Showing papers on "Nitrogen fixation published in 2018"
TL;DR: Recovery of population genomes from surface ocean samples identified non-cyanobacterial diazotrophs that were widespread and abundant, including Proteobacteria and Planctomycetes, indicating their importance for nitrogen fixation in this environment.
Abstract: Nitrogen fixation in the surface ocean impacts global marine nitrogen bioavailability and thus microbial primary productivity. Until now, cyanobacterial populations have been viewed as the main suppliers of bioavailable nitrogen in this habitat. Although PCR amplicon surveys targeting the nitrogenase reductase gene have revealed the existence of diverse non-cyanobacterial diazotrophic populations, subsequent quantitative PCR surveys suggest that they generally occur in low abundance. Here, we use state-of-the-art metagenomic assembly and binning strategies to recover nearly one thousand non-redundant microbial population genomes from the TARA Oceans metagenomes. Among these, we provide the first genomic evidence for non-cyanobacterial diazotrophs inhabiting surface waters of the open ocean, which correspond to lineages within the Proteobacteria and, most strikingly, the Planctomycetes. Members of the latter phylum are prevalent in aquatic systems, but have never been linked to nitrogen fixation previously. Moreover, using genome-wide quantitative read recruitment, we demonstrate that the discovered diazotrophs were not only widespread but also remarkably abundant (up to 0.3% of metagenomic reads for a single population) in both the Pacific Ocean and the Atlantic Ocean northwest. Our results extend decades of PCR-based gene surveys, and substantiate the importance of heterotrophic bacteria in the fixation of nitrogen in the surface ocean.
369 citations
TL;DR: In this paper, the co-association and assemblage process of diazotrophic community members in both rhizosphere and bulk soil of wheat fields was investigated, and it was found that deterministic versus stochastic community assemblages dominated in soils with neutral pH, while deterministic processes dominated in acidic or alkaline soils.
Abstract: Biological nitrogen fixation contributes to the pool of plant-available N in both bulk soil and the rhizosphere. Here we investigated the co-association and assemblage process of diazotrophic community members in both rhizosphere and bulk soil of wheat fields. The diazotrophic community structure in the rhizosphere was significantly different and comprised a less competitive and more stable network structure when compared with that of the bulk soil. Deterministic versus stochastic community assemblage processes were quantified using betaNTI scores, demonstrating that deterministic processes decreased in importance with distance from plant roots. Soil pH was correlated with diazotrophic community structure and diversity, and community structure showed greater connectivity and stability in soils with neutral pH relative to those in acidic or alkaline soils. Stochastic processes dominated the assemblage of the diazotrophic community in soils with neutral pH, while deterministic processes dominated in acidic or alkaline soils. These results suggest that soil pH may play an essential role in the interaction and assemblage processes of the diazotrophic community in the rhizosphere and bulk soils, which could enhance our understanding of biological nitrogen fixation in agricultural soils.
217 citations
TL;DR: Analysis of the mucilage microbiota of an indigenous landrace of maize grown in nitrogen-depleted soils in the Sierra Mixe region of Oaxaca, Mexico indicated that it was enriched in taxa for which many known species are diazotrophic, was enriched for homologs of genes encoding nitrogenase subunits, and harbored active nitrogenase activity.
Abstract: Plants are associated with a complex microbiota that contributes to nutrient acquisition, plant growth, and plant defense. Nitrogen-fixing microbial associations are efficient and well characterized in legumes but are limited in cereals, including maize. We studied an indigenous landrace of maize grown in nitrogen-depleted soils in the Sierra Mixe region of Oaxaca, Mexico. This landrace is characterized by the extensive development of aerial roots that secrete a carbohydrate-rich mucilage. Analysis of the mucilage microbiota indicated that it was enriched in taxa for which many known species are diazotrophic, was enriched for homologs of genes encoding nitrogenase subunits, and harbored active nitrogenase activity as assessed by acetylene reduction and 15N2 incorporation assays. Field experiments in Sierra Mixe using 15N natural abundance or 15N-enrichment assessments over 5 years indicated that atmospheric nitrogen fixation contributed 29%-82% of the nitrogen nutrition of Sierra Mixe maize.
