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At least 19 recordsLinked to original sources
PubMed2026

Mikania micrantha invasion restructures rhizosphere nitrogen cycling through enzyme activation, microbial recruitment, and allelopathic regulation.

BACKGROUND: Plant invasions profoundly influence terrestrial ecosystems by reshaping nutrient cycling processes. However, the mechanisms through which invasive plants such as Mikania micrantha modulate soil nitrogen (N) cycling and microbial communities remain insufficiently explored. Moreover, comparative studies with indigenous congener are scarce, limiting insights into whether such effects reflect species-specific strategies or genus-wide traits. This study investigates how M. micrantha modulates nitrogen metabolic pathways and rhizosphere microecology using combined metagenomic and metabolomic analyses. RESULTS: Integrated analyses revealed that M. micrantha established a distinctive "high total nitrogen-low mineral nitrogen" profile in the rhizosphere soil. Metagenomic profiling showed consistent enrichment of key ammonium assimilation enzymes, including glutamine synthetase and glutamate dehydrogenase, promoting enhanced incorporation of NH₄⁺ into organic nitrogen pools. In contrast, genes encoding nitrate reductase and nitrate transporters were significantly lower in relative abundance, limiting nitrate assimilation. Mikania micrantha also selectively enriched nitrogen-fixing microbes (notably rhizobia genera) and plant growth-promoting rhizobacteria (PGPR), thereby enhancing biological nitrogen fixation capacity. Metabolomic analysis further identified several allelopathic compounds in invaded soils at higher relative abundance, particularly epicatechin, which exhibited inhibitory effects on nitrifying bacteria. Compared with the congener Mikania cordata, which exerted weaker impacts on soil nitrogen cycling and microbial assembly, M. micrantha deployed a more comprehensive strategy integrating biochemical, microbial, and metabolic regulation. CONCLUSIONS: These findings demonstrate that under greenhouse-controlled conditions, M. micrantha reconfigures rhizosphere nitrogen cycling through a multi-dimensional strategy that couples biochemical regulation, microbial recruitment, and metabolite-mediated interference, thereby suggesting a potential mechanism that may contribute to its ecological advantage in natural settings. Video Abstract.

Rhizosphere↗
PubMed1992

Glutamine synthetase and nitrogen cycling in colonies of the marine diazotrophic cyanobacteria Trichodesmium spp.

We examined freshly collected samples of the colonial planktonic cyanobacterium Trichodesmium thiebautii to determine the pathways of recently fixed N within and among trichomes. High concentrations of glutamate and glutamine were found in colonies. Glutamate and glutamine uptake rates and concentrations in cells were low in the early morning and increased in the late morning to reach maxima near midday; then uptake and concentration again fell to low values. This pattern followed that previously observed for T. thiebautii nitrogenase activity. Our results suggest that recently fixed nitrogen is incorporated into glutamine in the N2-fixing trichomes and may be passed as glutamate to non-N2-fixing trichomes. The high transport rates and concentrations of glutamate may explain the previously observed absence of appreciable uptake of NH4+, NO3-, or urea by Trichodesmium spp. Immunolocalization, Western blots (immunoblots), and enzymatic assays indicated that glutamine synthetase (GS) was present in all cells during both day and night. GS appeared to be primarily contained in cells of T. thiebautii rather than in associated bacteria or cyanobacteria. Double immunolabeling showed that cells with nitrogenase (Fe protein) contained levels of the GS protein that were twofold higher than those in cells with little or no nitrogenase. GS activity and the uptake of glutamine and glutamate dramatically decreased in the presence of the GS inhibitor methionine sulfoximine. Since no glutamate dehydrogenase activity was detected in this species, GS appears to be the primary enzyme responsible for NH3 incorporation.

Animals↗
PubMed2026

Azotobacter vinelandii gene fitness following carbon shift from sucrose to acetate, succinate and glycerol.

Nitrogen-fixing microbes are a primary contributor of this important nutrient to the global nitrogen cycle. Biological nitrogen fixation (BNF) through the enzyme nitrogenase requires extensive energy that in whole cells is generally studied during the oxidation of carbohydrates such as sugars. The nitrogen-fixing bacterium Azotobacter vinelandii is a model diazotroph for the study of aerobic BNF. Much is known about metabolism in A. vinelandii when cultured on a simple medium where energy is provided primarily in the form of sucrose or glucose. Outside of the laboratory, this soil bacterium grows on metabolites primarily derived from plant root exudates or from the degradation of dead plant matter. In this work, we expand on previous studies looking at genes that are essential to BNF in A. vinelandii when grown on sucrose medium using transposon sequencing (Tn-seq). We applied Tn-seq to determine the genes essential to growth when the medium was shifted to acetate, succinate or glycerol as the primary carbon and energy source to fuel both growth and BNF. A global overview of the genes of central metabolism and those directing substrates toward central metabolism, along with a selection of unexpected genes that were essential for specific growth substrates, is provided.

