Decoupling of respiration rates and abundance in marine prokaryoplankton.

The ocean-atmosphere exchange of CO2 largely depends on the balance between marine microbial photosynthesis and respiration. Despite vast taxonomic and metabolic diversity among marine planktonic bacteria and archaea (prokaryoplankton)1-3, their respiration usually is measured in bulk and treated as a 'black box' in global biogeochemical models4; this limits the mechanistic understanding of the global carbon cycle. Here, using a technology for integrated phenotype analyses and genomic sequencing of individual microbial cells, we show that cell-specific respiration rates differ by more than 1,000× among prokaryoplankton genera. The majority of respiration was found to be performed by minority members of prokaryoplankton (including the Roseobacter cluster), whereas cells of the most prevalent lineages (including Pelagibacter and SAR86) had extremely low respiration rates. The decoupling of respiration rates from abundance among lineages, elevated counts of proteorhodopsin transcripts in Pelagibacter and SAR86 cells and elevated respiration of SAR86 at night indicate that proteorhodopsin-based phototrophy3,5-7 probably constitutes an important source of energy to prokaryoplankton and may increase growth efficiency. These findings suggest that the dependence of prokaryoplankton on respiration and remineralization of phytoplankton-derived organic carbon into CO2 for its energy demands and growth may be lower than commonly assumed and variable among lineages.

Species-resolved, single-cell respiration rates reveal dominance of sulfate reduction in a deep continental subsurface ecosystem.

Rates of microbial processes are fundamental to understanding the significance of microbial impacts on environmental chemical cycling. However, it is often difficult to quantify rates or to link processes to specific taxa or individual cells, especially in environments where there are few cultured representatives with known physiology. Here, we describe the use of the redox-enzyme-sensitive molecular probe RedoxSensor™ Green to measure rates of anaerobic electron transfer physiology (i.e., sulfate reduction and methanogenesis) in individual cells and link those measurements to genomic sequencing of the same single cells. We used this method to investigate microbial activity in hot, anoxic, low-biomass (~103 cells mL-1) groundwater of the Death Valley Regional Flow System, California. Combining this method with electron donor amendment experiments and metatranscriptomics confirmed that the abundant spore formers including Candidatus Desulforudis audaxviator were actively reducing sulfate in this environment, most likely with acetate and hydrogen as electron donors. Using this approach, we measured environmental sulfate reduction rates at 0.14 to 26.9 fmol cell-1 h-1. Scaled to volume, this equates to a bulk environmental rate of ~103 pmol sulfate L-1 d-1, similar to potential rates determined with radiotracer methods. Despite methane in the system, there was no evidence for active microbial methanogenesis at the time of sampling. Overall, this method is a powerful tool for estimating species-resolved, single-cell rates of anaerobic metabolism in low-biomass environments while simultaneously linking genomes to phenomes at the single-cell level. We reveal active elemental cycling conducted by several species, with a large portion attributable to Ca. Desulforudis audaxviator.

Leptothrix ochracea genomes reveal potential for mixotrophic growth on Fe(II) and organic carbon.

