Biological nitrogen fixation constitutes the main input of fixed nitrogen to Earth's ecosystems, and its isotope effect is a key parameter in isotope-based interpretations of the N cycle. The nitrogen isotopic composition (δ(15)N) of newly fixed N is currently believed to be ∼-1‰, based on measurements of organic matter from diazotrophs using molybdenum (Mo)-nitrogenases. We show that the vanadium (V)- and iron (Fe)-only "alternative" nitrogenases produce fixed N with significantly lower δ(15)N (-6 to -7‰). An important contribution of alternative nitrogenases to N2 fixation provides a simple explanation for the anomalously low δ(15)N (<-2‰) in sediments from the Cretaceous Oceanic Anoxic Events and the Archean Eon. A significant role for the alternative nitrogenases over Mo-nitrogenase is also consistent with evidence of Mo scarcity during these geologic periods, suggesting an additional dimension to the coupling between the global cycles of trace elements and nitrogen.
<p>Nitrogen is the primary limiting nutrient in high latitude ecosystems. Biological nitrogen fixation (BNF) by microorganisms associated with cryptogamic covers, such as cyanolichens and bryophytes, is an important source of new reactive nitrogen in pristine, high-latitude ecosystems. BNF is catalyzed by the enzyme nitrogenase, for which three isoforms have been described; the canonical molybdenum (Mo) nitrogenase which requires Mo in its active site and two alternative nitrogenases, the vanadium and iron-only nitrogenases. The low availability of Mo on land has been shown to limit BNF in many ecosystems from the tropical forest to the arctic tundra. Alternative nitrogenases have been suggested as viable alternatives to cope with Mo limitation of BNF, however, field data supporting this long-standing hypothesis have been lacking.</p><p>Here, we elucidated the contribution of the vanadium nitrogenase to BNF by cyanolichens across a 600 km latitudinal transect in eastern Canadian boreal forests. We report a widespread activity of the vanadium nitrogenase which contributed between 15 to 50% of total BNF rates on all sites. Vanadium nitrogenase contribution to BNF was more robust in the northern part of the transect. Vanadium nitrogenase contribution to BNF also changed during the growing season, with a three-fold increase between the early (May) and late (September) growing season. By including the contribution of the vanadium nitrogenase to BNF, estimates of new N input by cyanolichens increase by up to 30%, a significant change in these low N input ecosystems. Finally, we found that Mo availability was the primary driver for the contribution of the vanadium nitrogenase to BNF with a Mo threshold of ~ 250 ng.g<sub>lichen</sub><sup>-1</sup> for the onset of vanadium based BNF.</p><p>This study on N<sub>2</sub>-fixing cyanolichens provides extensive field evidence, at an ecosystem scale, that vanadium-based nitrogenase greatly contributes to BNF when Mo availability is limited. The results showcase the resilience of BNF to micronutrient limitation and reveal a strong link between the biogeochemical cycle of macro- and micronutrients in terrestrial ecosystems. Given widespread findings of Mo limitation of BNF in terrestrial ecosystems, additional consideration of vanadium-based BNF is required in experimental and modeling studies of terrestrial biogeochemistry.</p>
The bacterial Wood-Ljungdahl pathway for CO(2)-reductive acetogenesis is important for the nutritional mutualism occurring between wood-feeding insects and their hindgut microbiota. A key step in this pathway is the reduction of CO(2) to formate, catalysed by the enzyme formate dehydrogenase (FDH). Putative selenocysteine- (Sec) and cysteine- (Cys) containing paralogues of hydrogenase-linked FDH (FDH(H)) have been identified in the termite gut acetogenic spirochete, Treponema primitia, but knowledge of their relevance in the termite gut environment remains limited. In this study, we designed degenerate PCR primers for FDH(H) genes (fdhF) and assessed fdhF diversity in insect gut bacterial isolates and the gut microbial communities of termites and cockroaches. The insects examined herein represent three wood-feeding termite families, Termopsidae, Kalotermitidae and Rhinotermitidae (phylogenetically 'lower' termite taxa); the wood-feeding roach family Cryptocercidae (the sister taxon to termites); and the omnivorous roach family Blattidae. Sec and Cys FDH(H) variants were identified in every wood-feeding insect but not the omnivorous roach. Of 68 novel alleles obtained from inventories, 66 affiliated phylogenetically with enzymes from T. primitia. These formed two subclades (37 and 29 phylotypes) almost completely comprised of Sec-containing and Cys-containing enzymes respectively. A gut cDNA inventory showed transcription of both variants in the termite Zootermopsis nevadensis (family Termopsidae). The gene patterns suggest that FDH(H) enzymes are important for the CO(2)-reductive metabolism of uncultured acetogenic treponemes and imply that the availability of selenium, a trace element, shaped microbial gene content in the last common ancestor of dictyopteran, wood-feeding insects, and continues to shape it to this day.