Azotobacter vinelandii↗
PubMed2026

Ocean warming enhances iron use efficiencies of marine ammonia-oxidizing archaea.

Ammonia-oxidizing archaea (AOA) are among the most abundant microorganisms in the ocean, playing a fundamental role in the marine nitrogen cycle. Although temperature and trace metal availability each individually influence the growth and activity of marine AOA, there is only a very limited understanding of the interactive effects of these two major factors on AOA in the rapidly changing ocean. Here, we show that the iron requirements of the model marine AOA species Nitrosopumilus maritimus SCM1 are highly sensitive to temperature changes. A 5 °C increase in growth temperature reduced SCM1 iron requirements by >80%, and was associated with a substantial increase in iron use efficiencies (IUE, mol C fixed/h/mol cellular Fe) under iron-limited and warming conditions. A thermally enhanced IUE enables SCM1 to more efficiently utilize scarce available iron supplies to support its growth. Whole-cell proteomic analysis revealed that iron limitation decreased expression of a ferredoxin and increased expression of a copper-dependent plastocyanin that became more pronounced with warming, suggesting coordinated electron transport response regulation under combined iron and temperature stress. The global impacts of these temperature-dependent changes to AOA iron demands were assessed using sensitivity experiments with a state-of-the-art biogeochemical model. Simulations showed that impacts on nitrification were concentrated at higher latitudes, but the alterations to ammonia concentrations were redistributed toward lower latitudes by mode and intermediate water transport. These findings reveal a previously unrecognized mechanism by which ocean warming may alleviate iron limitation of AOA, enhance their ecological competitiveness, and reshape ocean nitrogen cycling throughout marine ecosystems.

Iron↗
PubMed2026

Manipulation of rhizosphere microbiome by Microbacterium sp. GB16_1_BI to promote plant growth.

AIM: The bioinoculant properties of a newly identified ammonium-releasing novel strain of Actinomycetota-Microbacterium bengalense sp. nov. GB16_1_BI (Accession number: SRX9280401) on the microbiome structure of rice rhizosphere were assessed. METHODS AND RESULT: GB16_1_BI may inhibit most bacteria present in the rice rhizosphere as well as encouraged the growth of rare bacteria specific to the waterlogged rice rhizosphere. The genome sequence as well as untargeted metabolome analyses of GB16_1_BI showed abundance of secondary metabolites with probable antimicrobial activity. Amplicon sequencing of the 16S rRNA V3-V4 region from the rhizosphere of the black rice showed inhibition of most bacteria by GB16_1_BI. Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) analysis showed increased abundance in the marker genes for nitrogen cycling (nifH, nrfA, and nrt) but not for nifD or nifK, which was also reflected in the ANOSIM analysis in the OTUs of the N-fixing bacteria. Higher abundance of the nitrogen-fixing methanotrophs, Methylosinus and Methylocystis in inoculated plants also led to study of the marker genes for methane metabolism. CONCLUSION: Microbes present in the rhizosphere contribute to the biogeochemical cycle by transforming unavailable minerals and by retaining nutrients for their growth, which get released after death for plant adsorption. However, not all microbes contribute positively to plant growth. Microbes compete with plants for nutrients, cause disease, or produce harmful greenhouse gases. Hence, GB16_1_BI could influence plant growth predominantly by suppressing microbes and encouraged niche-specific microbes specifically involved in nitrogen cycling.

Rhizosphere↗
PubMed2025

Depth-dependent multi-kingdom microbial interactions and biogeochemical cycling genes in eutrophic shallow lake sediments.

