The crane fly glycosylated triketide δ-lactone cornicinine elicits akinete differentiation of the cyanobiont in aquatic Azolla fern symbioses.

The restriction of plant-symbiont dinitrogen fixation by an insect semiochemical had not been previously described. Here we report on a glycosylated triketide δ-lactone from Nephrotoma cornicina crane flies, cornicinine, that causes chlorosis in the floating-fern symbioses from the genus Azolla. Only the glycosylated trans-A form of chemically synthesized cornicinine was active: 500 nM cornicinine in the growth medium turned all cyanobacterial filaments from Nostoc azollae inside the host leaf-cavities into akinetes typically secreting CTB-bacteriocins. Cornicinine further inhibited akinete germination in Azolla sporelings, precluding re-establishment of the symbiosis during sexual reproduction. It did not impact development of the plant Arabidopsis thaliana or several free-living cyanobacteria from the genera Anabaena or Nostoc but affected the fern host without cyanobiont. Fern-host mRNA sequencing from isolated leaf cavities confirmed high NH4-assimilation and proanthocyanidin biosynthesis in this trichome-rich tissue. After cornicinine treatment, it revealed activation of Cullin-RING ubiquitin-ligase-pathways, known to mediate metabolite signaling and plant elicitation consistent with the chlorosis phenotype, and increased JA-oxidase, sulfate transport and exosome formation. The work begins to uncover molecular mechanisms of cyanobiont differentiation in a seed-free plant symbiosis important for wetland ecology or circular crop-production today, that once caused massive CO2 draw-down during the Eocene geological past.

Azospirillum isscasi sp. nov., a bacterium isolated from rhizosphere soil of rice.

A Gram-negative and rod-shaped bacterium, designated C340-1T, was isolated and screened from paddy soil in Zhongshan County, Guangxi Province, PR China. This strain grew at 20-42 °C (optimum, 37 °C), pH 5.0-9.0 (optimum, pH 7.0) and 0-4 % (w/v) NaCl (optimum, 0-1 %) on Reasoner's 2A medium. The strain could fix atmospheric nitrogen and acetylene reduction activity was recorded up to 120.26 nmol ethylene h-1 (mg protein)-1. Q-10 was the only isoprenoid quinone component; phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, an unidentified aminolipid and an unidentified polar lipid were the major polar lipids. Summed feature 8 (C18 : 1 ω7c and/or C18 : 1 ω6c) and summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c) were the primary cellular fatty acids. The genome of strain C340-1T was 6.18 Mb, and the G+C content was 69.0 mol%. Phylogenetic tree analysis based on 16S rRNA gene and 92 core genes showed that strain C340-1T was closely related to and clustered with the type strains Azospirillum brasilense JCM 1224T, Azospirillum argentinense Az39T, Azospirillum baldaniorum Sp245T and Azospirillum formosense JCM 17639T. The average nucleotide identity (ANI), average amino acid identity (AAI) and digital DNA-DNA hybridization (dDDH) values between strain C340-1T and the closely related type strains mentioned above were significantly lower than the threshold values for species classification (95-96 %, 95-96 % and 70 %, respectively). Based on phylogenetic, genomic, phenotypic, physiological and biochemical data, we have reason to believe that C340-1T represents a new species of the genus Azospirillum, for which the name Azospirillum isscasi sp. nov. is proposed. The type strain is C340-1T(=CCTCC AB 2023105T=KCTC 8126T).

Novel pyrochlore type europium stannate - tungsten disulfide heterostructure for light driven carbon dioxide reduction and nitrogen fixation.