215 citations
TL;DR: This review will provide an update on the current knowledge of how the recognition specificity has evolved in the context of symbiosis signaling and plant immunity.
Abstract: Legumes are able to form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is to form nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant. Establishment of a successful symbiosis requires the two symbiotic partners to be compatible with each other throughout the process of symbiotic development. However, incompatibility frequently occurs, such that a bacterial strain is unable to nodulate a particular host plant or forms nodules that are incapable of fixing nitrogen. Genetic and molecular mechanisms that regulate symbiotic specificity are diverse, involving a wide range of host and bacterial genes/signals with various modes of action. In this review, we will provide an update on our current knowledge of how the recognition specificity has evolved in the context of symbiosis signaling and plant immunity.
167 citations
TL;DR: New diazotroph mutants with enhanced capabilities to excrete ammonium are being successfully used to promote plant growth as commensal bacteria.
Abstract: Cereals such as maize, rice, wheat and sorghum are the most important crops for human nutrition. Like other plants, cereals associate with diverse bacteria (including nitrogen-fixing bacteria called diazotrophs) and fungi. As large amounts of chemical fertilizers are used in cereals, it has always been desirable to promote biological nitrogen fixation in such crops. The quest for nitrogen fixation in cereals started long ago with the isolation of nitrogen-fixing bacteria from different plants. The sources of diazotrophs in cereals may be seeds, soils, and even irrigation water and diazotrophs have been found on roots or as endophytes. Recently, culture-independent molecular approaches have revealed that some rhizobia are found in cereal plants and that bacterial nitrogenase genes are expressed in plants. Since the levels of nitrogen-fixation attained with nitrogen-fixing bacteria in cereals are not high enough to support the plant's needs and never as good as those obtained with chemical fertilizers or with rhizobium in symbiosis with legumes, it has been the aim of different studies to increase nitrogen-fixation in cereals. In many cases, these efforts have not been successful. However, new diazotroph mutants with enhanced capabilities to excrete ammonium are being successfully used to promote plant growth as commensal bacteria. In addition, there are ambitious projects supported by different funding agencies that are trying to genetically modify maize and other cereals to enhance diazotroph colonization or to fix nitrogen or to form nodules with nitrogen-fixing symbiotic rhizobia.
162 citations
TL;DR: It is indicated that long-term fertilization strongly affected the diversity, community structure and assembly processes of soil diazotrophs, which may have implications for the rate of biological N2 fixation in agricultural systems.
Abstract: Unraveling the drivers of microbial community variation in response to different environmental conditions is a major goal in ecology. Although diazotrophs play a dominant role in global biological nitrogen (N2) fixation, the controls on soil diazotrophic community assembly are not fully understood. In this study, we investigated soil diazotrophic communities in field plots that received long-term (30 years) fertilization treatments with Illumina MiSeq sequencing. Long-term chemical fertilization significantly changed soil diazotrophic community structure and resulted in the decrease of diazotrophic diversity, while the addition of livestock manure could maintain the diversity. Diazotrophic community structure and diversity were mostly correlated with soil pH. Deterministic processes structured diazotrophic communities in both unfertilized and fertilized soils. However, the deterministic selection on phylogenetically non-conserved traits increased phylogenetic randomness in all fertilized treatments. These trends for diazotrophs differed from those for the entire bacterial community, which was structured through deterministic processes and exhibited phylogenetic nonrandomness in unfertilized and fertilized soils. Taken together, our results indicated that long-term fertilization strongly affected the diversity, community structure and assembly processes of soil diazotrophs, which may have implications for the rate of biological N2 fixation in agricultural systems.
