Michael P. Thorgersen

Eubacterium limosum is an abundant gut microbe that efficiently metabolizes lactate to produce the short-chain fatty acids, butyrate and acetate. The key oxidoreductase enzymes, formate dehydrogenase (FDH), carbon monoxide dehydrogenase (CODH) of the Wood-Ljungdahl pathway, pyruvate ferredoxin oxidoreductase (POR), and an electron-bifurcating tungsten-containing oxidoreductase (WOR) that detoxifies acetaldehyde produced as a byproduct of the POR reaction, are all utilized during lactate metabolism. Fermentative anaerobes typically utilize a single ferredoxin as an electron carrier for their primary pathways, which is replaced by flavodoxin under iron-limiting conditions. However, the E. limosum genome encodes two putative ferredoxins (Fd1 and Fd2) and three putative flavodoxins (Fld, Fld-like1, and Fld-like2). All five proteins were heterologously expressed in Escherichia coli , but the UV-visible absorption properties of purified Fld-like proteins 1 and 2 were not typical of canonical flavodoxins. Both POR and CODH reduced Fd1, Fd2, and Fld at comparable rates. Partially purified WOR and FDH had low activity using Fd1, Fd2, and Fld as electron carriers. With WOR, NAD-linked bifurcating activity could not be demonstrated with any of the electron carriers. Deletion mutants of E. limosum lacking Fd1, Fd2, or Fld exhibited similar lag phases during growth on glucose, and this increased on lactate for the Fld mutant but much less so for the Fd mutants. We conclude that E. limosum can utilize either Fd1, Fd2, or Fld as the primary redox protein, but that during growth on lactate, Fld plays a more prominent role than Fd1 or Fd2, even under iron-sufficient conditions. IMPORTANCE: Eubacterium limosum is an abundant gut microbe that is beneficial to human health due to its production of short-chain fatty acids particularly during growth on lactate. It is also of interest due to its ability to metabolize H ₂ /CO ₂ and C1 substrates. It was assumed that E. limosum , like other fermentative anaerobes, utilizes a single ferredoxin as an electron carrier during primary carbon metabolism and only utilizes flavodoxin under iron-limited conditions. However, we show here that this organism utilizes flavodoxin as its main electron carrier. This may be a significant advantage in the gut environment where competition with the host and with other gut microbes for iron is intense. This may have both biotechnological and health implications for this organism.

Eubacterium limosum is a dominant member of the human gut microbiome and produces short-chain fatty acids (SCFAs). These promote immune system function and inhibit inflammation, making this microbe important for human health. Lactate is a primary source of gut SCFAs but its utilization by E. limosum has not been explored. We show that E. limosum growing on lactate takes up added tungstate rather than molybdate and produces the SCFAs acetate and butyrate, but not propionate. The genes encoding an electron bifurcating, tungsten-containing oxidoreductase (WOR1) and a tungsten-containing formate dehydrogenase (FDH), along with an electron bifurcating lactate dehydrogenase (LCT), lactate permease, and enzymes of the propanediol pathway, are all up-regulated on lactate compared to growth on glucose. Lactate metabolism is controlled by a GntR-family repressor (LctR) and two global regulators, Rex and CcpA, where Rex in part controls W storage and tungstopyranopterin (Tuco) biosynthesis. Tuco-dependent riboswitches, along with CcpA, also control two iron transporters, consistent with the increased iron demand for many iron-containing enzymes, including WOR1 and FDH, involved in SCFA production. From intracellular aldehyde concentrations and the substrate specificity of WOR1, we propose that WOR1 is involved in detoxifying acetaldehyde produced during lactate degradation. Lactate to SCFA conversion by E. limosum is clearly highly tungstocentric and tungsten might be an overlooked micronutrient in the human microbiome and in overall human health.

Tungsten (W) is a metal that is generally thought to be seldom used in biology. We show here that a W-containing oxidoreductase (WOR) family is diverse and widespread in the microbial world. Surprisingly, WORs, along with the tungstate-specific transporter Tup, are abundant in the human gut microbiome, which contains 24 phylogenetically distinct WOR types. Two model gut microbes containing six types of WOR and Tup were shown to assimilate W. Two of the WORs were natively purified and found to contain W. The enzymes catalyzed the conversion of toxic aldehydes to the corresponding acid, with one WOR carrying out an electron bifurcation reaction coupling aldehyde oxidation to the simultaneous reduction of NAD⁺ and of the redox protein ferredoxin. Such aldehydes are present in cooked foods and are produced as antimicrobials by gut microbiome metabolism. This aldehyde detoxification strategy is dependent on the availability of W to the microbe. The functions of other WORs in the gut microbiome that do not oxidize aldehydes remain unknown. W is generally beyond detection (<6 parts per billion) in common foods and at picomolar concentrations in drinking water, suggesting that W availability could limit some gut microbial functions and might be an overlooked micronutrient. Significance The diverse microorganisms contained within the human gut are known to have significant effects on human health. Herein, we show that genes encoding members of the tungsten oxidoreductase (WOR) family of enzymes and a tungstate-specific transporter are prevalent in the human gut microbiome and metagenome. We demonstrate that two model gut microbes assimilate tungsten into multiple WOR enzymes and that some of these enzymes catalyze the conversion of gut aldehydes to the corresponding acid, likely as a detoxification strategy to remove these reactive compounds.