The reduction of carbon dioxide (CO2) and nitrogen (N2) to value-added products is a substantial area of research in the fields of sustainable chemistry and renewable energy that aims at reducing greenhouse gas emissions and the production of alternative fuels and chemicals. The current work deals with the synthesis of pyrochlore-type europium stannate (Eu2Sn2O7: EuSnO), tungsten disulfide (WS2:WS), and novel EuSnO/WS heterostructure by a simple and facile co-precipitation-aided hydrothermal method. Using different methods, the morphological and structural analyses of the prepared samples were characterized. It was confirmed that a heterostructure was formed between the cubic EuSnO and the layered WS. Synthesized materials were used for photocatalytic CO2 and N2 reduction under UV and visible light. The amount of CO and CH4 evolved due to CO2 reduction is high in EuSnO/WS (CO = 104, CH4 = 64 µmol h-1 g-1) compared to pure EuSnO (CO = 36, CH4 = 70 µmol h-1 g-1) and WS (CO = 22, CH4 = 1.8 µmol h-1 g-1) under visible light. The same trend was observed even in the N2 fixation reaction under visible light, and the amount of NH4+ produced was found to be 13, 26, and 41 µmol h-1 g-1 in the presence of WS, EuSnO and EuSnO/WS, respectively. Enhanced light-driven activity towards CO2 and N2 reduction reactions in EuSnO/WS is due to the efficient charge separation through the formation of type-II heterostructure, which is in part associated with photocurrent response, photoluminescence, and electrochemical impedence spectroscopic (EIS) results. The EuSnO/WS heterostructure's exceptional stability and reusability may pique the attention of pyrochlore-based composite materials in photocatalytic energy and environmental applications.

N2 Fixation, N Transfer, and Land Equivalent Ratio (LER) in Grain Legume-Wheat Intercropping: Impact of N Supply and Plant Density.

Intercropping legumes with cereals can lead to increased overall yield and optimize the utilization of resources such as water and nutrients, thus enhancing agricultural efficiency. Legumes possess the unique ability to acquire nitrogen (N) through both N2 fixation and from the available N in the soil. However, soil N can diminish the N2 fixation capacity of legumes. It is postulated that in intercropping, legumes uptake N mainly through N2 fixation, leaving more soil N available for cereals. The latter, in turn, has larger root systems, allowing it to explore greater soil volume and absorb more N, mitigating its adverse effects on N2 fixation in legumes. The goal of this study was to evaluate how the supply of N affects the intercropping of faba beans (Vicia faba L.) and peas (Pisum sativum L.) with wheat under varying plant densities and N levels. We measured photosynthetic traits, biomass production, the proportion of N derived from air (%Ndfa) in the shoot of the legumes, the N transferred to the wheat, and the land equivalent ratio (LER). The results revealed a positive correlation between soil N levels and the CO2 assimilation rate (An), chlorophyll content, and N balance index (NBI) in wheat. However, no significant effect was observed in legumes as soil N levels increased. Transpiration (E) increased in wheat intercropped with legumes, while stomatal conductance (gs) increased with N addition in all crops. Water use efficiency (WUE) decreased in faba beans intercropped with wheat as N increased, but it showed no significant change in wheat or peas. The shoot dry matter of wheat increased with the addition of N; however, the two legume species showed no significant changes. N addition reduced the %Ndfa of both legume species, especially in monoculture, with peas being more sensitive than faba beans. The intercropping of wheat alleviated N2 fixation inhibition, especially at high wheat density and increased N transfer to wheat, particularly with peas. The LER was higher in the intercropping treatments, especially under limited N conditions. It is concluded that in the intercropping of wheat with legumes, the N2 fixation inhibition caused by soil N is effectively reduced, as well as there being a significant N transfer from the legume to the wheat, with both process contributing to increase LER.

Fronts divide diazotroph communities in the Southern Indian Ocean.

Dinitrogen (N2) fixation represents a key source of reactive nitrogen in marine ecosystems. While the process has been rather well-explored in low latitudes of the Atlantic and Pacific Oceans, other higher latitude regions and particularly the Indian Ocean have been chronically overlooked. Here, we characterize N2 fixation and diazotroph community composition across nutrient and trace metals gradients spanning the multifrontal system separating the oligotrophic waters of the Indian Ocean subtropical gyre from the high nutrient low chlorophyll waters of the Southern Ocean. We found a sharp contrasting distribution of diazotroph groups across the frontal system. Notably, cyanobacterial diazotrophs dominated north of fronts, driving high N2 fixation rates (up to 13.96 nmol N l-1 d-1) with notable peaks near the South African coast. South of the fronts non-cyanobacterial diazotrophs prevailed without significant N2 fixation activity being detected. Our results provide new crucial insights into high latitude diazotrophy in the Indian Ocean, which should contribute to improved climate model parameterization and enhanced constraints on global net primary productivity projections.