131 citations
01 Oct 2018
TL;DR: The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms as mentioned in this paper, however, only 30% of the nitrogen added to fields is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts.
Abstract: The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms. To supplement natural nitrogen fixation, farmers add large amounts of nitrogen-containing fertilizer to their soils such that nitrogen never becomes a limiting nutrient for plant growth. However, of the nitrogen added to fields — most of which is in the form of NH3 and NO3− — only 30–50% is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts. The first of these processes, that is, nitrification, refers to the biological oxidation of NH3 to NO2− and NO3−, which have low retention in soil and pollute waterways, leading to downstream eutrophication and ultimately ‘dead zones’ (low oxygen zones) in coastal waters, for example, the Gulf of Mexico. In a second process, namely, denitrification, NO3− and NO2− undergo stepwise reduction to N2O and N2. Substantial amounts of the N2O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction. Recent results suggest that nitrification also affords N2O. This Review describes the enzymes involved in NH3 oxidation and N2O production and degradation in the nitrogen cycle. We pay particular attention to the active site structures, the associated coordination chemistry that enables the chemical transformations and the reaction mechanisms. Nitrification and denitrification are responsible for the processing of ammonia fertilizer, ultimately leading to the generation of environmental pollutants that accumulate in waterways and the atmosphere. This Review describes the enzymes involved in these processes, which fascinate with their unusual active sites and the surprising reactions that they catalyse.
122 citations
TL;DR: The current state of knowledge on the biochemistry of these complex systems highlighting the common and specific structural features and catalytic activities of the enzymes, the recent progress in defining the discrete set of genes associated to N2-fixation and the regulatory features that coordinate the differential expression of genes in response to metal availability are summarized.
Abstract: Most biological nitrogen fixation (BNF) results from the activity of the molybdenum nitrogenase (Mo-nitrogenase, Nif), an oxygen-sensitive metalloenzyme complex found in all known diazotrophs. Two alternative forms of nitrogenase, the vanadium nitrogenase (V-nitrogenase, Vnf) and the iron-only nitrogenase (Fe-only nitrogenase, Anf) have also been identified in the genome of some organisms that encode for Nif. It has been suggested that alternative nitrogenases were responsible for N2-fixation on early Earth because oceans were depleted of bioavailable Mo. Results of recent phylogenetic- and structure-based studies suggest, however, that such an evolutionary path is unlikely, and favor a new model for a stepwise evolution of nitrogenase where the V-nitrogenase and the Fe-only nitrogenase are not the ancestor of the Mo-nitrogenase. Rather, Mo-nitrogenase emerged within the methanogenic archaea and then gave rise to the alternative forms suggesting they arose later in response to the availability of fixed N2 and local environmental factors that influenced metal availability. This review summarizes the current state of knowledge on (1) the biochemistry of these complex systems highlighting the common and specific structural features and catalytic activities of the enzymes, (2) the recent progress in defining the discrete set of genes associated to N2-fixation and the regulatory features that coordinate the differential expression of genes in response to metal availability, and (3) the diverse taxonomic and phylogenic distribution of nitrogenase enzymes and the evolutionary history of BNF from the perspective of metal content and metal availability.
110 citations
TL;DR: It is suggested that nutrient limitation is an intrinsic property of the biochemical demands of N fixation, constraining free-living N fixation in the terrestrial biosphere and has implications for understanding the causes and consequences of N limitation in coupled nutrient cycles, as well as modeling and forecasting nutrient controls over carbon-climate feedbacks.
