A novel chemoheterotrophic iron-reducing micro-organism, designated as strain LSZ-M11000T, was isolated from sediment of the Marianas Trench. Phylogenetic analysis based on the 16S rRNA gene revealed that strain LSZ-M11000T belonged to genus Tepidibacillus, with 97 % identity to that of Tepidibacillus fermentans STGHT, a mesophilic bacterium isolated from the Severo-Stavropolskoye underground gas storage facility in Russia. The polar lipid profile of strain LSZ-M11000T consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, as well as other unidentified phospholipids and lipids. The major fatty acids were C16 : 0 (28.4 %), C18 : 0 (15.8 %), iso-C15 : 0 (12.9 %), and anteiso-C15 : 0 (12.0 %). Strain LSZ-M11000T had no menaquinone. Genome sequencing revealed that the genome size of strain LSZ-M11000T was 2.97 Mb and the DNA G+C content was 37.9 mol%. The average nucleotide identity values between strain LSZ-M11000T and its close phylogenetic relatives, Tepidibacillus fermentans STGHT and Tepidibacillus decaturensis Z9T, were 76.4 and 72.6 %, respectively. The corresponding DNA-DNA hybridization estimates were 20.9 and 23.4 %, respectively. Cells of strain LSZ-M11000T were rod-shaped (1.0-1.5×0.3-0.5 µm). Using pyruvate as an electron donor, it was capable of reducing KMnO4, MnO2, As(V), NaNO3, NaNO2, Na2SO4, Na2S2O3, and K2Cr2O7. Based on phenotypic, genotypic, and phylogenetic evidence, strain LSZ-M11000T is proposed to be a novel strain of the genus Tepidibacillus, for which the name Tepdibacillus marianensis is proposed. The type strain is LSZ-M11000T (=CCAM 1008T=JCM 39431T).

The Fe content and the morphometry of asbestos are two major factors linked to its toxicity. This study explored the use of microbe-mineral interactions between asbestos (and asbestos-like) minerals and thermophilic chemolithoautotrophic microorganisms as possible mineral dissolution treatments targeting their toxic properties. The removal of Fe from crocidolite was tested through chemolithoautotrophic Fe(III) reduction activities at 60°C. Chrysotile and tremolite-actinolite were tested for dissolution and potential release of elements like Si and Mg through biosilicification processes at 75°C. Our results show that chemolithoautotrophic Fe(III) reduction activities by Deferrisoma palaeochoriense were supported with crocidolite as the sole source of Fe(III) used as a terminal electron acceptor during respiration. Microbial Fe(III) reduction activities resulted in higher Fe release rates from crocidolite in comparison to previous studies on Fe leaching from crocidolite through Fe assimilation activities by soil fungi. Evidence of biosilicification in Thermovibrio ammonificans did not correspond with increased Si and Mg release from chrysotile or tremolite-actinolite dissolution. However, overall Si and Mg release from chrysotile into our experimental medium outmatched previously reported capabilities for Si and Mg release from chrysotile by fungi. Differences in the profiles of elements released from chrysotile and tremolite-actinolite during microbe-mineral experiments with T. ammonificans underscored the relevance of underlying crystallochemical differences in driving mineral dissolution and elemental bioavailability. Experimental studies targeting the interactions between chemolithoautotrophs and asbestos (or asbestos-like) minerals offer new access to the mechanisms behind crystallochemical mineral alterations and their role in the development of tailored asbestos treatments. IMPORTANCE We explored the potential of chemosynthetic microorganisms growing at high temperatures to induce the release of key elements (mainly iron, silicon, and magnesium) involved in the known toxic properties (iron content and fibrous mineral shapes) of asbestos minerals. We show for the first time that the microbial respiration of iron from amphibole asbestos releases some of the iron contained in the mineral while supporting microbial growth. Another microorganism imposed on the two main types of asbestos minerals (serpentines and amphiboles) resulted in distinct elemental release profiles for each type of asbestos during mineral dissolution. Despite evidence of microbially mediated dissolution in all minerals, none of the microorganisms tested disrupted the structure of the asbestos mineral fibers. Further constraints on the relationships between elemental release rates, amount of starting asbestos, reaction volumes, and incubation times will be required to better compare asbestos dissolution treatments studied to date.

