Precipitation of barite by Myxococcus xanthus: possible implications for the biogeochemical cycle of barium

Bacterial precipitation of barite (BaSO(4)) under laboratory conditions is reported for the first time. The bacterium Myxococcus xanthus was cultivated in a solid medium with a diluted solution of barium chloride. Crystallization occurred as a result of the presence of live bacteria and the bacterial metabolic activity. A phosphorous-rich amorphous phase preceded the more crystalline barite formation. These experiments may indicate the involvement of bacteria in the barium biogeochemical cycle, which is closely related to the carbon cycle.     

Barite encrustation of benthic sulfur‐oxidizing bacteria at a marine cold seep

Crusts and chimneys composed of authigenic barite are found at methane seeps and hydrothermal vents that expel fluids rich in barium. Microbial processes have not previously been associated with barite precipitation in marine cold seep settings. Here, we report on the precipitation of barite on filaments of sulfide‐oxidizing bacteria at a brine seep in the Gulf of Mexico. Barite‐mineralized bacterial filaments in the interiors of authigenic barite crusts resemble filamentous sulfide‐oxidizing bacteria of the genus Beggiatoa. Clone library and iTag amplicon sequencing of the 16S rRNA gene show that the barite crusts that host these filaments also preserve DNA of Candidatus Maribeggiatoa, as well as sulfate‐reducing bacteria. Isotopic analyses show that the sulfur and oxygen isotope compositions of barite have lower δ³⁴S and δ¹⁸O values than many other marine barite crusts, which is consistent with barite precipitation in an environment in which sulfide oxidation was occurring. Laboratory experiments employing isolates of sulfide‐oxidizing bacteria from Gulf of Mexico seep sediments showed that under low sulfate conditions, such as those encountered in brine fluids, sulfate generated by sulfide‐oxidizing bacteria fosters rapid barite precipitation localized on cell biomass, leading to the encrustation of bacteria in a manner reminiscent of our observations of barite‐mineralized Beggiatoa in the Gulf of Mexico. The precipitation of barite directly on filaments of sulfide‐oxidizing bacteria, and not on other benthic substrates, suggests that sulfide oxidation plays a role in barite formation at certain marine brine seeps where sulfide is oxidized to sulfate in contact with barium‐rich fluids, either prior to, or during, the mixing of those fluids with sulfate‐containing seawater in the vicinity of the sediment/water interface. As with many other geochemical interfaces that foster mineral precipitation, both biological and abiological processes likely contribute to the precipitation of barite at marine brine seeps such as the one studied here.     

The Importance of Understanding the Chemical Form of a Metal in the Environment: The Case of Barium Sulfate (Barite)

Environmental assessments of metals are often simplified by referring to metals in terms of their elemental names without further consideration of specific metal compounds. Such simplifications can obscure important information on the fate and effects of the particular metal compounds. Barium sulfate (barite) provides a good example of the importance of knowing the specific compound when evaluating exposure, effects, and risks. We illustrate this by comparing the solubility and toxicity of barite to the soluble barium salts used to derive the U.S. Environmental Protection Agency barium ecological soil screening levels (Eco-SSLs). For ecologically relevant exposure pathways, the toxicity of barium compounds is largely controlled by their solubility. In contrast to soluble barium salts (e.g., barium acetate, barium chloride, and barium nitrate), barite is sparingly soluble and practically nontoxic to invertebrates, plants, or wildlife. No-effect levels for soil invertebrates (17,000 to 1,000,000 mg/kg) are several orders of magnitude higher than the barium Eco-SSL for soil invertebrates (330 mg/kg). And, low solubility likely limits the bioaccumulation of barium associated with barite and potential for effects on wildlife species. The importance of knowing the form of a metal compound is underscored by the fact that barite is one of the most commonly occurring barium compounds in soils and is widely used in commerce. The large differences between barite and the soluble barium compounds used to derive the Eco-SSL indicate that this screening tool cannot be usefully applied to soils containing the most commonly found barium compound, barite. The value of considering the specific metal compounds present in exposure media applies to other metals and indicates a need to go beyond simplified characterizations of metals expressed solely in terms of their elemental names.     