Abstract: Nitrogen (N) fixation by free-living bacteria is a primary N input pathway in many ecosystems and sustains global plant productivity. Uncertainty exists over the importance of N, phosphorus (P) and molybdenum (Mo) availability in controlling free-living N fixation rates. Here, we investigate the geographic occurrence and variability of nutrient constraints to free-living N fixation in the terrestrial biosphere. We compiled data from studies measuring free-living N fixation in response to N, P and Mo fertilizers. We used meta-analysis to quantitatively determine the extent to which N, P and Mo stimulate or suppress N fixation, and if environmental variables influence the degree of nutrient limitation of N fixation. Across our compiled dataset, free-living N fixation is suppressed by N fertilization and stimulated by Mo fertilization. Additionally, free-living N fixation is stimulated by P additions in tropical forests. These findings suggest that nutrient limitation is an intrinsic property of the biochemical demands of N fixation, constraining free-living N fixation in the terrestrial biosphere. These findings have implications for understanding the causes and consequences of N limitation in coupled nutrient cycles, as well as modeling and forecasting nutrient controls over carbon-climate feedbacks.
103 citations
TL;DR: Members of the plant family Leguminosae (Fabaceae) are unique in that they have evolved a symbiotic relationship with rhizobia, a group of soil bacteria that can fix atmospheric nitrogen.
Abstract: Members of the plant family Leguminosae (Fabaceae) are unique in that they have evolved a symbiotic relationship with rhizobia (a group of soil bacteria that can fix atmospheric nitrogen). Rhizobia infect and form root nodules on their specific host plants before differentiating into bacteroids, the symbiotic form of rhizobia. This complex relationship involves the supply of C4-dicarboxylate and phosphate by the host plants to the microsymbionts that utilize them in the energy-intensive process of fixing atmospheric nitrogen into ammonium, which are in turn made available to the host plants as a source of nitrogen, a macronutrient for growth. Although nitrogen-fixing bacteroids are no longer growing, they are metabolically active. The symbiotic process is complex and tightly regulated by both the host plants and the bacteroids. The metabolic pathways of carbon, nitrogen, and phosphate are heavily regulated in the host plants, as they need to strike a fine balance between satisfying their own needs as well as those of the microsymbionts. A network of transporters for the various metabolites are responsible for the trafficking of these essential molecules between the two partners through the symbiosome membrane (plant-derived membrane surrounding the bacteroid), and these are in turn regulated by various transcription factors that control their expressions under different environmental conditions. Understanding this complex process of symbiotic nitrogen fixation is vital in promoting sustainable agriculture and enhancing soil fertility.
TL;DR: It is shown that mutation or downregulation of NLP genes prevents nitrate inhibition of infection, nodule formation and nitrogen fixation and that NLP1 is required for the expression of nitrate-responsive genes and that nitrate triggers N LP1 re-localization from the cytosol to the nucleus.
Abstract: Legume plants can assimilate inorganic nitrogen and have access to fixed nitrogen through symbiotic interaction with diazotrophic bacteria called rhizobia. Symbiotic nitrogen fixation is an energy-consuming process and is strongly inhibited when sufficient levels of fixed nitrogen are available, but the molecular mechanisms governing this regulation are largely unknown. The transcription factor nodule inception (NIN) is strictly required for nodulation and belongs to a family of NIN-like proteins (NLPs), which have been implicated in the regulation of nitrogen homeostasis in Arabidopsis. Here, we show that mutation or downregulation of NLP genes prevents nitrate inhibition of infection, nodule formation and nitrogen fixation. We find that NIN and NLPs physically interact through their carboxy-terminal PB1 domains. Furthermore, we find that NLP1 is required for the expression of nitrate-responsive genes and that nitrate triggers NLP1 re-localization from the cytosol to the nucleus. Finally, we show that NLP1 can suppress NIN activation of CRE1 expression in Nicotiana benthamiana and Medicago truncatula. Our findings highlight a central role for NLPs in the suppression of nodulation by nitrate.
TL;DR: The findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil d Diazotroph distribution on the Tibetan Plateau.
TL;DR: In this article, an experimentally available 2D MoC6 was discovered as a nitrogen reduction reaction (NRR) electrocatalyst, which exhibits excellent catalytic activity for N2 fixation at room temperature with a low potential of −0.54 V.