Microorganisms are pivotal to lake ecosystem biogeochemical cycles, yet existing research often focuses on single microbial kingdoms or surface sediments, neglecting multi-kingdom interactions and depth-resolved dynamics. To address these gaps, we used metagenomic sequencing to characterize microbial communities and their functional associations across overlying water and 0-45 cm sediments in four shallow lakes of the middle Yangtze River basin, China. Despite increasing bacterial and fungal diversity with depth, the 0-9 cm surface sediments exhibited the strongest multi-kingdom network connectivity and the greatest microbial stability. Functional genes exhibited clear depth-dependent patterns: nitrogen cycling genes, including those involved in dissimilatory nitrate reduction to ammonium, were most enriched in the upper 0-9 cm of sediment; methane cycling genes were positively correlated with depth; phosphorus cycling genes and some sulfur cycling genes, such as assimilatory sulphate reduction, declined with depth. Sediment microbial assembly was dominated by deterministic processes, in which the vertical distribution of functional genes was primarily dictated by heavy metals and conventional environmental indicators. These findings highlight depth-specific multi-kingdom microbial interactions and their associations with biogeochemical cycling, advancing lacustrine microbial ecology understanding and providing references for lake conservation under environmental change.

Lakes↗
PubMed2026

Elevated water levels drive greenhouse gas mitigation in the riparian zone profile.

Wetlands are critical for climate regulation, with their hyporheic zone serving as sensitive interfaces for groundwater-soil-atmosphere exchange. These zones are active hotspots for carbon-nitrogen cycling and greenhouse gas (GHG) emissions (CO2, CH4, N2O), yet the impact of water level fluctuations on these emissions and their microbial drivers in freshwater wetlands remains poorly understood. This study investigated the spatiotemporal dynamics of GHG emissions and carbon-nitrogen coupling processes along riparian soil profiles of Baiyangdian Lake during water level fluctuations. Employing static chamber measurements, microcosms, quantitative PCR, Metagenome-Assembled genome (MAG) analyses, and Structural Equation Modeling (SEM), we observed that GHG emissions were significantly affected by water level fluctuations. Specifically, CO2 and N2O fluxes, as well as CO2 production potential were significantly lower at high-water-level conditions. Water level also emerged as a key driver of microbial community structure, with Methylococcaceae and Methanosarcinaceae as key regulators of CH4 emission, and Anaeromyxobacteraceae as central to N2O dynamics. A high-quality Methylomirabilales-like MAG, possessing the complete pathway for coupled nitrate reduction and methane oxidation, was identified. Its abundance negatively correlated with water level, suggesting that these C-N coupling bacteria contribute to reducing GHG emissions. This study provides crucial theoretical insights and identifies microbial targets for mitigating wetland GHG emission through hydrological management.

Greenhouse Gases↗
PubMed1975

Recent research into the nutrition of the horse.

The majority of recent experiments in equine nutrition relates to the metabolism of protein, non-protein nitrogen, carbohydrate, calcium and phosphorus. An understanding of the significance of the nitrogen cycle is emerging in which there is a two-way movement of compounds across the wall of the G.I. tract. Some quantitative estimates of the role of microflora in this process have been made for comparison with similar processes in ruminants. However, the pathway by which the microflora contributes to the nitrogen economy and the extent to which this occurs is not established in the horse. It is well established that both glucose and volatile fatty acids are common substrates in tissue metabolism; but in contrast to evidence in the ruminant the ratio of starch to fibre in the diet does not affect the efficiency fo fibre digestibility. This, and similar observations, are explicable in terms of the anatomy of the G.I. tract of the horse. There is a scarcity of information on the secretion of digestive enzymes by the horse and on the relationship between intestinal function and certain digestive upsets. Furthermore, there is an urgent need for accurate clinical evidence for the significance of calcium and phosphorus nutrition to the development of leg abnormalities in horses. The irrelevance, for diagnosing purposes, of estimates of serum Ca and P is concluded, especially where no other relevant information is available. Haematological procedures, of diagnostic value in this respect, are available for use at least in controlled clinical conditions.

Amino Acids↗
PubMed2026

Spiramycin fermentation residue-derived biochar regulates soil nutrient cycling, microbial communities, and antibiotic resistance gene dynamics.