Leptothrix ochracea creates distinctive iron-mineralized mats that carpet streams and wetlands. Easily recognized by its iron-mineralized sheaths, L. ochracea was one of the first microorganisms described in the 1800s. Yet it has never been isolated and does not have a complete genome sequence available, so key questions about its physiology remain unresolved. It is debated whether iron oxidation can be used for energy or growth and if L. ochracea is an autotroph, heterotroph, or mixotroph. To address these issues, we sampled L. ochracea-rich mats from three of its typical environments (a stream, wetlands, and a drainage channel) and reconstructed nine high-quality genomes of L. ochracea from metagenomes. These genomes contain iron oxidase genes cyc2 and mtoA, showing that L. ochracea has the potential to conserve energy from iron oxidation. Sox genes confer potential to oxidize sulfur for energy. There are genes for both carbon fixation (RuBisCO) and utilization of sugars and organic acids (acetate, lactate, and formate). In silico stoichiometric metabolic models further demonstrated the potential for growth using sugars and organic acids. Metatranscriptomes showed a high expression of genes for iron oxidation; aerobic respiration; and utilization of lactate, acetate, and sugars, as well as RuBisCO, supporting mixotrophic growth in the environment. In summary, our results suggest that L. ochracea has substantial metabolic flexibility. It is adapted to iron-rich, organic carbon-containing wetland niches, where it can thrive as a mixotrophic iron oxidizer by utilizing both iron oxidation and organics for energy generation and both inorganic and organic carbon for cell and sheath production. IMPORTANCE: Winogradsky's observations of L. ochracea led him to propose autotrophic iron oxidation as a new microbial metabolism, following his work on autotrophic sulfur-oxidizers. While much culture-based research has ensued, isolation proved elusive, so most work on L. ochracea has been based in the environment and in microcosms. Meanwhile, the autotrophic Gallionella became the model for freshwater microbial iron oxidation, while heterotrophic and mixotrophic iron oxidation is not well-studied. Ecological studies have shown that Leptothrix overtakes Gallionella when dissolved organic carbon content increases, demonstrating distinct niches. This study presents the first near-complete genomes of L. ochracea, which share some features with autotrophic iron oxidizers, while also incorporating heterotrophic metabolisms. These genome, metabolic modeling, and transcriptome results give us a detailed metabolic picture of how the organism may combine lithoautotrophy with organoheterotrophy to promote Fe oxidation and C cycling and drive many biogeochemical processes resulting from microbial growth and iron oxyhydroxide formation in wetlands.

Phosphorus Interactions with Iron in Undisturbed and Disturbed Arctic Tundra Ecosystems.

Phosphorus (P) limitation often constrains biological processes in Arctic tundra ecosystems. Although adsorption to soil minerals may limit P bioavailability and export from soils into aquatic systems, the contribution of mineral phases to P retention in Arctic tundra is poorly understood. Our objective was to use X-ray absorption spectroscopy to characterize P speciation and associations with soil minerals along hillslope toposequences and in undisturbed and disturbed low-lying wet sedge tundra on the North Slope, AK. Biogenic mats comprised of short-range ordered iron (Fe) oxyhydroxides were prevalent in undisturbed wet sedge meadows. Upland soils and pond sediments impacted by gravel mining or thermokarst lacked biogenic Fe mats and were comparatively iron poor. Phosphorus was primarily contained in organic compounds in hillslope soils but associated with Fe(III) oxyhydroxides in undisturbed wet sedge meadows and calcium (Ca) in disturbed pond sediments. We infer that phosphate mobilized through organic decomposition binds to Fe(III) oxyhydroxides in wet sedge, but these associations are disrupted by physical disturbance that removes Fe mats. Increasing disturbances of the Arctic tundra may continue to alter the mineralogical composition of soils at terrestrial-aquatic interfaces and binding mechanisms that could inhibit or promote transport of bioavailable P from soils to aquatic ecosystems.

Enhancing hydrogen-dependent growth of and carbon dioxide fixation by Clostridium ljungdahlii through nitrate supplementation.

Synthesis gas (syngas) fermentation via the Wood-Ljungdahl pathway is receiving growing attention as a possible platform for the fixation of CO2 and renewable production of fuels and chemicals. However, the pathway operates near the thermodynamic limit of life, resulting in minimal adenosine triphosphate (ATP) production and long doubling times. This calls into question the feasibility of producing high-energy compounds at industrially relevant levels. In this study, we investigated the possibility of co-utilizing nitrate as an inexpensive additional electron acceptor to enhance ATP production during H2 -dependent growth of Clostridium ljungdahlii, Moorella thermoacetica, and Acetobacterium woodii. In contrast to other acetogens tested, growth rate and final biomass titer were improved for C. ljungdahlii growing on a mixture of H2 and CO2 when supplemented with nitrate. Transcriptomic analysis, 13CO2 labeling, and an electron balance were used to understand how electron flux was partitioned between CO2 and nitrate. We further show that, with nitrate supplementation, the ATP/adenosine diphosphate (ADP) ratio and acetyl-CoA pools were increased by fivefold and threefold, respectively, suggesting that this strategy could be useful for the production of ATP-intensive heterologous products from acetyl-CoA. Finally, we propose a pathway for enhanced ATP production from nitrate and use this as a basis to calculate theoretical yields for a variety of products. This study demonstrates a viable strategy for the decoupling of ATP production from carbon dioxide fixation, which will serve to significantly improve the CO2 fixation rate and the production metrics of other chemicals from CO2 and H2 in this host.