The biological production of marine suspended barite and the barium cycle in the Western Mediterranean Sea

Suspended particulate barium was measured in the Western Mediterranean along 4 profiles sampled during the PHYCEMED 1 cruise in 1981. The non-terrigenous fraction of particulate barium (i.e. excess Ba; Ba_xs) can account for up to 96% of total barium. This fraction follows the barite settling and dissolution rate model we described earlier for the Atlantic Ocean, confirming barite as the carrier of excess barium. We propose that these Ba_xs concentrations, which are higher in the Western Mediterranean than in the overall Atlantic, may represent an adaptation of the microplankton to the high densities and density gradients of surface Mediterranean waters.A mass-balance for Ba in the Mediterranean Sea shows: (1) that the atmospheric source must be much more important than previously reported: (2) that contrary to the Atlantic situation, the internal recycling of barium is of little importance compared to the flow of barium through the system.     

Dissolution of Barium from Barite in Sewage Sludges and Cultures of Desulfovibrio desulfuricans

High concentrations of total barium, ranging from 0.42 to 1.58 mg(middot)g(sup-1) (dry weight) were found in sludges of two sewage treatment plants near Florence, Italy. Barium concentrations in the suspended matter decreased as redox potential values changed from negative to positive. An anoxic sewage sludge sample was aerated, and 30% of the total barium was removed in 24 h. To demonstrate that barium was solubilized from barite by sulfate-reducing bacteria, a strain of Desulfovibrio desulfuricans was used to study the solubilization of barium from barite under laboratory conditions. During cell growth with different concentrations of barite from 0.01 to 0.3 g(middot)liter(sup-1) (the latter is the MIC) as the only source of sulfates in the cultures, the D. desulfuricans strain accumulated barium up to 0.58 (mu)g(middot)mg(sup-1) (dry weight). Three times the quantity of barium was dissolved by bacteria than in the uninoculated medium (control). The unexpectedly low concentration of soluble barium (1.2 mg of Ba(middot)liter(sup-1)) with respect to the quantity expected (109 mg of Ba(middot)liter(sup-1)), calculated on the basis of the free H(inf2)S evolved from the dissimilatory reduction of sulfate from barite, was probably due to the formation of other barium compounds, such as witherite (BaCO(inf3)) and the transient species barium sulfide (BaS). The D. desulfuricans strain, growing on barite, formed visible aggregates. Confocal microscopy analysis showed that aggregates consisted of bacteria and barite. After 3 days of incubation, several autofluorescent crystals surrounded by a dissolution halo were observed. The crystals were identified as BaS by comparison with the commercial compound.     

Barite in hydrothermal environments as a recorder of subseafloor processes: a multiple‐isotope study from the Loki's Castle vent field

Barite chimneys are known to form in hydrothermal systems where barium‐enriched fluids generated by leaching of the oceanic basement are discharged and react with seawater sulfate. They also form at cold seeps along continental margins, where marine (or pelagic) barite in the sediments is remobilized because of subseafloor microbial sulfate reduction. We test the possibility of using multiple sulfur isotopes (δ³⁴S, Δ³³S, ?³⁶S) of barite to identify microbial sulfate reduction in a hydrothermal system. In addition to multiple sulfur isotopes, we present oxygen (δ¹⁸O) and strontium (⁸⁷Sr/⁸⁶Sr) isotopes for one of numerous barite chimneys in a low‐temperature (~20 °C) venting area of the Loki's Castle black smoker field at the ultraslow‐spreading Arctic Mid‐Ocean Ridge (AMOR). The chemistry of the venting fluids in the barite field identifies a contribution of at least 10% of high‐temperature black smoker fluid, which is corroborated by ⁸⁷Sr/⁸⁶Sr ratios in the barite chimney that are less radiogenic than in seawater. In contrast, oxygen and multiple sulfur isotopes indicate that the fluid from which the barite precipitated contained residual sulfate that was affected by microbial sulfate reduction. A sulfate reduction zone at this site is further supported by the multiple sulfur isotopic composition of framboidal pyrite in the flow channel of the barite chimney and in the hydrothermal sediments in the barite field, as well as by low SO₄ and elevated H₂S concentrations in the venting fluids compared with conservative mixing values. We suggest that the mixing of ascending H₂‐ and CH₄‐rich high‐temperature fluids with percolating seawater fuels microbial sulfate reduction, which is subsequently recorded by barite formed at the seafloor in areas where the flow rate is sufficient. Thus, low‐temperature precipitates in hydrothermal systems are promising sites to explore the interactions between the geosphere and biosphere in order to evaluate the microbial impact on these systems.     