Seed-Coat Pigmentation Plays a Crucial Role in Partner Selection and N2 Fixation in Legume-Root-Microbe Associations in African Soils.

Legume-rhizobia symbiosis is the most important plant-microbe interaction in sustainable agriculture due to its ability to provide much needed N in cropping systems. This interaction is mediated by the mutual recognition of signaling molecules from the two partners, namely legumes and rhizobia. In legumes, these molecules are in the form of flavonoids and anthocyanins, which are responsible for the pigmentation of plant organs, such as seeds, flowers, fruits, and even leaves. Seed-coat pigmentation in legumes is a dominant factor influencing gene expression relating to N2 fixation and may be responsible for the different N2-fixing abilities observed among legume genotypes under field conditions in African soils. Common bean, cowpea, Kersting's groundnut, and Bambara groundnut landraces with black seed-coat color are reported to release higher concentrations of nod-gene-inducing flavonoids and anthocyanins compared with the Red and Cream landraces. Black seed-coat pigmentation is considered a biomarker for enhanced nodulation and N2 fixation in legumes. Cowpea, Bambara groundnut, and Kersting's bean with differing seed-coat colors are known to attract different soil rhizobia based on PCR-RFLP analysis of bacterial DNA. Even when seeds of the same legume with diverse seed-coat colors were planted together in one hole, the nodulating bradyrhizobia clustered differently in the PCR-RFLP dendrogram. Kersting's groundnut, Bambara groundnut, and cowpea with differing seed-coat colors were selectively nodulated by different bradyrhizobial species. The 16S rRNA amplicon sequencing also found significant selective influences of seed-coat pigmentation on microbial community structure in the rhizosphere of five Kersting's groundnut landraces. Seed-coat color therefore plays a dominant role in the selection of the bacterial partner in the legume-rhizobia symbiosis.

Degradation of Complex Carbon Sources in Organic Fertilizers Facilitates Nitrogen Fixation.

Biological nitrogen fixation is crucial for agriculture and improving fertilizer efficiency, but organic fertilizers in enhancing this process remain debated. Here, we investigate the impact of organic fertilizers on biological nitrogen fixation through experiments and propose a new model where bacterial interactions with complex carbon sources enhance nitrogen fixation. Field experiments showed that adding organic fertilizers increased the nitrogenase activity by 57.85%. Subculture experiments revealed that organic fertilizer addition enriched genes corresponding to complex carbon and energy metabolism, as well as nifJ involved in electron transfer for nitrogenase. It also enhanced bacterial interactions and enhanced connectors associated with complex carbon degradation. Validation experiments demonstrated that combinations increased nitrogenase activity by 2.98 times compared to the single. Our findings suggest that organic fertilizers promoted nitrogen fixation by enhancing microbial cooperation, improved the degradation of complex carbon sources, and thereby provided utilizable carbon sources, energy, and electrons to N-fixers, thus increasing nitrogenase activity and nitrogen fixation.

Predominant heterotrophic diazotrophic bacteria are involved in Sargassum proliferation in the Great Atlantic Sargassum Belt.