Abstract: Nitrogen fixation under mild conditions has been one of the most important issues and a long-standing challenge in chemistry. By means of density functional theory (DFT) calculations, an experimentally available 2D MoC6 was discovered as a nitrogen reduction reaction (NRR) electrocatalyst. Our results show that MoC6 with high stability can be prepared experimentally. In particular, MoC6 exhibits excellent catalytic activity for N2 fixation at room temperature with a low potential of −0.54 V. The optimal active site, high utilization, selective stabilization of N2H* species and destabilization of the species is responsible for the high activity of MoC6. Our findings provide a rational strategy for nitrogen activation and ammonia production.
01 Dec 2018
TL;DR: A review of the research progress in the field of plasma-assisted nitrogen fixation achieved in the past five years is presented in this paper, where both the production of NOx and the synthesis of ammonia are included, and discussion on plasma reactors, operation parameters and plasma-catalysts are given.
Abstract: Nitrogen is an essential element to plants, animals, human beings and all the other living things on earth. Nitrogen fixation, which converts inert atmospheric nitrogen into ammonia or other valuable substances, is a very important part of the nitrogen cycle. The Haber-Bosch process plays the dominant role in the chemical nitrogen fixation as it produces a large amount of ammonia to meet the demand from the agriculture and chemical industries. However, due to the high energy consumption and related environmental concerns, increasing attention is being given to alternative (greener) nitrogen fixation processes. Among different approaches, plasma-assisted nitrogen fixation is one of the most promising methods since it has many advantages over others. These include operating at mild operation conditions, a green environmental profile and suitability for decentralized production. This review covers the research progress in the field of plasma-assisted nitrogen fixation achieved in the past five years. Both the production of NOx and the synthesis of ammonia are included, and discussion on plasma reactors, operation parameters and plasma-catalysts are given. In addition, outlooks and suggestions for future research are also given.
TL;DR: The recent progress in catalytic nitrogen fixation using transition metal-dinitrogen complexes as catalysts may provide a new approach to the development of economical nitrogen fixation to replace the energy-consuming Haber-Bosch process.
Abstract: This paper describes our recent progress in catalytic nitrogen fixation using transition metal–dinitrogen complexes as catalysts. Our research group has recently developed novel reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and hydrazine using molybdenum–, iron–, cobalt– and vanadium–dinitrogen complexes under mild reaction conditions. The new findings presented in this paper may provide a new approach to the development of economical nitrogen fixation to replace the energy-consuming Haber–Bosch process.
TL;DR: It is shown that light regulates symbiotic nitrogen fixation more strongly than does soil nitrogen and that light mediates the response of symbiotics nitrogen fixation to soil nitrogen availability, and can resolve a long-standing biogeochemical paradox.
Abstract: Nitrogen limits primary production in almost every biome on Earth1,2. Symbiotic nitrogen fixation, conducted by certain angiosperms and their endosymbiotic bacteria, is the largest potential natural source of new nitrogen into the biosphere3, influencing global primary production, carbon sequestration and element cycling. Because symbiotic nitrogen fixation represents an alternative to soil nitrogen uptake, much of the work on symbiotic nitrogen fixation regulation has focused on soil nitrogen availability4-8. However, because symbiotic nitrogen fixation is an energetically expensive process9, light availability to the plant may also regulate symbiotic nitrogen fixation rates10,11. Despite the importance of symbiotic nitrogen fixation to biosphere functioning, the environmental factors that most strongly regulate this process remain unresolved. Here we show that light regulates symbiotic nitrogen fixation more strongly than does soil nitrogen and that light mediates the response of symbiotic nitrogen fixation to soil nitrogen availability. In a shadehouse experiment, low light levels (comparable with forest understories) completely shut down symbiotic nitrogen fixation, whereas soil nitrogen levels that far exceeded plant demand did not fully downregulate symbiotic nitrogen fixation at high light. For in situ forest seedlings, light was a notable predictor of symbiotic nitrogen fixation activity, but soil-extractable nitrogen was not. Light as a primary regulator of symbiotic nitrogen fixation is a departure from decades of focus on soil nitrogen availability. This shift in our understanding of symbiotic nitrogen fixation regulation can resolve a long-standing biogeochemical paradox12, and it will improve our ability to predict how symbiotic nitrogen fixation will fuel the global forest carbon sink and respond to human alteration of the global nitrogen cycle.