Integrated bioprocess for conversion of gaseous substrates to liquids.

In the quest for inexpensive feedstocks for the cost-effective production of liquid fuels, we have examined gaseous substrates that could be made available at low cost and sufficiently large scale for industrial fuel production. Here we introduce a new bioconversion scheme that effectively converts syngas, generated from gasification of coal, natural gas, or biomass, into lipids that can be used for biodiesel production. We present an integrated conversion method comprising a two-stage system. In the first stage, an anaerobic bioreactor converts mixtures of gases of CO2 and CO or H2 to acetic acid, using the anaerobic acetogen Moorella thermoacetica The acetic acid product is fed as a substrate to a second bioreactor, where it is converted aerobically into lipids by an engineered oleaginous yeast, Yarrowia lipolytica We first describe the process carried out in each reactor and then present an integrated system that produces microbial oil, using synthesis gas as input. The integrated continuous bench-scale reactor system produced 18 g/L of C16-C18 triacylglycerides directly from synthesis gas, with an overall productivity of 0.19 g⋅L(-1)⋅h(-1) and a lipid content of 36%. Although suboptimal relative to the performance of the individual reactor components, the presented integrated system demonstrates the feasibility of substantial net fixation of carbon dioxide and conversion of gaseous feedstocks to lipids for biodiesel production. The system can be further optimized to approach the performance of its individual units so that it can be used for the economical conversion of waste gases from steel mills to valuable liquid fuels for transportation.

Modelling Thermally Induced Non-Equilibrium Gas Flows by Coupling Kinetic and Extended Thermodynamic Methods.

Thermally induced non-equilibrium gas flows have been simulated in the present study by coupling kinetic and extended thermodynamic methods. Three different types of thermally induced gas flows, including temperature-discontinuity- and temperature-gradient-induced flows and radiometric flow, have been explored in the transition regime. The temperature-discontinuity-induced flow case has shown that as the Knudsen number increases, the regularised 26 (R26) moment equation system will gradually loss its accuracy and validation. A coupling macro- and microscopic approach is employed to overcome these problems. The R26 moment equations are used at the macroscopic level for the bulk flow region, while the kinetic equation associated with the discrete velocity method (DVM) is applied to describe the gas close to the wall at the microscopic level, which yields a hybrid DVM/R26 approach. The numerical results have shown that the hybrid DVM/R26 method can be faithfully used for the thermally induced non-equilibrium flows. The proposed scheme not only improves the accuracy of the results in comparison with the R26 equations, but also extends their capability with a wider range of Knudsen numbers. In addition, the hybrid scheme is able to reduce the computational memory and time cost compared to the DVM.

Rediverting carbon flux in Clostridium ljungdahlii using CRISPR interference (CRISPRi).