Sulfate-Reducing Bacteria Release Barium and Radium from Naturally Occurring Radioactive Material in Oil-Field Barite

Scale and sludge deposits formed during oil production can contain elevated levels of Ra, often coprecipitated with barium sulfate (barite). The potential for sulfate-reducing bacteria to release ²²⁶Ra and Ba (a Ra analog) from oil-field barite was evaluated. The concentration of dissolved Ba increased when samples containing pipe scale, tank sludge, or oil-field brine pond sediment were incubated with sulfate-reducing bacteria Desulfovibrio sp., Str LZK1, isolated from an oil-field brine pond. However, Ba release was not stoichiometric with sulfide production in oil-field samples, and < 0.1% of the Ba was released. Potential for the release of ²²⁶Ra was demonstrated, and the ²²⁶Ra release associated with sulfate-reducing activity was predictable from the amount of Ba released. As with Ba, only a fraction of the ²²⁶Ra expected from the amount of sulfide produced was released, and most of the Ra remained associated with the solid material.     

Determination of the sensitivity of bacteria to barium ions,a taxonomic marker of the genus <Emphasis Type="Italic">Pseudomonas</Emphasis>

Comparative efficacy of the determination of the sensitivity of bacterial cells to barium ions was evaluated on a synthetic nutrient medium, FMH agar, Mueller-Hinton agar, and AGV agar. The synthetic nutrient medium developed for this study contained L-proline and L-glutamine as the sole nitrogen and carbon source, which promoted growth of all Pseudomonas strains and ensured the minimal level of barium binding. The sensitivity of 80 strains belonging to 11 Pseudomonas species, including the type strains, as well as of 80 strains of 22 other bacterial species, was studied. The sensitivity of bacteria to barium ions was determined by using serial dilutions of barium chloride in the nutrient medium. The highest level of analytical sensitivity of pseudomonads to barium ions was determined on the synthetic nutrient medium: the minimal inhibitory concentration (MIC) values of barium chloride ranged from 0.5 to 6 g/L, the MIC₉₀ value was 2 g/L. At the same time, 86.1% of all strains of fluorescent Pseudomonas species produced fluorescein on the control BaCl₂-free synthetic nutrient medium. For representatives of other genera grown on all the studied nutrient media, the MIC values of barium chloride ranged from 20 to 50 g/L. The proposed method for determination of the sensitivity of bacteria to barium ions using the synthetic nutrient medium with 6 g/L of barium chloride as a criterion for the classification of barium-sensitive strains to the genus Pseudomonas is suitable for standardization.     

Carbon Flow of Heliobacteria Is Related More to Clostridia than to the Green Sulfur Bacteria

The recently discovered heliobacteria are the only Gram-positive photosynthetic bacteria that have been cultured. One of the unique features of heliobacteria is that they have properties of both the photosynthetic green sulfur bacteria (containing the type I reaction center) and Clostridia (forming heat-resistant endospores). Most of the previous studies of heliobacteria, which are strict anaerobes and have the simplest known photosynthetic apparatus, have focused on energy and electron transfer processes. It has been assumed that like green sulfur bacteria, the major carbon flow in heliobacteria is through the (incomplete) reductive (reverse) tricarboxylic acid cycle, whereas the lack of CO₂-enhanced growth has not been understood. Here, we report studies to fill the knowledge gap of heliobacterial carbon metabolism. We confirm that the CO₂-anaplerotic pathway is active during phototrophic growth and that isoleucine is mainly synthesized from the citramalate pathway. Furthermore, to our surprise, our results suggest that the oxidative (forward) TCA cycle is operative and more active than the previously reported reductive (reverse) tricarboxylic acid cycle. Both isotopomer analysis and activity assays suggest that citrate is produced by a putative (Re)-citrate synthase and then enters the oxidative (forward) TCA cycle. Moreover, in contrast to (Si)-citrate synthase, (Re)-citrate synthase produces a different isomer of 2-fluorocitrate that is not expected to inhibit the activity of aconitase.     

