, and the termite hindgut

Bovine rumen fluid and slurried hamster feces completely reduced millimolar levels of arsenate to arsenite upon incubation under anoxic conditions. This activity was strongly inhibited by autoclaving or aerobic conditions, and partially inhibited by tungstate or chloramphenicol. The rate of arsenate reduction was faster in feces from a population of arsenate-watered (100 ppm) hamsters compared to a control group watered without arsenate. Using radioisotope methods, arsenate reductase activity in hamster feces was also detected at very low concentrations of added arsenate (∼10 μM). Bacterial cultures were isolated from these materials, as well as from the termite hindgut, that grew using H 2 as their electron donor, acetate as their carbon source, and arsenate as their respiratory electron acceptor. The three cultures aligned phylogenetically either with well-established enteric bacteria, or with an organism associated with feedlot fecal wastes. Because arsenite is transported across the gut epithelium more readily than arsenate, microbial dissimilatory reduction of arsenate in the gut may promote the body's absorption of arsenic and hence potentiate its toxicity.

FEMS Microbiology Ecology↗