TL;DR: Current understanding of crop productivity in high Al soils suggests that a much greater future accumulation of Al is likely to occur in agricultural soils globally if crop irrigation is increased under a changing climate.
Abstract: Recent findings on the effect of aluminium (Al) on the functioning of legumes and their associated microsymbionts are reviewed here. Al represents 7% of solid matter in the Earth’s crust and is an important abiotic factor that alters microbial and plant functioning at very early stages. The trivalent Al (Al3+) dominates at pH < 5 in soils and becomes a constraint to legume productivity through its lethal effect on rhizobia, the host plant and their interaction. Al3+ has lethal effects on many aspects of the rhizobia/legume symbiosis, which include a decrease in root elongation and root hair formation, lowered soil rhizobial population, and suppression of nitrogen metabolism involving nitrate reduction, nitrite reduction, nitrogenase activity and the functioning of uptake of hydrogenases (Hup), ultimately impairing the N2 fixation process. At the molecular level, Al is known to suppress the expression of nodulation genes in symbiotic rhizobia, as well as the induction of genes for the formation of hexokinase, phosphodiesterase, phosphooxidase and acid/alkaline phosphatase. Al toxicity can also induce the accumulation of reactive oxygen species and callose, in addition to lipoperoxidation in the legume root elongation zone. Al tolerance in plants can be achieved through over-expression of citrate synthase gene in roots and/or the synthesis and release of organic acids that reverse Al-induced changes in proteins, as well as metabolic regulation by plant-secreted microRNAs. In contrast, Al tolerance in symbiotic rhizobia is attained via the production of exopolysaccharides, the synthesis of siderophores that reduce Al uptake, induction of efflux pumps resistant to heavy metals and the expression of metal-inducible (dmeRF) gene clusters in symbiotic Rhizobiaceae. In soils, Al toxicity is usually ameliorated through liming, organic matter supply and use of Al-tolerant species. Our current understanding of crop productivity in high Al soils suggests that a much greater future accumulation of Al is likely to occur in agricultural soils globally if crop irrigation is increased under a changing climate.
TL;DR: In this paper, the authors examined the ANF potential of switchgrass (Panicum virgatum L.), a North American prairie grass whose productivity is often unresponsive to N fertilizer addition, via separate short-term 15N2 incubations of rhizosphere soils and excised roots four times during the growing season.
Abstract: Associative N fixation (ANF), the process by which dinitrogen gas is converted to ammonia by bacteria in casual association with plants, has not been well-studied in temperate ecosystems. We examined the ANF potential of switchgrass (Panicum virgatum L.), a North American prairie grass whose productivity is often unresponsive to N fertilizer addition, via separate short-term 15N2 incubations of rhizosphere soils and excised roots four times during the growing season. Measurements occurred along N fertilization gradients at two sites with contrasting soil fertility (Wisconsin, USA Mollisols and Michigan, USA Alfisols). In general, we found that ANF potentials declined with long-term N addition, corresponding with increased soil N availability. Although we hypothesized that ANF potential would track plant N demand through the growing season, the highest root fixation rates occurred after plants senesced, suggesting that root diazotrophs exploit carbon (C) released during senescence, as C is translocated from aboveground tissues to roots for wintertime storage. Measured ANF potentials, coupled with mass balance calculations, suggest that ANF appears to be an important source of N to unfertilized switchgrass, and, by extension, to temperate grasslands in general.