Both Forward and Reverse TCA Cycles Operate in Green Sulfur Bacteria

The anoxygenic green sulfur bacteria (GSBs) assimilate CO₂ autotrophically through the reductive (reverse) tricarboxylic acid (RTCA) cycle. Some organic carbon sources, such as acetate and pyruvate, can be assimilated during the phototrophic growth of the GSBs, in the presence of CO₂ or HCO₃⁻. It has not been established why the inorganic carbonis required for incorporating organic carbon for growth and how the organic carbons are assimilated. In this report, we probed carbon flux during autotrophic and mixotrophic growth of the GSB Chlorobaculum tepidum. Our data indicate the following: (a) the RTCA cycle is active during autotrophic and mixotrophic growth; (b) the flux from pyruvate to acetyl-CoA is very low and acetyl-CoA is synthesized through the RTCA cycle and acetate assimilation; (c) pyruvate is largely assimilated through the RTCA cycle; and (d) acetate can be assimilated via both of the RTCA as well as the oxidative (forward) TCA (OTCA) cycle. The OTCA cycle revealed herein may explain better cell growth during mixotrophic growth with acetate, as energy is generated through the OTCA cycle. Furthermore, the genes specific for the OTCA cycle are either absent or down-regulated during phototrophic growth, implying that the OTCA cycle is not complete, and CO₂ is required for the RTCA cycle to produce metabolites in the TCA cycle. Moreover, CO₂ is essential for assimilating acetate and pyruvate through the CO₂-anaplerotic pathway and pyruvate synthesis from acetyl-CoA.     

Community Structures and Distribution of Anaerobic Ammonium Oxidizing and nirS-Encoding Nitrite-Reducing Bacteria in Surface Sediments of the South China Sea

Anaerobic ammonium oxidation (anammox) and denitrification are two important processes responsible for nitrogen loss; monitoring of microbial communities carrying out these two processes offers a unique opportunity to understand the microbial nitrogen cycle. The aim of the current study was to characterize community structures and distribution of anammox and nirS-encoding nitrite-reducing bacteria in surface sediments of the northern South China Sea (SCS). The consistent phylogenetic results of three biomarkers of anammox bacteria, including 16S rRNA, hzo, and Scalindua-nirS genes, showed that Scalindua-like bacteria were the only anammox group presenting in surface sediments of the SCS. However, a relatively high micro-diversity was found within this group, including several SCS habitat-specific phylotypes, Candidatus “Scalindua zhenghei”. Comparing to 16S rRNA gene, hzo and Scalindua-nirS genes provided a relatively higher resolution to elucidate anammox bacteria. For the nirS-encoding nitrite-reducing bacteria, the detected nirS gene sequences were closely related to various marine nirS denitrifiers, especially those which originated from coastal and estuarine sediments with a much higher diversity than anammox bacteria. Anammox bacterial communities shifted along with the seawater depth, while nirS-encoding nitrite-reducing bacteria did not. Although nirS-encoding nitrite-reducing bacteria have a much higher abundance and diversity than anammox bacteria, they showed similar abundance variation patterns in research sites, suggesting the two microbial groups might be affected by the similar environmental factors. The significant correlations among the abundance of the two microbial groups with the molar ratio of NH₄ ⁺ to (NO₂ ^??+?NO₃ ^?), pH, and organic matters of sediments strongly supported this hypothesis.     

Synthesis and structure of two polymeric barium-oxydiacetates: [Ba(oda) · H2O]n and [Ba(Hoda)2]n (H2oda: oxydiacetic acid)

The reaction of barium carbonate or hydroxide with oxydiacetic acid leads to the self-assembly of two barium oxydiacetate polymers in good yield: [Ba(oda) · H₂O]_n (1) and [Ba(Hoda)₂]_n (2). The products have been characterized by elemental analysis, IR, TGA and single crystal X-ray diffraction studies. The central barium atom in each mononuclear fragment is nine-coordinate in 1 and 10-coordinate in 2. These fragments are bridged by carboxylato groups in anti-anti conformation and through H-bonds bonding interactions forming complex 3D networks.     