Abstract Microorganisms have evolved diverse strategies to acquire the vital element nitrogen (N) from the environment. Ecological and physiological controls on the distribution of these strategies among microbes remain unclear. In this study, we examine the distribution of 10 major N acquisition strategies in taxonomically and metabolically diverse microbial genomes, including those from the Genomic Catalogue of Earth's Microbiomes dataset. We utilize a marker gene‐based approach to assess relationships between N acquisition strategy prevalence and microbial life history strategies. Our results underscore energetic costs of assimilation as a broad control on strategy distribution. The most prevalent strategies are the uptake of ammonium and simple amino acids, which have relatively low energetic costs, while energy‐intensive biological nitrogen fixation is the least common. Deviations from the energy‐based framework include the higher‐than‐expected prevalence of the assimilatory pathway for chitin, a large organic polymer. Energy availability is also important, with aerobic chemoorganotrophs and oxygenic phototrophs notably possessing ~2‐fold higher numbers of total strategies compared to anaerobic microbes. Environmental controls are evidenced by the enrichment of inorganic N assimilation strategies among free‐living taxa compared to host‐associated taxa. Physiological constraints such as pathway incompatibility add complexity to N acquisition strategy distributions. Finally, we discuss the necessity for microbially‐relevant spatiotemporal environmental metadata for improving mechanistic and prediction‐oriented analyses of genomic data.
Methyl phenolic resin and boron modified methyl phenolic resin were synthesized by solution polymerization using m-cresol, formaldehyde and boric acid as raw materials, and quartz fiber reinforced resin matrix composites were prepared using the two resins as matrix. The structure of the resin was characterized, and its solubility, mechanical properties, heat resistance and ablative heat insulation properties of the composite were tested. The effect of resin structure on its properties was studied. Compared with methyl phenolic resin, boron modified phenolic resin not only has better solubility and mechanical properties, but also increases the initial decomposition temperature and peak decomposition temperature by 170°C, the carbon residue rate by 11.11%, and improves the ablative heat insulation performance of the composite. The linear ablative rate is 0.0003333 mm s-l, and the mass ablative rate is 0.0002733 g s-l, The back temperature of 4mm sheet is 110°C after 10 min of gas oxygen ablation, which is expected to be used as the matrix resin of thermal protection materials.
Significance Most environments host a poorly understood microbial diversity. In recent years, work on gene inventories and metagenomics has revealed much about the microbial species and metabolic genes that may be present in situ; however, connecting microbial species with environmental function has lagged. Here a combination of emerging single-cell and other approaches revealed the gut microbes that may catalyze a key activity in their termite hosts. The results implicate a previously unappreciated deltaproteobacterium living on a gut protist.
The safe operation of railway train is very important to the safety of the railway system. Therefore, it is necessary to master the failure rules and causes of the railway train devices. However, the current common fault analysis methods have some shortcomings, such as complicated compilation process and insufficient expression. Therefore, we choose to use the knowledge graph technology. Firstly, the common faults of railway train devices are obtained through the statistics of real fault data; secondly, the causes of faults are analyzed and the fault tree is established; thirdly, the ontology of railway train system is modeled according to the idea of fault tree construction; finally, the logical relationship between ontology is established according to the ontology model, and the railway train device fault causality model based on knowledge graph is established.
Abstract Controllable synthesis of the two vanadium‐containing polyoxoniobate‐based three‐dimensional hybrid compounds (IV) and (V) is achieved by simply changing the hydrothermal temperature or adding N‐(aminoethyl)piperazine as an additive.
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