Increasing CO2 emission has resulted in pressing climate and environmental issues. While abiotic and biotic processes mediating the fate of CO2 have been studied separately, their interactions and combined effects have been poorly understood. To explore this knowledge gap, an iron-reducing organism, Orenia metallireducens, was cultured under 18 conditions that systematically varied in headspace CO2 concentrations, ferric oxide loading, and dolomite (CaMg(CO3)2) availability. The results showed that abiotic and biotic processes interactively mediate CO2 acidification and sequestration through \"chain reactions\", with pH being the dominant variable. Specifically, dolomite alleviated CO2 stress on microbial activity, possibly via pH control that transforms the inhibitory CO2 to the more benign bicarbonate species. The microbial iron reduction further impacted pH via the competition between proton (H+) consumption during iron reduction and H+ generation from oxidization of the organic substrate. Under Fe(III)-rich conditions, microbial iron reduction increased pH, driving dissolved CO2 to form bicarbonate. Spectroscopic and microscopic analyses showed enhanced formation of siderite (FeCO3) under elevated CO2, supporting its incorporation into solids. The results of these CO2-microbe-mineral experiments provide insights into the synergistic abiotic and biotic processes that alleviate CO2 acidification and favor its sequestration, which can be instructive for practical applications (e.g., acidification remediation, CO2 sequestration, and modeling of carbon flux).

Many naturally seleniferous soils are faced with Cd contamination problem, which severely limits crop cultivation in these areas. Straw returning has been widely applied in agricultural production due to its various benefits to soil physicochemical properties, soil fertility, and crops yield. However, effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils remain largely unclear. Therefore, the effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils were investigated in this study. The results showed that iron reduction driven by Clostridium and Anaeromyxbacter was responsible for the variations in Se and Cd fates in soil. Straw application respectively increased the gene copy numbers of Clostridium and Anaeromyxbacter by 19.5-56.3% and 33.6-39.8%, thus promoting iron reductive dissolution, eventually resulting in a high release amount of Se and Cd from Fe(III) (oxyhydr) oxides. Under reducing conditions, the released Cd was adsorbed by the newly formed metal sulfides or reacted with sulfides to generate CdS precipitates. Straw application decreased the soil exchangeable Se and soil exchangeable Cd concentration during flooding phase. However, straw application significantly increased Se/Cd in soil solution which had the highest bioavailability during flooding. In addition, straw application increased soil exchangeable Se concentration, but it had no significant effects on soil exchangeable Cd concentration after soil drainage. Taken together, straw application increased Se bioavailability and Cd mobility. Therefore, straw application is an effective method for improving Se bioavailability, but it is not suitable for the application to Cd-contaminated paddy soils. In the actual agricultural production, straw could be applied in seleniferous soils to improve Se bioavailability. At the same time, straw application should be cautious to avoid the release of Cd from Cd-contaminated soil.

Microbial iron reduction is a widespread and ancient metabolism on Earth, and may plausibly support microbial life on Mars and beyond. Yet, the extreme limits of this metabolism are yet to be defined. To investigate this, we surveyed the recorded limits to microbial iron reduction in a wide range of characterized iron-reducing microorganisms (n = 141), with a focus on pH and temperature. We then calculated Gibbs free energy of common microbially mediated iron reduction reactions across the pH-temperature habitability space to identify thermodynamic limits. Comparing predicted and observed limits, we show that microbial iron reduction is generally reported at extremes of pH or temperature alone, but not when these extremes are combined (with the exception of a small number of acidophilic hyperthermophiles). These patterns leave thermodynamically favourable combinations of pH and temperature apparently unoccupied. The empty spaces could be explained by experimental bias, but they could also be explained by energetic and biochemical limits to iron reduction at combined extremes. Our data allow for a review of our current understanding of the limits to microbial iron reduction at extremes and provide a basis to test more general hypotheses about the extent to which biochemistry establishes the limits to life.

Sediment interfaces in alluvial aquifers have a disproportionately large influence on biogeochemical activity and, therefore, on groundwater quality. Previous work showed that exports from fine-grained, organic-rich zones sustain reducing conditions in downstream coarse-grained aquifers beyond the influence of reduced aqueous products alone. Here, we show that sustained anaerobic activity can be attributed to the export of organic carbon, including live microorganisms, from fine-grained zones. We used a dual-domain column system with ferrihydrite-coated sand and embedded reduced, fine-grained lenses from Slate River (Crested Butte, CO) and Wind River (Riverton, WY) floodplains. After 50 d of groundwater flow, 8.8 ± 0.7% and 14.8 ± 3.1% of the total organic carbon exported from the Slate and Wind River lenses, respectively, had accumulated in the sand downstream. Furthermore, higher concentrations of dissolved Fe(II) and lower concentrations of dissolved organic carbon in the sand compared to total aqueous transport from the lenses suggest that Fe(II) was produced in situ by microbial oxidation of organic carbon coupled to iron reduction. This was further supported by an elevated abundance of 16S rRNA and iron-reducing (gltA) gene copies. These findings suggest that organic carbon transport across interfaces contributes to downstream biogeochemical reactions in natural alluvial aquifers.