TL;DR: Nitrogenfixing mutualistic relationships between plant roots and bacteria have evolved multiple times in both partners and are widely distributed across all terrestrial biomes and continents apart from Antarctica.
Abstract: Root symbiotic associations with Nfixing bacteria and mycorrhizal fungi are important evolutionary adaptations of plants to compete for nutrients. Nitrogenfixing plant–bacterial associations are widely distributed across all terrestrial biomes and continents apart from Antarctica. Nodulated plants form important components of plant communities, especially in Nlimited early successional ecosystems, riparian habitats and tropical savanna and shrubland biomes (Cleveland et al., 1999). In early successional habitats, Nfixing plants and their root symbiotic microbes contribute to soil development and facilitate recruitment of other plant species and consumers (Walker, Clarkson, Silvester, & Clarkson, 2003). The global symbiotic biological N fixation amounts roughly to 45 Mt annually, which is the main contributor to natural terrestrial N sources (Vitousek, Menge, Reed, & Cleveland, 2013). Nitrogenfixing mutualistic relationships between plant roots and bacteria have evolved multiple times in both partners (Rai, Söderbäck, & Bergman, 2000; Santi, Bogusz, & Franche, 2013; Werner, Cornwell, Sprent, Kattge, & Kiers, 2014; Doyle, 2016). The differentiated forms of associations occur as root (or additionally stem) nodules, but in multiple instances plants host Nfixing bacteria in undifferentiated leaf, stem or root tissues (Vessey, Pawlowski, & Bergman, 2005; Santi et al., 2013). Rhizobiaceae (αproteobacteria) and Burkholderiaceae (βproteobacteria) are the most well known Nfixing bacterial groups that nodulate mostly legumes (Fabaceae; Sprent, Ardley, & James, 2017). A small genus Parasponia (Cannabaceae) has evolved independently symbiotic associations with Rhizobiaceae (Trinick, 1980). In addition, rhizobial root nodules have been reported in three zygophyllaceous genera, Tribulus, Fagonia and Zygophyllum (Mostafa & Mahmoud, 1951), but Received: 20 November 2017 | Accepted: 12 February 2018 DOI: 10.1111/jvs.12627
TL;DR: This is the first report of Lysinibacillus sphaericus as a nitrogen fixing and plant growth promoting endophyte with biocontrol activity.
TL;DR: Mimosa species have the ability to associate with different types of rhizobia (α- and β-proteobacteria), suggesting low specificity between host and bacterium in experimental conditions and soil factors seem to favour the predominance of certain types of Rhizobia, thus influencing the establishment of symbiotic relationships.
Abstract: To evaluate the influence of soil type on the symbiosis between Mimosa spp. and rhizobia. A greenhouse experiment was carried out with trap plants using seeds of six species of Mimosa and soils from three different locations in central Brazil: Posse, Brasilia and Cavalcante. Plant dry biomass and number of nodules were measured after four months. Symbiotic bacteria were isolated from nodules and their molecular identification was performed. Three housekeeping genes (16S rRNA, recA and gyrB) plus the nodC and nifH symbiotic genes were used to determine the identity of the symbionts and to reconstruct the phylogenetic relationships among the isolated nitrogen-fixing bacteria. Rhizobia from the Betaproteobacterial genus Paraburkholderia (former Burkholderia) and the Alphaproteobacterial genus Rhizobium were isolated from different species of Mimosa. As in previous studies, the phylogenies of their symbiosis-essential genes, nodC and nifH, were broadly congruent with their core housekeeping genes (16S rRNA, recA and gyrB), which suggests limited or no horizontal gene transfer. Edaphic factors such as pH and fertility influenced the occurrence of these unrelated rhizobial types in the nodules on these Mimosa spp. Mimosa species have the ability to associate with different types of rhizobia (α- and β-proteobacteria), suggesting low specificity between host and bacterium in experimental conditions. Soil factors such as pH, nitrogen and fertility seem to favour the predominance of certain types of rhizobia, thus influencing the establishment of symbiotic relationships.