Since 2011, the Caribbean coasts have been subject to episodic influxes of floating Sargassum seaweed of unprecedented magnitude originating from a new area "the Great Atlantic Sargassum Belt" (GASB), leading in episodic influxes and mass strandings of floating Sargassum. For the biofilm of both holopelagic and benthic Sargassum as well as in the surrounding waters, we characterized the main functional groups involved in the microbial nitrogen cycle. The abundance of genes representing nitrogen fixation (nifH), nitrification (amoA), and denitrification (nosZ) showed the predominance of diazotrophs, particularly within the GASB and the Sargasso Sea. In both location, the biofilm associated with holopelagic Sargassum harboured a more abundant proportion of diazotrophs than the surrounding water. The mean δ15N value of the GASB seaweed was very negative (-2.04‰), and lower than previously reported, reinforcing the hypothesis that the source of nitrogen comes from the nitrogen-fixing activity of diazotrophs within this new area of proliferation. Analysis of the diversity of diazotrophic communities revealed for the first time the predominance of heterotrophic diazotrophic bacteria belonging to the phylum Proteobacteria in holopelagic Sargassum biofilms. The nifH sequences belonging to Vibrio genus (Gammaproteobacteria) and Filomicrobium sp. (Alphaproteobacteria) were the most abundant and reached, respectively, up to 46.0% and 33.2% of the community. We highlighted the atmospheric origin of the nitrogen used during the growth of holopelagic Sargassum within the GASB and a contribution of heterotrophic nitrogen-fixing bacteria to a part of the Sargassum proliferation.

Integrated Proteomics and Metabolomics Reveal Altered Metabolic Regulation of Xanthobacter autotrophicus under Electrochemical Water-Splitting Conditions.

Biological-inorganic hybrid systems are a growing class of technologies that combine microorganisms with materials for a variety of purposes, including chemical synthesis, environmental remediation, and energy generation. These systems typically consider microorganisms as simple catalysts for the reaction of interest; however, other metabolic activity is likely to have a large influence on the system performance. The investigation of biological responses to the hybrid environment is thus critical to the future development and optimization. The present study investigates this phenomenon in a recently reported hybrid system that uses electrochemical water splitting to provide reducing equivalents to the nitrogen-fixing bacteria Xanthobacter autotrophicus for efficient reduction of N2 to biomass that may be used as fertilizer. Using integrated proteomic and metabolomic methods, we find a pattern of differentiated metabolic regulation under electrochemical water-splitting (hybrid) conditions with an increase in carbon fixation products glycerate-3-phosphate and acetyl-CoA that suggests a high energy availability. We further report an increased expression of proteins of interest, namely, those responsible for nitrogen fixation and assimilation, which indicate increased rates of nitrogen fixation and support previous observations of faster biomass accumulation in the hybrid system compared to typical planktonic growth conditions. This work complicates the inert catalyst view of biological-inorganic hybrids while demonstrating the power of multiomics analysis as a tool for deeper understanding of those systems.

Effects of methane to oxygen ratio on cell growth and polyhydroxybutyrate synthesis in high cell density cultivation of Methylocystis sp. MJC1.

Polyhydroxybutyrate (PHB) production through CH4 conversion by methanotrophs offers a solution for greenhouse gas emissions and plastic waste concerns. In this study, we aimed to achieve high cell density cultivation of Methylocystis sp. MJC1 for efficient PHB production. Cultivating MJC1 using CH4 and air (3:7, v/v) yielded a final cell density of 52.9 g/L with a 53.7% (28.4 g/L) PHB content after 210 h, showcasing PHB mass production potential. However, long-term cultivation led to a low volumetric productivity of 0.200 g/L/h. To address this, we conducted cultivation at various O2/CH4 ratios using O2 instead of air, which significantly improved the PHB productivity. Under high O2 conditions (O2/CH4 ratio of 1.5), biomass productivity increased 1.51-fold compared to that under low O2 conditions in the same time frame; however, PHB accumulation was delayed. Using an equal ratio of CH4 and O2 induced active cell growth and selective PHB production, achieving the highest PHB productivity (0.365 g/L/h) with a final cell density of 55.9 g/L and PHB content of 61.7% (34.5 g/L) in 162 h. This study highlighted the significance of the O2/CH4 ratio in CH4 conversion and PHB production by M. sp. MJC1.

Integrating bimetallic borides with g-C3N4 containing cyanamide defects for efficient photocatalytic nitrogen fixation.