Clostridium ljungdahlii has emerged as an attractive candidate for the bioconversion of synthesis gas (CO, CO2, H2) to a variety of fuels and chemicals through the Wood-Ljungdahl pathway. However, metabolic engineering and pathway elucidation in this microbe is limited by the lack of genetic tools to downregulate target genes. To overcome this obstacle, here we developed an inducible CRISPR interference (CRISPRi) system for C. ljungdahlii that enables efficient (> 94%) transcriptional repression of several target genes, both individually and in tandem. We then applied CRISPRi in a strain engineered for 3-hydroxybutyrate (3HB) production to examine targets for increasing carbon flux toward the desired product. Downregulating phosphotransacetylase (pta) with a single sgRNA led to a 97% decrease in enzyme activity and a 2.3-fold increase in titer during heterotrophic growth. However, acetate production still accounted for 40% of the carbon flux. Repression of aldehyde:ferredoxin oxidoreductase (aor2), another potential route for acetate production, led to a 5% reduction in acetate flux, whereas using an additional sgRNA targeted to pta reduced the enzyme activity to 0.7% of the wild-type level, and further reduced acetate production to 25% of the carbon flux with an accompanying increase in 3HB titer and yield. These results demonstrate the utility of CRISPRi for elucidating and controlling carbon flow in C. ljungdahlii.

The role of iron-oxidizing bacteria in biocorrosion: a review.

Lithotrophic iron-oxidizing bacteria depend on reduced iron, Fe(II), as their primary energy source, making them natural candidates for growing in association with steel infrastructure and potentially contributing to microbially influenced corrosion (MIC). This review summarizes recent work on the role of iron-oxidizing bacteria (FeOB) in MIC. By virtue of producing complex 3-dimensional biofilms that result from the accumulation of iron-oxides, FeOB may aid in the colonization of steel surfaces by other microbes involved in MIC. Evidence points to a successional pattern occurring whereby FeOB are early colonizers of mild steel (MS), followed by sulfate-reducing bacteria and other microbes, although studies of aged corrosion products indicate that FeOB do establish a long-term presence. There is evidence that only specific clades of FeOB, with unique adaptations for growing on steel surfaces are part of the MIC community. These are discussed in the context of the larger MIC microbiome.

Bringing microbial diversity into focus: high-resolution analysis of iron mats from the Lo'ihi Seamount.

Thirty kilometers south of the island of Hawai'i lies the Lo'ihi Seamount, an active submarine volcano that hosts a network of low-temperature hydrothermal vents enriched in ferrous iron that supports extensive microbial mats. These mats, which can be a half a meter deep, are composed of ferric iron bound to organic polymers - the metabolic byproduct of iron-oxidizing Zetaproteobacteria. Though the role of Zetaproteobacteria in mat formation is well established, we have a limited understanding of how differences in diversity are related to mat morphology. We used Minimum Entropy Decomposition and ZetaOtu classification to demonstrate cryptic diversity between closely related Zetaproteobacteria while showing habitat and geographic specificity. Veiled mats, common structures at Lo'ihi, exhibit distinct community composition and contain diversity not detected in other mat types, including specific Zetaproteobacteria and an unclassified Gammaproteobacteria. Our analyses also indicate that diversity can change dramatically across small spatial transects from points of active venting, yet we found comparatively few differences between major sampling sites. This study provides a better picture of the microbiome responsible for iron mat production at Lo'ihi and has broad implications for our understanding of these globally distributed communities.

Peeking under the Iron Curtain: Development of a Microcosm for Imaging the Colonization of Steel Surfaces by Mariprofundus sp. Strain DIS-1, an Oxygen-Tolerant Fe-Oxidizing Bacterium.

Microbially influenced corrosion (MIC) is a major cause of damage to steel infrastructure in the marine environment. Despite their ability to grow directly on Fe(II) released from steel, comparatively little is known about the role played by neutrophilic iron-oxidizing bacteria (FeOB). Recent work has shown that FeOB grow readily on mild steel (1018 MS) incubated in situ or as a substrate for pure cultures in vitro; however, details of how they colonize steel surfaces are unknown yet are important for understanding their effects. In this study, we combine a novel continuously upwelling microcosm with confocal laser scanning microscopy (CLSM) to determine the degree of colonization of 1018 MS by the marine FeOB strain DIS-1. 1018 MS coupons were incubated with sterile seawater (pH 8) inoculated with strain DIS-1. Incubations were performed both under oxic conditions and in an anoxic-to-oxic gradient. Following incubations of 1 to 10 days, the slides were removed from the microcosms and stained to visualize both cells and stalk structures. Stained coupons were visualized by CLSM after being mounted in a custom frame to preserve the three-dimensional structure of the biofilm. The incubation of 1018 MS coupons with strain DIS-1 under oxic conditions resulted in initial attachment of cells within 2 days and nearly total coverage of the coupon with an ochre film within 5 days. CLSM imaging revealed a nonadherent biofilm composed primarily of the Fe-oxide stalks characteristic of strain DIS-1. When incubated with elevated concentrations of Fe(II), DIS-1 colonization of 1018 MS was inhibited. IMPORTANCE: These experiments describe the growth of a marine FeOB in a continuous culture system and represent direct visualizations of steel colonization by FeOB. We anticipate that these experiments will lay the groundwork for studying the mechanisms by which FeOB colonize steel and help to elucidate the role played by marine FeOB in MIC. These observations of the interaction between an FeOB, strain DIS-1, and steel suggest that this experimental system will provide a useful model for studying the interactions between microbes and solid substrates.