Comparative study of metabolism of the purple photosynthetic bacteria grown in the light and in the dark under anaerobic and aerobic conditions

For three species of anoxygenic phototrophic alphaproteobacteria differing in their reaction to oxygen and light, physiological characteristics (capacity for acetate assimilation, activity of the tricarboxylic acid (TCA) cycle enzymes, respiration, and the properties of the oxidase systems) were studied. Nonsulfur purple bacteria Rhodobacter sphaeroides, Rhodobaca bogoriensis, and aerobic anoxygenic phototrophic bacteria Roseinatronobacter thiooxidans were the subjects of investigation. All of these organisms were able to grow under aerobic conditions in the dark using the respiratory system with cytochrome aa ₃ as the terminal oxidase. They differed, however, in their capacity for growth in the light, bacteriochlorophyll synthesis, and regulation of activity of the TCA cycle enzymes. Oxygen suppressed bacteriochlorophyll synthesis by Rha. sphaeroides and Rbc. bogoriensis both in the dark and in the light. Bacteriochlorophyll synthesis in Rna. thiooxidans occurred only in the dark and was suppressed by light. The results on acetate assimilation by the studied strains reflected the degree of their adaptation to aerobic growth in the dark. Acetate assimilation by light-grown Rha. sphaeroides was significantly higher than by the dark-grown ones. Unlike Rha. sphaeroides, acetate assimilation by Rbc. bogoriensis in the light under anaerobic and aerobic conditions was much less dependent on the growth conditions. Aerobic acetate assimilation by all studied bacteria was promoted by light. In Rha. sphaeroides, activity of the TCA cycle enzymes increased significantly in the cells grown aerobically in the dark. In Rbc. bogoriensis, activity of most of the TCA cycle enzymes under aerobic conditions either decreased or remained unchanged. Our results confirm the origin of modern chemoorganotrophs from anoxygenic phototrophic bacteria. The evolution from anoxygenic photoorganotrophs to aerobic chemoorganotrophs included several stages: nonsulfur purple bacteria → nonsulfur purple bacteria similar to Rbc. bogoriensis → aerobic anoxygenic phototrophs → chemoorganotrophs.     

Some Enzymes and Properties of the Reductive Carboxylic Acid Cycle Are Present in the Green Alga Chlamydomonas reinhardtii F-60

The reductive carboxylic acid cycle, the autotrophic pathway of CO₂ assimilation in prokaryotes (photosynthetic and nonphotosynthetic autotrophic bacteria), was investigated in Chlamydomonas reinhardtii F-60, an algal mutant lacking a complete photosynthetic carbon reduction pathway (C₃) due to a deficiency in phosphoribulokinase. Evidence was obtained consistent with the presence of the reductive carboxylic acid cycle in F-60. This conclusion is based on the fact that: (a) acetate approximately doubled CO₂ fixation in whole cells (4 micromoles per milligram chlorophyll per hour) and in chloroplasts (32 nanomoles per milligram chlorophyll per hour); and (b) pyruvate synthase, α-ketoglutarate synthase, and ATP-citrate lyase, three indicators of the cycle, were found in cell-free extracts.     

Biochemical properties and phylogeny of hydroxypyruvate reductases from methanotrophic bacteria with different c<Subscript>1</Subscript>-assimilation pathways