Spiramycin fermentation residues (SFR) are hazardous wastes enriched with residual antibiotics, yet they can serve as potential feedstocks for resource recovery after appropriate treatment. In this study, SFR-derived biochar (SFR-BC) was produced by pyrolysis and applied to agricultural soil to evaluate its effects on soil properties, microbial communities, potential pathogenic bacteria, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs). A 60-day soil incubation experiment was conducted with one control and three SFR-BC application rates of 0.5%, 1.0%, and 2.0%. SFR-BC improved soil physicochemical properties, nutrient status, enzyme activities, and microbial alpha diversity. Metagenomic analysis showed that SFR-BC altered the abundance of functional genes associated with carbon and nitrogen cycling, indicating shifts in microbial functional potential. SFR-BC also changed bacterial co-occurrence patterns, with the high-dose treatment showing a more complex and highly connected network structure during incubation. In addition, high-dose SFR-BC reduced several potential pathogenic bacteria, including major plant pathogenic taxa. SFR-BC decreased soil ARG abundance by 9.38%-33.67% and MGE abundance by 6.49%-27.89% relative to the control, showing a dose-dependent reduction in antibiotic resistance-related genetic elements. Network and PLS-PM analyses further indicated that ARG variation was statistically associated with soil physicochemical properties, microbial diversity, potential bacterial hosts, and MGEs. Overall, these results suggest that SFR-BC can improve short-term soil nutrient status and reduce ARGs, MGEs, and several potential pathogenic taxa under controlled incubation conditions, providing useful evidence for the potential valorization of antibiotic fermentation residues through pyrolysis.

Charcoal↗
PubMed2026

Bioplastic biodegradability shapes microbial communities in a coastal brackish environment.

Microorganisms are metabolically versatile and central to marine ecosystems, yet the potential of marine microbial communities to degrade different bioplastics and the effect of environmental factors are poorly understood. Employing multi-seasonal in situ and in vitro experiments, we assessed the biodegradation of six commonly used bio-based bioplastic materials at a coastal site in the brackish Baltic Sea and characterized the associated microbial communities using metagenomics and metatranscriptomics. Cellulose acetate (CA), polybutylene succinate (PBS), and polyhydroxybutyrate/valerate (PHB) degraded at varying rates across materials, seasons, and experimental settings, with up to 28% weight attrition after 97 weeks in situ (CA) and 56% carbon loss as CO2 after 4 weeks in vitro (PBS). The three biodegraded plastics developed similar microbial communities that differed markedly from those on the other materials (cellulose acetate propionate, polyamide, and polyethylene) and in the water column. The main microbial populations on the biodegraded plastics included aerobic and facultative anaerobic heterotrophs with a broad capacity for carbohydrate metabolism. Populations with the potential for nitrogen fixation and denitrification were more prevalent on the biodegraded plastics, suggesting that bioplastic biodegradation is constrained by and coupled to the marine nitrogen cycle. Based on the metatranscriptomic signal of key genes involved in the initial hydrolysis of CA, PBS, and PHB, we identified diverse microbial populations that can potentially drive the biodegradation of these materials in the Baltic Sea, many of which encoded the potential to degrade multiple bioplastics. We propose the term 'bioplastisphere' to denote the distinctive microbial communities associated with biodegradable plastics.

Seawater↗
PubMed2026

Lifestyle Differentiation Among Marine Denitrifying Microorganisms.

Microorganisms carrying out denitrification in marine anoxic zones drive bioavailable nitrogen loss. Sequencing datasets have demonstrated the modularity of denitrification, with most populations having the genetic capability for only a subset of the pathway (NO3-➔NO2-➔NO➔N2O➔N2). Although previous work provided ecological explanations for this diversity among the functional modules, large trait variations exist within each functional module, and this within-module diversity and its biogeochemical implications remain unexplored. Here, we combine genomic data and modeling to explore how metabolic "lifestyle" strategies influence denitrifier community structure. We build a comprehensive genomic database of marine denitrifiers, and identify lifestyle differentiation among denitrifier functional groups. We then extend a mathematical ecosystem model by resolving two microbial functional types for each module representing a metabolic trade-off: a copiotroph, optimized for fast growth, and an oligotroph, optimized for high nutrient affinity. In the model, as the supply of organic matter relative to nitrate increases, the degree of copiotrophy among the community increases and then decreases. This suggests that oligotrophs are associated with either organic-matter- or nitrate-limiting conditions, whereas copiotrophic lifestyles are associated with an intermediate regime. Our model further associates NO2- reducers with oligotrophy and NO3- reducers with copiotrophy, particularly those producing greenhouse gas nitrous oxide (N2O), linking N2O production to substrate-replete conditions, which is consistent with our genome-based lifestyle estimates. Results provide insight into denitrifier ecological niches and thus the biogeochemical conditions that are associated with the production of intermediates, such as N2O, improving our understanding of how nitrogen cycling will change in a warming ocean.

Marine denitrifiers↗
PubMed2026

Ecological Filtering by Tuber Compartments Shapes Stable Core Microbiomes That Underpin Potato Plant Growth Across Environments.