An SOS Regulon under Control of a Noncanonical LexA-Binding Motif in the Betaproteobacteria.

UNLABELLED: The SOS response is a transcriptional regulatory network governed by the LexA repressor that activates in response to DNA damage. In the Betaproteobacteria, LexA is known to target a palindromic sequence with the consensus sequence CTGT-N8-ACAG. We report the characterization of a LexA regulon in the iron-oxidizing betaproteobacterium Sideroxydans lithotrophicus. In silico and in vitro analyses show that LexA targets six genes by recognizing a binding motif with the consensus sequence GAACGaaCGTTC, which is strongly reminiscent of the Bacillus subtilis LexA-binding motif. We confirm that the closely related Gallionella capsiferriformans shares the same LexA-binding motif, and in silico analyses indicate that this motif is also conserved in the Nitrosomonadales and the Methylophilales. Phylogenetic analysis of LexA and the alpha subunit of DNA polymerase III (DnaE) reveal that the organisms harboring this noncanonical LexA form a compact taxonomic cluster within the Betaproteobacteria. However, their lexA gene is unrelated to the standard Betaproteobacteria lexA, and there is evidence of its spread through lateral gene transfer. In contrast to other reported cases of noncanonical LexA-binding motifs, the regulon of S. lithotrophicus is comparable in size and function to that of many other Betaproteobacteria, suggesting that a convergent SOS regulon has reevolved under the control of a new LexA protein. Analysis of the DNA-binding domain of S. lithotrophicus LexA reveals little sequence similarity with that of other LexA proteins targeting similar binding motifs, suggesting that network structure may limit site evolution or that structural constrains make the B. subtilis-type motif an optimal interface for multiple LexA sequences. IMPORTANCE: Understanding the evolution of transcriptional systems enables us to address important questions in microbiology, such as the emergence and transfer potential of different regulatory systems to regulate virulence or mediate responses to stress. The results reported here constitute the first characterization of a noncanonical LexA protein regulating a standard SOS regulon. This is significant because it illustrates how a complex transcriptional program can be put under the control of a novel transcriptional regulator. Our results also reveal a substantial degree of plasticity in the LexA recognition domain, raising intriguing questions about the space of protein-DNA interfaces and the specific evolutionary constrains faced by these elements.

Microbial iron oxidation in the Arctic tundra and its implications for biogeochemical cycling.