In the aerobic methanotrophic bacteria Methylomicrobium alcaliphilum 20Z, Methylococcus capsulatus Bath, and Methylosinus trichosporium OB3b, the biochemical properties of hydroxypyruvate reductase (Hpr), an indicator enzyme of the serine pathway for assimilation of reduced C₁-compounds, were comparatively analyzed. The recombinant Hpr obtained by cloning and heterologous expression of the hpr gene in Escherichia coli catalyzed NAD(P)H-dependent reduction of hydroxypyruvate or glyoxylate, but did not catalyze the reverse reactions of D-glycerate or glycolate oxidation. The absence of the glycerate dehydrogenase activity in the methanotrophic Hpr confirmed a key role of the enzyme in utilization of C₁-compounds via the serine cycle. The enzyme from Ms. trichosporium OB3b realizing the serine cycle as a sole assimilation pathway had much higher special activity and affinity in comparison to Hpr from Mm. alcaliphilum 20Z and Mc. capsulatus Bath assimilating carbon predominantly via the ribulose monophosphate (RuMP) cycle. The hpr gene was found as part of gene clusters coding the serine cycle enzymes in all sequenced methanotrophic genomes except the representatives of the Verrucomicrobia phylum. Phylogenetic analyses revealed two types of Hpr: (i) Hpr of methanotrophs belonging to the Gammaproteobacteria class, which use the serine cycle along with the RuMP cycle, as well as of non-methylotrophic bacteria belonging to the Alphaproteobacteria class; (ii) Hpr of methylotrophs from Alpha- and Betaproteobacteria classes that use only the serine cycle and of non-methylotrophic representatives of Betaproteobacteria. The putative role and origin of hydroxypyruvate reductase in methanotrophs are discussed.     

Long-term field fertilization alters the diversity of autotrophic bacteria based on the ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) large-subunit genes in paddy soil

Carbon dioxide (CO₂) assimilation by autotrophic bacteria is an important process in the soil carbon cycle with major environmental implications. The long-term impact of fertilizer on CO₂ assimilation in the bacterial community of paddy soils remains poorly understood. To narrow this knowledge gap, the composition and abundance of CO₂-assimilating bacteria were investigated using terminal restriction fragment length polymorphism and quantitative PCR of the cbbL gene [that encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO)] in paddy soils. Soils from three stations in subtropical China were used. Each station is part of a long-term fertilization experiment with three treatments: no fertilizer (CK), chemical fertilizers (NPK), and NPK combined with rice straw (NPKM). At all of the stations, the cbbL-containing bacterial communities were dominated by facultative autotrophic bacteria such as Rhodopseudomonas palustris, Bradyrhizobium japonicum, and Ralstonia eutropha. The community composition in the fertilized soil (NPK and NPKM) was distinct from that in unfertilized soil (CK). The bacterial cbbL abundance (3–8?×?10⁸ copies g soil^?1) and RubisCO activity (0.40–1.76 nmol CO₂ g soil^?1 min^?1) in paddy soils were significantly positively correlated, and both increased with the addition of fertilizer. Among the measured soil parameters, soil organic carbon and pH were the most significant factors influencing the community composition, abundance, and activity of the cbbL-containing bacteria. These results suggest that long-term fertilization has a strong impact on the activity and community of cbbL-containing bacterial populations in paddy soils, especially when straw is combined with chemical fertilizers.     

Enzymes Induced in a Bacterium by Growth on Sodium Dodecyl Sulfate

Alkyl sulfatase was induced by growth on nutrient broth plus sodium dodecyl sulfate (SDS) in a bacterium we have designated Pseudomonas C12B. Measurement of the radioactivity of S³⁵O₄⁼ released from SDS³⁵ by the enzyme in cell-free extracts provided an effective assay technique. The barium chloranilate assay for release of SO₄⁼ from SDS was somewhat less sensitive but effective if analyzed at 332 mμ. This test was only approximately 55% as sensitive if analyzed at 530 mμ. The activity of the glyoxylate bypass enzymes, isocitrate lyase and malate synthetase, was significantly stimulated by growth of the bacteria on SDS as the sole carbon source, but not by growth on nutrient broth or nutrient broth plus SDS.     

Calvin Cycle Mutants of Photoheterotrophic Purple Nonsulfur Bacteria Fail To Grow Due to an Electron Imbalance Rather than Toxic Metabolite Accumulation