The sustainable generation of ammonia by photocatalytic nitrogen fixation under mild conditions is fascinating compared to conventional industrial processes. Nevertheless, owing to the low charge transfer efficiency, the insufficient light absorption capacity and limited active sites of the photocatalyst cause the difficult adsorption and activation of N2 molecules, thereby resulting in a low photocatalytic conversion efficiency. Herein, a novel bimetallic CoMoB nanosheets (CoMoB) co-catalyst modified carbon nitride with dual moiety defects (CN-TH3/3) Schottky junction photocatalyst is designed for photocatalytic nitrogen reduction reaction (NRR). The photocatalytic nitrogen reduction rate of the optimized CoMoB/CN-TH3/3 photocatalyst is 4.81 mM·g-1·h-1, which is 6.2 and 2.2 times higher than carbon nitride (CN) (0.78 mM·g-1·h-1) and CN-TH3/3 (2.21 mM·g-1·h-1), respectively. The excellent photocatalytic NRR performance is ascribed not only to the introduction of dual moiety defects (cyano and cyanamide groups) that extends the visible light absorption range and promotes exciton polarization dissociation, but also to the formation of interfacial electric field between CoMoB and CN-TH3/3, which effectively facilitates the interfacial charge transfer. Thus, the synergistic interaction between CN-TH3/3 and CoMoB further increases the electron numble of CoMoB active sites, which effectively strengthens the adsorption and activation of N2 and weakens the NN triple bond, thereby enhancing the photocatalytic NRR activity. This work highlights the introduced dual moiety defects and bimetallic CoMoB co-catalyst to synergistically enhance the photocatalytic nitrogen reduction performance.

Nitrogen-fixing and non-nitrogen-fixing legume plants differ in leaf nutrient concentrations and relationships between photosynthetic and hydraulic traits.

Legumes account for a significant proportion of plants in the terrestrial ecosystems. Nitrogen (N)-fixing capability of certain legumes is a pivotal trait that contributes to their ecological dominance. Yet, the functional traits and trait relationships between N-fixer and non-N-fixer legumes are poorly understood. Here, we investigated 27 functional traits associated with morphology, nutrients, hydraulic conductance and photosynthesis in 42 woody legumes (19 N-fixers and 23 non-N-fixers) in a common garden. Our results showed that N-fixers had higher specific leaf area, photosynthetic phosphorus (P)-use efficiency, leaf N, and iron concentrations on both area and mass basis, N/P ratio, and carbon (C) to P ratio, but lower wood density, area-based maximum photosynthetic rate (Aa), photosynthetic N-use efficiency, leaf mass- and area-based P and molybdenum and area-based boron concentrations, and C/N ratio, compared with non-N-fixers. The mass-based maximum photosynthetic rate (Am), stomatal conductance (gs), intrinsic water-use efficiency (WUEi), mass- and area-based leaf potassium and mass-based boron concentrations, leaf hydraulic conductance (Kleaf), and whole-shoot hydraulic conductance (Kshoot) showed no difference between N-fixers and non-N-fixers. Significant positive associations between all hydraulic and photosynthetic trait pairs were found in N-fixers, but only one pair (Kshoot-Aa) in non-N-fixers, suggesting that hydraulic conductance plays a more important role in mediating photosynthetic capacity in N-fixers compared with non-N-fixers. Higher mass-based leaf N was linked to lower time-integrated gs and higher WUEi among non-N-fixer legumes or all legumes pooled after phylogeny was considered. Moreover, mass-based P concentration was positively related to Am and gs in N-fixers, but not in non-N-fixers, indicating that the photosynthetic capacity and stomatal conductance in N-fixers were more dependent on leaf P status than in non-N-fixers. These findings expand our understanding of the trait-based ecology within and across N-fixer and non-N-fixer legumes in tropics.

Correlation of methane production with physiological traits in Trichodesmium IMS 101 grown with methylphosphonate at different temperatures.