The role that neutrophilic iron-oxidizing bacteria play in the Arctic tundra is unknown. This study surveyed chemosynthetic iron-oxidizing communities at the North Slope of Alaska near Toolik Field Station (TFS) at Toolik Lake (lat 68.63, long -149.60). Microbial iron mats were common in submerged habitats with stationary or slowly flowing water, and their greatest areal extent is in coating plant stems and sediments in wet sedge meadows. Some Fe-oxidizing bacteria (FeOB) produce easily recognized sheath or stalk morphotypes that were present and dominant in all the mats we observed. The cool water temperatures (9 to 11°C) and reduced pH (5.0 to 6.6) at all sites kinetically favor microbial iron oxidation. A microbial survey of five sites based on 16S rRNA genes found a predominance of Proteobacteria, with Betaproteobacteria and members of the family Comamonadaceae being the most prevalent operational taxonomic units (OTUs). In relative abundance, clades of lithotrophic FeOB composed 5 to 10% of the communities. OTUs related to cyanobacteria and chloroplasts accounted for 3 to 25% of the communities. Oxygen profiles showed evidence for oxygenic photosynthesis at the surface of some mats, indicating the coexistence of photosynthetic and FeOB populations. The relative abundance of OTUs belonging to putative Fe-reducing bacteria (FeRB) averaged around 11% in the sampled iron mats. Mats incubated anaerobically with 10 mM acetate rapidly initiated Fe reduction, indicating that active iron cycling is likely. The prevalence of iron mats on the tundra might impact the carbon cycle through lithoautotrophic chemosynthesis, anaerobic respiration of organic carbon coupled to iron reduction, and the suppression of methanogenesis, and it potentially influences phosphorus dynamics through the adsorption of phosphorus to iron oxides.

New Insight into Microbial Iron Oxidation as Revealed by the Proteomic Profile of an Obligate Iron-Oxidizing Chemolithoautotroph.

Microaerophilic, neutrophilic, iron-oxidizing bacteria (FeOB) grow via the oxidation of reduced Fe(II) at or near neutral pH, in the presence of oxygen, making them relevant in numerous environments with elevated Fe(II) concentrations. However, the biochemical mechanisms for Fe(II) oxidation by these neutrophilic FeOB are unknown, and genetic markers for this process are unavailable. In the ocean, microaerophilic microorganisms in the genus Mariprofundus of the class Zetaproteobacteria are the only organisms known to chemolithoautotrophically oxidize Fe and concurrently biomineralize it in the form of twisted stalks of iron oxyhydroxides. The aim of this study was to identify highly expressed proteins associated with the electron transport chain of microaerophilic, neutrophilic FeOB. To this end, Mariprofundus ferrooxydans PV-1 was cultivated, and its proteins were extracted, assayed for redox activity, and analyzed via liquid chromatography-tandem mass spectrometry for identification of peptides. The results indicate that a cytochrome c4, cbb3-type cytochrome oxidase subunits, and an outer membrane cytochrome c were among the most highly expressed proteins and suggest an involvement in the process of aerobic, neutrophilic bacterial Fe oxidation. Proteins associated with alternative complex III, phosphate transport, carbon fixation, and biofilm formation were abundant, consistent with the lifestyle of Mariprofundus.

Iron-oxidizing bacteria: an environmental and genomic perspective.

In the 1830s, iron bacteria were among the first groups of microbes to be recognized for carrying out a fundamental geological process, namely the oxidation of iron. Due to lingering questions about their metabolism, coupled with difficulties in culturing important community members, studies of Fe-oxidizing bacteria (FeOB) have lagged behind those of other important microbial lithotrophic metabolisms. Recently, research on lithotrophic, oxygen-dependent FeOB that grow at circumneutral pH has accelerated. This work is driven by several factors including the recognition by both microbiologists and geoscientists of the role FeOB play in the biogeochemistry of iron and other elements. The isolation of new strains of obligate FeOB allowed a better understanding of their physiology and phylogeny and the realization that FeOB are abundant at certain deep-sea hydrothermal vents. These ancient microorganisms offer new opportunities to learn about fundamental biological processes that can be of practical importance.

Iron cycling at corroding carbon steel surfaces.