Purple nonsulfur bacteria grow photoheterotrophically by using light for energy and organic compounds for carbon and electrons. Disrupting the activity of the CO₂-fixing Calvin cycle enzyme, ribulose 1,5-bisphosphate carboxylase (RubisCO), prevents photoheterotrophic growth unless an electron acceptor is provided or if cells can dispose of electrons as H₂. Such observations led to the long-standing model wherein the Calvin cycle is necessary during photoheterotrophic growth to maintain a pool of oxidized electron carriers. This model was recently challenged with an alternative model wherein disrupting RubisCO activity prevents photoheterotrophic growth due to the accumulation of toxic ribulose-1,5-bisphosphate (RuBP) (D. Wang, Y. Zhang, E. L. Pohlmann, J. Li, and G. P. Roberts, J. Bacteriol. 193:3293-3303, 2011, http://dx.doi.org/10.1128/JB.00265-11). Here, we confirm that RuBP accumulation can impede the growth of Rhodospirillum rubrum (Rs. rubrum) and Rhodopseudomonas palustris (Rp. palustris) RubisCO-deficient (ΔRubisCO) mutants under conditions where electron carrier oxidation is coupled to H₂ production. However, we also demonstrate that Rs. rubrum and Rp. palustris Calvin cycle phosphoribulokinase mutants that cannot produce RuBP cannot grow photoheterotrophically on succinate unless an electron acceptor is provided or H₂ production is permitted. Thus, the Calvin cycle is still needed to oxidize electron carriers even in the absence of toxic RuBP. Surprisingly, Calvin cycle mutants of Rs. rubrum, but not of Rp. palustris, grew photoheterotrophically on malate without electron acceptors or H₂ production. The mechanism by which Rs. rubrum grows under these conditions remains to be elucidated.     

Impact of antimicrobial usage on the transmission dynamics of antimicrobial resistant bacteria among pigs

There is increasing evidence showing that antimicrobial consumption provides a powerful selective force that promotes the emergence of resistance in pathogenic, commensal as well as zoonotic bacteria in animals. The main aim of this study was to develop a modeling framework that can be used to assess the impact of antimicrobial usage in pigs on the emergence and transmission of resistant bacteria within a finisher pig farm. The transmission dynamics of drug-sensitive and drug-resistant bacteria among pigs in the herd were characterized by studying the local and global stability properties of steady state solutions of the system. Numerical simulations demonstrating the influence of factors such as initial prevalence of infection, presence of pre-existing antimicrobial resistant mutants, and frequency of treatment on predicted prevalence were performed. Sensitivity analysis revealed that two parameters had a huge influence on the predicted proportion of pigs carrying resistant bacteria: (a) the transmission coefficient between uninfected pigs and those infected with drug-resistant bacteria during treatment (β₂) and after treatment stops (β₃), and (b) the spontaneous clear-out rate of drug-resistant bacteria during treatment (γ₂) and immediately after treatment stops (γ₃). Control measures should therefore be geared towards reducing the magnitudes of β₂ and β₃ or increasing those of γ₂ and γ₃.     

In situ S‐isotope compositions of sulfate and sulfide from the 3.2 Ga Moodies Group,South Africa: A record of oxidative sulfur cycling

Sulfate minerals are rare in the Archean rock record and largely restricted to the occurrence of barite (BaSO₄). The origin of this barite remains controversially debated. The mass‐independent fractionation of sulfur isotopes in these and other Archean sedimentary rocks suggests that photolysis of volcanic aerosols in an oxygen‐poor atmosphere played an important role in their formation. Here, we report on the multiple sulfur isotopic composition of sedimentary anhydrite in the ca. 3.22 Ga Moodies Group of the Barberton Greenstone Belt, southern Africa. Anhydrite occurs, together with barite and pyrite, in regionally traceable beds that formed in fluvial settings. Variable abundances of barite versus anhydrite reflect changes in sulfate enrichment by evaporitic concentration across orders of magnitude in an arid, nearshore terrestrial environment, periodically replenished by influxes of seawater. The multiple S‐isotope compositions of anhydrite and pyrite are consistent with microbial sulfate reduction. S‐isotope signatures in barite suggest an additional oxidative sulfate source probably derived from continental weathering of sulfide possibly enhanced by microbial sulfur oxidation. Although depositional environments of Moodies sulfate minerals differ strongly from marine barite deposits, their sulfur isotopic composition is similar and most likely reflects a primary isotopic signature. The data indicate that a constant input of small portions of oxidized sulfur from the continents into the ocean may have contributed to the observed long‐term increase in Δ³³S_sulfate values through the Paleoarchean.