The diazotrophic cyanobacterium Trichodesmium has been recognized as a potentially significant contributor to aerobic methane generation via several mechanisms including the utilization of methylphophonate (MPn) as a source of phosphorus. Currently, there is no information about how environmental factors regulate methane production by Trichodesmium. Here, we grew Trichodesmium IMS101 at five temperatures ranging from 16 to 31°C, and found that its methane production rates increased with rising temperatures to peak (1.028 ± 0.040 nmol CH4 µmol POC-1 day-1) at 27°C, and then declined. Its specific growth rate changed from 0.03 ± 0.01 d-1 to 0.34 ± 0.02 d-1, with the optimal growth temperature identified between 27 and 31°C. Within the tested temperature range the Q10 for the methane production rate was 4.6 ± 0.7, indicating a high sensitivity to thermal changes. In parallel, the methane production rates showed robust positive correlations with the assimilation rates of carbon, nitrogen, and phosphorus, resulting in the methane production quotients (molar ratio of carbon, nitrogen, or phosphorus assimilated to methane produced) of 227-494 for carbon, 40-128 for nitrogen, and 1.8-3.4 for phosphorus within the tested temperature range. Based on the experimental data, we estimated that the methane released from Trichodesmium can offset about 1% of its CO2 mitigation effects.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2.

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50-500 µg cyanobacterial chlorophyll-a L-1) and lake CO2 levels were undersaturated (≤300 µatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Biomass and nitrogen fixation dataset of Pisum sativum L. and Vicia faba L. cultivated under elevated CO2 and nitrogen addition.

It is expected that CO2 concentration will increase in the air, thereby stimulating the photosynthesis process and, hence, plant biomass production. In the case of legumes, increased biomass due to higher CO2 concentration can stimulate atmospheric nitrogen (N2) fixation in the nodules. However, N2 fixation is inhibited by external N supply. Thus, biomass production and N2 fixation were analysed in two legumes (Pisum sativum L. and Vicia faba L.) grown at two levels of CO2 and three N levels. P. sativum reduces fixation with high soil N (facultative), while V. faba maintains high fixation regardless of soil N levels (obligate). The N2 fixation and plant and nodule biomass of the two species were evaluated in a pot experiment under controlled conditions using growth chambers with artificial CO2 supply and N addition. The proportion of N derived from the air (%Ndfa) present in the plants' biomass was calculated from the natural abundance of 15N and the N concentration of plant tissues using nonlegumes reference plants. Additionally, N content data are presented for both species growing at two levels of air CO2. The data may be useful for plant physiologists, especially those working on biological N2 fixation with non-model legumes at elevated CO2.

Intensive N2 fixation accelerates microbial turnover in cropland soils.

Biological nitrogen fixation (BNF) is strongly affected by the carbon (C) and nitrogen (N) stoichiometry in soil and depends on the input of organic C. Due to the high metabolic costs of nitrogenase activity, however, the response of BNF to organic C input and its impact on microbial turnover remain unclear. To address this knowledge gap, we combined 15N2 tracing with high-throughput sequencing by adding glucose or glucose plus mineral N fertilizer for a 12-day incubation in three cropland soils. Glucose addition alone strongly changed the BNF activity (0.76-2.51 mg N kg-1 d-1), while BNF was completely absent after mineral N fertilization. This switch-on of BNF by glucose addition supported equally high rates of microbial growth and organic C mineralization compared with the direct mineral N assimilation by microorganisms. Glucose-induced BNF was predominantly catalyzed by Azotobacter-affiliated free-living diazotrophs (>50 % of the total nifH genes), which increased with diverse nondiazotrophs such as Nitrososphaera, Bacillus and Pseudoxanthomonas. Structural equation models (SEMs) and random forest (RF) analyses consistently revealed that the soil C:N ratio and Azotobacter-affiliated diazotrophic abundances were the key factors affecting glucose-induced BNF. Our findings emphasize the importance of free-living diazotrophs for microbial turnover of organic C in soil.

Projecting global biological N2 fixation under climate warming across land and ocean.

Biological N2 fixation sustains the global inventory of nitrogenous nutrients essential for the productivity of terrestrial and marine ecosystems. Like most metabolic processes, rates of biological N2 fixation vary strongly with temperature, making it sensitive to climate change, but a global projection across land and ocean is lacking. Here we use compilations of field and laboratory measurements to reveal a relationship between N2 fixation rates and temperature that is similar in both domains despite large taxonomic and environmental differences. Rates of N2 fixation increase gradually to a thermal optimum around ~25°C, and decline more rapidly toward a thermal maximum, which is lower in the ocean than on land. In both realms, the observed temperature sensitivities imply that climate warming this century could decrease N2 fixation rates by ~50% in the tropics while increasing rates by ~50% in higher latitudes. We propose a conceptual framework for understanding the physiological and ecological mechanisms that underpin and modulate the observed temperature dependence of global N2 fixation rates, facilitating cross-fertilization of marine and terrestrial research to assess its response to climate change.