Biochar was proved as an electron shuttle to facilitate extracellular electron transfer (EET) of electrochemically active bacteria (EAB); however, its underlying mechanism was not fully understood. In this study, we aimed to further explore how the regulation of surface functional groups of biochar would affect the microbial iron reduction process of Geobacter sulfurreducens as a typical EAB. Two modified biochars were achieved after HNO3 (NBC) and NaBH4 (RBC) pretreatments, and a control biochar was produced after deionized water (WBC) washing. Results showed that WBC and RBC significantly accelerated microbial iron reduction of G. sulfurreducens PCA, while had no effect in the final Fe (II) minerals (e.g., vivianite and green rust (CO32-)). Besides, Brunauer-Emmett-Teller (BET) surface area, electron spin resonance (ESR) and electrochemical measurements showed that larger surface area, lower redox potential, and more redox-active groups (e.g., aromatic structures and quinone/hydroquinone moieties) in RBC explained its better electron transfer performance comparing to WBC. Interestingly, NBC completely suppressed the Fe (III) reduction process, mainly due to the production of reactive oxygen species which inhibited the growth of G. sulfurreducens PCA. Overall, this work paves a feasible way to regulate the surface functional groups for biochar, and comprehensively revealed its effect on EET process of microorganisms.

Microbial iron reduction (MIR) is an important and ubiquitous natural process in the biogeochemical cycling of iron and carbon in anaerobic sedimentary and subsurface environments. The objectives of this study were (1) to determine if the MIR process can enhance the inactivation of Escherichia coli cells under anaerobic conditions and (2) to identify potential inactivation mechanisms. Laboratory microcosm experiments showed that the presence of MIR activity significantly enhanced E. coli inactivation, and the inactivation rate under the MIR condition was significantly larger than those under other anaerobic redox conditions. Under anoxic condition, higher Fe2+concentrations exhibited a linear function to larger E. coli inactivation rates, indicating that the production of Fe2+by MIR was one of the important roles in E. coli inactivation. When E. coli cells were amended as the sole electron source to the MIR process, increased Fe2+ production was observed, which corresponded to decreasing TOC concentration. Together, the results suggest that MIR enhanced E. coli inactivation through the production of Fe2+ as metabolic waste, and the inactivation benefited the MIR process as the inactivated cells were used as an electron source, which represents a potential new mechanism for bacterial inter-species competition. This knowledge could further improve our understanding of the fate of fecal bacteria in natural environments where the MIR process is prevalent, and may also be explored for enhanced removal of bacterial pathogens in engineering processes.

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

This paper discusses the abiotic and biotic processes in the in-situ control of biogenic hydrogen sulfide generated from microbial sulfate reduction by ferric (FeIII) (hydr)oxides (FeOOH) granules in the sediments of polluted urban waters. Granular ferric hydroxide (GFH, β-FeOOH) and granular ferric oxide (GFO, α-FeOOH) dosed in the organic- and sulfate-rich sediments had 180% and 19% higher sulfide removal capacities than those used for the purely abiotic removal of dissolved sulfide, respectively. The enhancement was attributable to the involvement of the biotic pathways, besides the abiotic pathways (mainly sulfide oxidation). The FeOOH granules stimulated the microbial reduction of surface FeIII by iron-reducing bacteria (e.g., Desulfovibrio and Carnobacterium), and increased the microbial sulfate reduction by 24%-30% under an organic-rich condition, likely due to the enhanced organic fermentation. The microbial iron reduction significantly enhanced the removal of the formed biogenic hydrogen sulfide through increasing sulfide precipitation because it remarkably promoted the release of Fe2+ ions from the granule surface, likely due to the involvement of siderophores as ligands. This biotic pathway led to the formation of amorphous FeS(s) as a major sulfur product (56%-81%), instead of elemental sulfur. The enhancement in the sulfide control performance was much more pronounced when the poorly ordered GFH was used, because of the faster Fe2+ release, compared to the highly ordered GFO. The abiotic and biotic mechanisms elucidated in this study provide insights into the iron-sulfur chemistry in the sediments of various polluted waters (e.g., storm drains, urban rivers, and estuary), where the manually-dosed and naturally-occurring FeIII (hydr)oxides control biogenic hydrogen sulfide.