Surfaces of carbon steel (CS) exposed to mixed cultures of iron-oxidizing bacteria (FeOB) and dissimilatory iron-reducing bacteria (FeRB) in seawater media under aerobic conditions were rougher than surfaces of CS exposed to pure cultures of either type of microorganism. The roughened surface, demonstrated by profilometry, is an indication of loss of metal from the surface. In the presence of CS, aerobically grown FeOB produced tight, twisted helical stalks encrusted with iron oxides. When CS was exposed anaerobically in the presence of FeRB, some surface oxides were removed. However, when the same FeOB and FeRB were grown together in an aerobic medium, FeOB stalks were less encrusted with iron oxides and appeared less tightly coiled. These observations suggest that iron oxides on the stalks were reduced and solubilized by the FeRB. Roughened surfaces of CS and denuded stalks were replicated with culture combinations of different species of FeOB and FeRB under three experimental conditions. Measurements of electrochemical polarization resistance established different rates of corrosion of CS in aerobic and anaerobic media, but could not differentiate rate differences between sterile controls and inoculated exposures for a given bulk concentration of dissolved oxygen. Similarly, total iron in the electrolyte could not be used to differentiate treatments. The experiments demonstrate the potential for iron cycling (oxidation and reduction) on corroding CS in aerobic seawater media.

Microbial iron cycling is prevalent in water-logged Alaskan Arctic tundra habitats, but sensitive to disturbance.

Water logged habitats in continuous permafrost regions provide extensive oxic-anoxic interface habitats for iron cycling. The iron cycle interacts with the methane and phosphorus cycles, and is an important part of tundra biogeochemistry. Our objective was to characterize microbial communities associated with the iron cycle within natural and disturbed habitats of the Alaskan Arctic tundra. We sampled aquatic habitats within natural, undisturbed and anthropogenically disturbed areas and sequenced the 16S rRNA gene to describe the microbial communities, then supported these results with process rate and geochemical measurements. Undisturbed habitats have microbial communities that are significantly different than disturbed habitats. Microbial taxa known to participate in the iron and methane cycles are significantly associated with natural habitats, whereas they are not significantly associated with disturbed sites. Undisturbed habitats have significantly higher extractable iron and are more acidic than disturbed habitats sampled. Iron reduction is not measurable in disturbed aquatic habitats and is not stimulated by the addition of biogenic iron mats. Our study highlights the prevalence of Fe-cycling in undisturbed water-logged habitats, and demonstrates that anthropogenic disturbance of the tundra, due to legacy gravel mining, alters the microbiology of aquatic habitats and disrupts important biogeochemical cycles in the Arctic tundra.

Psychological Distress, Burnout, and Business Student Turnover: The Role of Resilience as a Coping Mechanism.

This study's purpose is to examine whether resilience, conceptualized by Connor and Davidson (2003) as one's capacity to persevere and rebound under adversity, was a potential mitigating and/or moderating factor in the dynamic between both psychological distress and academic burnout, and student attrition. We concurrently distributed a survey containing a series of psychometric instruments to a convenience sample of 1,119 students pursuing various business majors at four geographically diverse U.S. universities. Via structural equations modeling analysis, we measured the associations between psychological distress, academic burnout, and departure intentions, and investigated whether student resilience levels are associated with lower distress, burnout, and departure intentions levels. The results indicated significant positive associations between psychological distress and each of the elements of academic burnout, and significant positive associations between the academic burnout elements and departure intentions. However, while resilience did not moderate those associations, it did attenuate them through its direct negative associations with both psychological distress and the cynicism and academic inefficacy elements of academic burnout. Based on these findings, we discuss implications for business educators seeking to enhance individual resilience levels as a coping strategy to combat voluntary student turnover, and better prepare students for the demands of the workplace.

Yoga vs Cognitive Processing Therapy for Military Sexual Trauma-Related Posttraumatic Stress Disorder: A Randomized Clinical Trial.