Harnessing plant microbiomes for sustainable agriculture requires understanding not only whether they can boost crop performance, but also how ecological processes govern their assembly, stability, and functional contributions across environments. While we previously showed that seed tuber microbiomes can predict potato vigour using machine learning, it remained unclear how ecological processes shape tuber microbiome stability and functionality across host genotypes, tuber compartments, soil types, and years. Here, we analyzed the national-scale dataset of 240 field-collected potato seedlots, spanning six genotypes, two soil types, and two growing years, with a focus on the spatially distinct heel and eye compartments of the potato tuber. By profiling over 1200 bacterial and fungal communities and linking microbiome composition to plant performance, we show that plant genotype and tuber compartment are the strongest determinants of microbial diversity and composition. Compartment-specific enrichment of functional traits revealed spatial partitioning of microbial functions, with organic compound conversion and nitrogen cycling dominant in the heel, and energy metabolism enriched in the eye. Applying a macroecological abundance-occupancy framework, we identified a stable core microbiome of bacterial and fungal taxa that persisted across all environments and years. These core members were more strongly associated with plant growth-related traits than non-core taxa, and core taxa in different tuber compartments showed distinct correlations with taxa of potential pathogenic relevance. Together, our findings demonstrate that tuber compartments act as ecological filters that structure persistent, functionally specialised microbiomes linked to plant growth-related traits across environments. By providing an ecological and functional framework for compartment-resolved, stable core microbiomes, this study advances mechanistic understanding of plant-microbe interactions and identifies stable microbial partners as promising targets for improving potato resilience and productivity.

Journal Article↗
PubMed2026

A conserved partner-switching system controls terminal differentiation in multicellular cyanobacteria.

UNLABELLED: Canonical partner-switching systems (PSSs) regulate sigma factor activity through reversible phosphorylation, but their established roles have been largely limited to stress responses and sporulation in Firmicutes. Whether this regulatory mechanism also controls developmental cell fate decisions in other bacterial phyla has remained unknown. Here, we identify a canonical PSS that governs heterocyst differentiation in the multicellular cyanobacterium Anabaena sp. PCC 7120. This system comprises the anti-sigma factor All2284 (NfsS) and the anti-anti-sigma factor All2283 (NfsR). Structural predictions and biochemical assays showed that NfsS phosphorylates NfsR on a conserved serine residue, whereas bacterial two-hybrid and co-purification assays demonstrated that NfsS binds the developmental sigma factors SigC and SigE. Deletion of nfsR abolished heterocyst formation and diazotrophic growth, and transcriptomic analysis revealed broad failure to induce late heterocyst genes, including nitrogen fixation functions such as nifHDK and fdxH. Phylum-wide comparative genomics further showed that PSS genes and putative functional clusters are strongly enriched in filamentous and heterocyst-forming taxa, indicating an association between the expansion of these signaling modules and the emergence of multicellularity and developmental specialization. Together, these findings establish a PSS as a direct regulator of terminal cell differentiation in a gram-negative bacterium and reveal partner switching as a conserved regulatory principle linking environmental signaling to developmental fate in a major bacterial phylum. IMPORTANCE: While partner-switching systems are classically associated with stress responses and sporulation control in Firmicutes, whether this regulatory logic governs developmental decisions in other bacterial phyla has remained unknown. Here, we establish that a related partner-switching mechanism operates in a distinct bacterial lineage, the cyanobacteria, where it controls a major developmental transition involving terminal cell differentiation. Specifically, we show that a phosphorylation-dependent checkpoint involving the anti-sigma factor NfsS and the anti-anti-sigma factor NfsR directly regulates heterocyst formation. Disruption of this switch abolishes cell differentiation and diazotrophic growth, revealing that this system is an obligate gatekeeper for terminal differentiation. Conceptually, these findings substantially extend the known functional repertoire of partner-switching circuits: rather than controlling stress adaptation or spore dormancy, this module has been co-opted to govern a complex, multicellular developmental program in an organism that underpins global carbon and nitrogen cycles. This work, therefore, establishes a new paradigm for phosphorylation-based control of developmental sigma factors and provides a tractable model for dissecting how conserved signaling modules are rewired to drive lineage-specific innovations across the bacterial domain.

cell differentiation↗
PubMed2026

Proteobacteria with chemosynthetic potential are highly prevalent in the gills of Hypoplectrus reef fishes.