Autochthonous carbon loading of macroalgae stimulates benthic biological nitrogen fixation rates in shallow coastal marine sediments.

Macroalgae, commonly known as seaweed, are foundational species in coastal ecosystems and contribute significantly to coastal primary production globally. However, the impact of macroalgal decomposition on benthic biological nitrogen fixation (BNF) after deposition to the seafloor remains largely unexplored. In this study, we measure BNF rates at three different sites at the Big Fisherman's Cove on Santa Catalina Island, CA, USA, which is representative of globally distributed rocky bottom macroalgal habitats. Unamended BNF rates varied among sites (0.001-0.05 nmol N g-1 h -1) and were generally within the lower end of previously reported ranges. We hypothesized that the differences in BNF between sites were linked to the availability of organic matter. Indeed, additions of glucose, a labile carbon source, resulted in 2-3 orders of magnitude stimulation of BNF rates in bottle incubations of sediment from all sites. To assess the impact of complex, autochthonous organic matter, we simulated macroalgal deposition and remineralization with additions of brown (i.e., Macrocystis pyrifera and Dictyopteris), green (i.e., Codium fragile), and red (i.e., Asparagopsis taxiformis) macroalgae. While brown and green macroalgal amendments resulted in 53- to 520-fold stimulation of BNF rates-comparable to the labile carbon addition-red alga was found to significantly inhibit BNF rates. Finally, we employed nifH sequencing to characterize the diazotrophic community associated with macroalgal decomposition. We observed a distinct community shift in potential diazotrophs from primarily Gammaproteobacteria in the early stages of remineralization to a community dominated by Deltaproteobacteria (e.g., sulfate reducers), Bacteroidia, and Spirochaeta toward the latter phase of decomposition of brown, green, and red macroalgae. Notably, the nifH-containing community associated with red macroalgal detritus was distinct from that of brown and green macroalgae. Our study suggests coastal benthic diazotrophs are limited by organic carbon and demonstrates a significant and phylum-specific effect of macroalgal loading on benthic microbial communities.

Estimation of Nitrogenase Enzyme Activities and Plant Growth of Legume and Non-legume Inoculated with Diazotrophic Bacteria

Biological Nitrogen Fixation (BNF) process benefits the agriculture sector especially for reducing cost of nitrogen fertilizer. In the process, the diazotrophs convert N2 into ammonia (NH3) which is useable by plants. The BNF process is catalysed by nitrogenase enzyme that involved protons and electrons together with evolution of H2 therefore, the assessment of N2 fixation is also available via H2 production and electron allocation analysis. Thus, the aims of this experiment were to estimate the nitrogenase enzyme activities and observe the influence of diazothrophs on growth of legume (soybean) and non legume (rice) plants. Host plants were inoculated with respective inocula; Bradyrhizobium japonicum (strain 532C) for soybean while Azospirillum brasilense (Sp7) and locally isolated diazotroph (isolate 5) for rice. At harvest, the plants were observed for plant growth parameters, H2 evolution, N2 fixation and electron allocation coefficient (EAC) values. The experiment recorded N2 fixation activities of inoculated soybean plants at 141.2 μmol N2 h-1 g-1 dry weight nodule, and the evolution of H2 at 144.4 μmol H2 h-1 g-1 dry weight nodule. The electron allocation coefficient (EAC) of soybean was recorded at 0.982. For inoculated rice plants, none of the observations was successfully recorded. However, results for chlorophyll contents and plant dry weight of both plants inoculated with respective inocula were similar to the control treatments supplied with full nitrogen fertilization (+N). The experiment clearly showed that inoculation of diazotrophic bacteria could enhance growth of the host plants similar to plants treated with nitrogenous fertilizer due to efficient N2 fixation process