Importance: First-line treatment for posttraumatic stress disorder (PTSD) in the US Department of Veterans Affairs (VA), ie, trauma-focused therapy, while effective, is limited by low treatment initiation, high dropout, and high treatment refraction. Objective: To evaluate the effectiveness of Trauma Center Trauma-Sensitive Yoga (TCTSY) vs first-line cognitive processing therapy (CPT) in women veterans with PTSD related to military sexual trauma (MST) and the hypothesis that PTSD outcomes would differ between the interventions. Design, Setting, and Participants: This multisite randomized clinical trial was conducted from December 1, 2015, to April 30, 2022, within 2 VA health care systems located in the southeast and northwest. Women veterans aged 22 to 71 years with MST-related PTSD were enrolled and randomized to TCTSY or CPT. Interventions: The TCTSY intervention (Hatha-style yoga focusing on interoception and empowerment) consisted of 10 weekly, 60-minute group sessions, and the CPT intervention (cognitive-based therapy targeting modification of negative posttraumatic thoughts) consisted of 12 weekly, 90-minute group sessions. Main Outcome and Measures: Sociodemographic data were collected via self-report survey. The primary outcome, PTSD symptom severity, was assessed using the Clinician-Administered PTSD Scale for DSM-5 (CAPS-5) and PTSD Checklist for DSM-5 (PCL-5). Assessments were conducted at baseline, midintervention, 2 weeks post intervention, and 3 months post intervention. Results: Of 200 women veterans who consented to participate, the intent-to-treat sample comprised 131 participants (mean [SD] age, 48.2 [11.2] years), with 72 randomized to TCTSY and 59 randomized to CPT. Treatment was completed by 47 participants (65.3%) in the TCTSY group and 27 (45.8%) in the CPT group, a 42.6% higher treatment completion rate in the TCTSY group (P = .03). Both treatment groups improved over time on the CAPS-5 (mean [SD] scores at baseline: 36.73 [8.79] for TCTSY and 35.52 [7.49] for CPT; mean [SD] scores at 3 months: 24.03 [11.55] for TCTSY and 22.15 [13.56]) and the PCL-5 (mean [SD] scores at baseline: 49.62 [12.19] for TCTSY and 48.69 [13.62] for CPT; mean [SD] scores at 3 months: 36.97 [17.74] for TCTSY and 31.76 [12.47]) (P < .001 for time effects). None of the group effects or group-by-time effects were significant. Equivalence analyses of change scores were not significantly different between the TCTSY and CPT groups, and the two one-sided test intervals fell within the equivalence bounds of plus or minus 10 for CAPS-5 for all follow-up time points. Conclusions and Relevance: In this comparative effectiveness randomized clinical trial, TCTSY was equivalent to CPT in reducing PTSD symptom severity, with both groups improving significantly. The higher treatment completion rate for TCTSY indicates its higher acceptability as an effective and acceptable PTSD treatment for women veterans with PTSD related to MST that could address current VA PTSD treatment limitations. Trial Registration: ClinicalTrials.gov Identifier: NCT02640690.

Biogeochemistry and microbiology of microaerobic Fe(II) oxidation.

Today high Fe(II) environments are relegated to oxic-anoxic habitats with opposing gradients of O2 and Fe(II); however, during the late Archaean and early Proterozoic eons, atmospheric O2 concentrations were much lower and aqueous Fe(II) concentrations were significantly higher. In current Fe(II)-rich environments, such as hydrothermal vents, mudflats, freshwater wetlands or the rhizosphere, rusty mat-like deposits are common. The presence of abundant biogenic microtubular or filamentous iron oxyhydroxides readily reveals the role of FeOB (iron-oxidizing bacteria) in iron mat formation. Cultivation and cultivation-independent techniques, confirm that FeOB are abundant in these mats. Despite remarkable similarities in morphological characteristics between marine and freshwater FeOB communities, the resident populations of FeOB are phylogenetically distinct, with marine populations related to the class Zetaproteobacteria, whereas freshwater populations are dominated by members of the Gallionallaceae, a family within the Betaproteobacteria. Little is known about the mechanism of how FeOB acquire electrons from Fe(II), although it is assumed that it involves electron transfer from the site of iron oxidation at the cell surface to the cytoplasmic membrane. Comparative genomics between freshwater and marine strains reveals few shared genes, except for a suite of genes that include a class of molybdopterin oxidoreductase that could be involved in iron oxidation via extracellular electron transport. Other genes are implicated as well, and the overall genomic analysis reveals a group of organisms exquisitely adapted for growth on iron.