The processing of sediment to accurately characterize the spatially-resolved depth profiles of geophysical and geochemical properties along with signatures of microbial density and activity remains a challenge especially in complex contaminated environments. To provide site assessment for a larger study, we processed cores from two sediment boreholes from background and contaminated core sediments and surrounding groundwater from the ENIGMA Field Research Site at the United States Department of Energy (DOE) Oak Ridge Reservation (ORR). We compared fresh core sediments by depth to capture the changes in sediment structure, sediment minerals, biomass, and pore water geochemistry in terms of major and trace elements including contaminants, cations, anions, and organic acids. Soil porewater samples were matched to groundwater level, flow rate, and preferential flows and compared to homogenized groundwater-only samples from neighboring monitoring wells. This environmental systems approach provided detailed site-specific biogeochemical information from the various properties of subsurface media to reveal the influences of solid, liquid, and gas phases. Groundwater analysis of nearby wells only revealed high sulfate and nitrate concentrations while the same analysis using sediment pore water samples with depth was able to suggest areas high in sulfate- and nitrate- reducing bacteria based on their decreased concentration and production of reduced by-products that could not be seen in the groundwater samples. Positive correlations among porewater content, total organic carbon, trace metals and clay minerals revealed a more complicated relationship among contaminant, sediment texture, groundwater table, and biomass. This suggested that groundwater predominantly flowed through preferential paths with high flux and little mixing with water in the interstices of sediment particles, which could impact microbial activity. The abundant clay minerals with high surface area and high water-holding capacity of micro-pores of the fine clay rich layer suggest suppression of nutrient supply to microbes from the surface. The fluctuating capillary interface had high concentrations of Fe and Mn-oxides combined with trace elements including U, Th, Sr, Ba, Cu, and Co. This suggests the mobility of highly toxic elements, sediment structure, and biogeochemical factors are all linked together to impact microbial communities, emphasizing that solid interfaces play an important role in determining the abundance of bacteria in the sediments.

Summary: Anthropogenic nitrate contamination is a serious problem in many natural environments. Nitrate removal by microbial action is dependent on the metal molybdenum (Mo), which is required by nitrate reductase for denitrification and dissimilatory nitrate reduction to ammonium. The soluble form of Mo, molybdate (MoO ₄ ²⁻ ), is incorporated into and adsorbed by iron (Fe) and aluminium (Al) (oxy) hydroxide minerals. Herein we used Oak Ridge Reservation (ORR) as a model nitrate‐contaminated acidic environment to investigate whether the formation of Fe‐ and Al‐precipitates could impede microbial nitrate removal by depleting Mo. We demonstrate that Fe and Al mineral formation that occurs as the pH of acidic synthetic groundwater is increased, decreases soluble Mo to low picomolar concentrations, a process proposed to mimic environmental diffusion of acidic contaminated groundwater. Analysis of ORR sediments revealed recalcitrant Mo in the contaminated core that co‐occurred with Fe and Al, consistent with Mo scavenging by Fe/Al precipitates. Nitrate removal by ORR isolate Pseudomonas fluorescens N2A2 is virtually abolished by Fe/Al precipitate‐induced Mo depletion. The depletion of naturally occurring Mo in nitrate‐ and Fe/Al‐contaminated acidic environments like ORR or acid mine drainage sites has the potential to impede microbial‐based nitrate reduction thereby extending the duration of nitrate in the environment.

Chromium and uranium are highly toxic metals that contaminate many natural environments. We investigated their mechanisms of toxicity under anaerobic conditions using nitrate-reducing Pseudomonas stutzeri RCH2, which was originally isolated from a chromium-contaminated aquifer. A random barcode transposon site sequencing library of RCH2 was grown in the presence of the chromate oxyanion (Cr[VI]O42-) or uranyl oxycation (U[VI]O22+). Strains lacking genes required for a functional nitrate reductase had decreased fitness as both metals interacted with heme-containing enzymes required for the later steps in the denitrification pathway after nitrate is reduced to nitrite. Cr[VI]-resistance also required genes in the homologous recombination and nucleotide excision DNA repair pathways, showing that DNA is a target of Cr[VI] even under anaerobic conditions. The reduced thiol pool was also identified as a target of Cr[VI] toxicity and psest_2088, a gene of previously unknown function, was shown to have a role in the reduction of sulfite to sulfide. U[VI] resistance mechanisms involved exopolysaccharide synthesis and the universal stress protein UspA. As the first genome-wide fitness analysis of Cr[VI] and U[VI] toxicity under anaerobic conditions, this study provides new insight into the impact of Cr[VI] and U[VI] on an environmental isolate from a chromium contaminated site, as well as into the role of a ubiquitous protein, Psest_2088.