Fishes host a diverse microbiome in their gills, but a broad characterization of this microbiome at the metagenomic level is lacking. Here, we apply genome-resolved metagenomics to the gills of the hamlets (Hypoplectrus spp), a group of reef fishes from the Greater Caribbean. The analysis of 353 gill samples from 15 hamlet species collected at eight locations over 13 years revealed a stark contrast between the gill microbiota and reef water microbial communities, indicating a distinct and specific gill microbiome. A total of 70 gill-associated metagenome-assembled genomes (MAGs) were recovered. These MAGs belong to 17 lineages, most of which are novel. They relate to known fish gill pathogens, fish gut microbes, free-living and biofilm-associated taxa, indicating that the gill microbiome was assembled from a collection of distinct eco-evolutionary trajectories. The MAGs harbor diverse metabolic modules, involved notably in nitrogen cycling, antibiotic production and biofilm formation, revealing a highly dynamic microbial ecosystem. One lineage in the Burkholderiaceae family was outstandingly prevalent across fish host species, sampling locations and years. Its genome encoded complete metabolic modules for carbon fixation and sulfur oxidation, indicating chemosynthetic potential. To the best of our knowledge, this is the first line of evidence that fishes may host sulfur-oxidizing chemosynthetic bacteria in their gills. The functional significance of this chemosynthetic potential for the fish host or other members of the gill microbiome remains to be established. The high prevalence of this lineage allowed to build a pangenome. It revealed large-scale geographic structure (western Caribbean, eastern Caribbean and Gulf of Mexico), which parallels the phylogenomic pattern observed in the hamlets. Overall, our findings point to complex fish host-microbe and microbe-microbe eco-evolutionary interactions in the gills that may influence fish physiology, homeostasis and immune response.

Animals↗
PubMed2026

Long-term PFOA and cadmium Co-contamination alters soil carbon, nitrogen, and phosphorus cycling: Insights from metagenomics and metabolomics.

The co-existence of perfluorooctanoic acid (PFOA) and cadmium (Cd) in soil poses a combined threat to microbial communities. However, the ecological effects and underlying mechanisms of their long-term combined exposure remain poorly understood. This study conducted a 90-day soil microcosm experiment to systematically investigate the effects of individual and combined effects of PFOA and Cd on microbial communities. Our results demonstrated that combined pollution of PFOA and Cd significantly affected four soil enzyme activities associated with carbon, nitrogen, and phosphorus cycling. It also influenced microbial thermal activity with an IC50 of PFOA at 0.94 mg/kg. The toxic interaction between PFOA and Cd varied with both toxicity indicators and exposure time. At the community level, PFOA and Cd synergistically reduced bacterial diversity and richness, while exerting more complex interactive effects on fungal communities. Metagenomic analysis revealed that PFOA and Cd significantly affected carbon, nitrogen, and phosphorus cycling by inhibiting inorganic phosphorus solubilization genes (gcd, pqqC) and altering key genes in carbon fixation and nitrogen transformation. Metabolomic profiling further demonstrated that PFOA disrupted membrane lipid homeostasis and amino acid metabolism. Meanwhile, the co-existence of Cd exacerbated disturbances in sugar and carbon metabolism. Our findings provide genetic-level insights into microbial responses to long-term PFOA and Cd co-contamination. These results are essential for risk assessment at such co-contamination sites.

Cadmium↗
PubMed1978

The hydrogen cycle in nitrogen-fixing Azotobacter chroococcum.

H2 will support nitrogenase activity (C2H2 reduction) in Azotobacter chroococcum with or without added carbon substrate. Results show that H2 is metabolised to transfer electrons to nitrogenase and to the respiratory chain to produce ATP. H2-supported nitrogenase activity is most significant at low carbon substrate concentrations, but also occurs at saturating concentration. Continuous cultures of N2-fixing A. chroococcum evolved H2 from nitrogenase under O2-N2- and C-limited conditions. This H2 represented a significant proportion of nitrogenase activity. Hydrogenase activity was consistently high under C-limited conditions, but low or undetectable under O2- and N2-limitations. Pre-treatment with 40 per cent C2H2 inhibited hydrogenase activity in C-limited cultures, and H2 evolution increased under air and under Ar:O2 (4:1) mixtures. We deduce that hydrogenase : I, recycles H2 produced by nitrogenase to provide electrons and energy for N2 reduction: II, supports respiratory protection for nitrogenase under C-limited conditions, and III, does not act to prevent any inhibition of N2 reduction by H2 produced by nitrogenase. A scheme for the H2 cycle in N2-fixing A. chroococcum is proposed.

Azotobacter↗