Cell suspensions of Pelosinus sp. strain UFO1 were previously shown, using spectroscopic analysis, to sequester uranium as U(IV) complexed with carboxyl and phosphoryl group ligands on proteins. The goal of our present study was to characterize the proteins involved in uranium binding. Virtually all of the uranium in UFO1 cells was associated with a heterodimeric protein, which was termed the uranium-binding complex (UBC). The UBC was composed of two S-layer domain proteins encoded by UFO1_4202 and UFO1_4203. Samples of UBC purified from the membrane fraction contained 3.3 U atoms/heterodimer, but significant amounts of phosphate were not detected. The UBC had an estimated molecular mass by gel filtration chromatography of 15 MDa, and it was proposed to contain 150 heterodimers (UFO1_4203 and UFO1_4202) and about 500 uranium atoms. The UBC was also the dominant extracellular protein, but when purified from the growth medium, it contained only 0.3 U atoms/heterodimer. The two genes encoding the UBC were among the most highly expressed genes within the UFO1 genome, and their expressions were unchanged by the presence or absence of uranium. Therefore, the UBC appears to be constitutively expressed and is the first line of defense against uranium, including by secretion into the extracellular medium. Although S-layer proteins were previously shown to bind U(VI), here we showed that U(IV) binds to S-layer proteins, we identified the proteins involved, and we quantitated the amount of uranium bound. IMPORTANCE Widespread uranium contamination from industrial sources poses hazards to human health and to the environment. Herein, we identified a highly abundant uranium-binding complex (UBC) from Pelosinus sp. strain UFO1. The complex makes up the primary protein component of the S-layer of strain UFO1 and binds 3.3 atoms of U(IV) per heterodimer. While other bacteria have been shown to bind U(VI) on their S-layer, we demonstrate here an example of U(IV) bound by an S-layer complex. The UBC provides a potential tool for the microbiological sequestration of uranium for the cleaning of contaminated environments.

Metal ion transport systems have been studied extensively, but the specificity of a given transporter is often unclear from amino acid sequence data alone. In this study, predicted Cu ²⁺ and Zn ²⁺ resistance systems in Pseudomonas stutzeri strain RCH2 are compared with those experimentally implicated in Cu ²⁺ and Zn ²⁺ resistance, as determined by using a DNA-barcoded transposon mutant library. Mutant fitness data obtained under denitrifying conditions are combined with regulon predictions to yield a much more comprehensive picture of Cu ²⁺ and Zn ²⁺ resistance in strain RCH2. The results not only considerably expand what is known about well-established metal ion exporters (CzcCBA, CzcD, and CusCBA) and their accessory proteins (CzcI and CusF), they also reveal that isolates with mutations in some predicted Cu ²⁺ resistance systems do not show decreased fitness relative to the wild type when exposed to Cu ²⁺ . In addition, new genes are identified that have no known connection to Zn ²⁺ ( corB , corC , Psest_3226, Psest_3322, and Psest_0618) or Cu ²⁺ resistance (Mrp antiporter subunit gene, Psest_2850, and Psest_0584) but are crucial for resistance to these metal cations. Growth of individual deletion mutants lacking corB , corC , Psest_3226, or Psest_3322 confirmed the observed Zn-dependent phenotypes. Notably, to our knowledge, this is the first time a bacterial homolog of TMEM165, a human gene responsible for a congenital glycosylation disorder, has been deleted and the resulting strain characterized. Finally, the fitness values indicate Cu ²⁺ - and Zn ²⁺ -based inhibition of nitrite reductase and interference with molybdenum cofactor biosynthesis for nitrate reductase. These results extend the current understanding of Cu ²⁺ and Zn ²⁺ efflux and resistance and their effects on denitrifying metabolism. IMPORTANCE In this study, genome-wide mutant fitness data in P. stutzeri RCH2 combined with regulon predictions identify several proteins of unknown function that are involved in resisting zinc and copper toxicity. For zinc, these include a member of the UPF0016 protein family that was previously implicated in Ca ²⁺ /H ⁺ antiport and a human congenital glycosylation disorder, CorB and CorC, which were previously linked to Mg ²⁺ transport, and Psest_3322 and Psest_0618, two proteins with no characterized homologs. Experiments using mutants lacking Psest_3226, Psest_3322, corB , corC , or czcI verified their proposed functions, which will enable future studies of these little-characterized zinc resistance determinants. Likewise, Psest_2850, annotated as an ion antiporter subunit, and the conserved hypothetical protein Psest_0584 are implicated in copper resistance. Physiological connections between previous studies and phenotypes presented here are discussed. Functional and mechanistic understanding of transport proteins improves the understanding of systems in which members of the same protein family, including those in humans, can have different functions.