TL;DR: The obtained results demonstrated that the dual inoculation of pea plants significantly increased the plant biomass, photosynthetic rate, nodulation, and nitrogen fixation activity in comparison with single inoculation with Rhizobium leguminosarum bv.
Abstract: The study evaluated the response of pea ( Pisum sativum cv. Avola) to arbuscular mycorrhizal fungi (AM) species Glomus mosseae and Glomus intraradices and Rhizobium leguminosarum bv. viceae, strain D 293, regarding the growth, photosynthesis, nodulation and nitrogen fixation activity. Pea plants were grown in a glasshouse until the flowering stage (35 days), in 4 kg plastic pots using leached cinnamonic forest soil (Chromic Luvisols – FAO) at P levels 13.2 (P1) and 39.8 (P2) mg P/kg soil. The obtained results demonstrated that the dual inoculation of pea plants significantly increased the plant biomass, photosynthetic rate, nodulation, and nitrogen fixation activity in comparison with single inoculation with Rhizobium leguminosarum bv. viceae strain D 293. On the other hand, coinoculation significantly increased the total phosphorus content in plant tissue, acid phosphatase activity and percentage of root colonization. The effectiveness of coinoculation with Rhizobium leguminosarum and Glomus mosseae was higher at the low phosphorus level while the coinoculation with Glomus intraradices appeared to be the most effective at higher phosphorus level.
TL;DR: The competitiveness for nodulation of a given pea-Rlv association evaluated in the multi-inoculated experiment was poorly correlated with its nitrogen fixation efficiency determined in mono- inoculation.
Abstract: Pea forms symbiotic nodules with Rhizobium leguminosarum sv. viciae (Rlv). In the field, pea roots can be exposed to multiple compatible Rlv strains. Little is known about the mechanisms underlying the competitiveness for nodulation of Rlv strains and the ability of pea to choose between diverse compatible Rlv strains. The variability of pea-Rlv partner choice was investigated by co-inoculation with a mixture of five diverse Rlv strains of a 104-pea collection representative of the variability encountered in the genus Pisum. The nitrogen fixation efficiency conferred by each strain was determined in additional mono-inoculation experiments on a subset of 18 pea lines displaying contrasted Rlv choice. Differences in Rlv choice were observed within the pea collection according to their genetic or geographical diversities. The competitiveness for nodulation of a given pea-Rlv association evaluated in the multi-inoculated experiment was poorly correlated with its nitrogen fixation efficiency determined in mono-inoculation. Both plant and bacterial genetic determinants contribute to pea-Rlv partner choice. No evidence was found for co-selection of competitiveness for nodulation and nitrogen fixation efficiency. Plant and inoculant for an improved symbiotic association in the field must be selected not only on nitrogen fixation efficiency but also for competitiveness for nodulation.
TL;DR: It is found that carbon dots (CDs) could significantly enhance the nitrogen-fixing activity of azotobacter chroococcum, in which the activity of Azotobacteria treated with CDs was increased by 158% compared to that of the control one.
Abstract: Biological nitrogen fixation is critical for the nitrogen cycle on the earth. Nitrogen-fixing bacteria, as an environmentally friendly microorganism, convert atmospheric nitrogen to available nitrogen source for plants. In this study, we found that carbon dots (CDs) could significantly enhance the nitrogen-fixing activity of azotobacter chroococcum, in which the activity of azotobacter treated with CDs (4 μg/mL) was increased by 158% compared to that of the control one. A series of experiments suggest that CDs can combine with the nitrogenase, affect the secondary structure of nitrogenase, improve the electron transfer in the biocatalytic process, and finally improve nitrogenase activity for nitrogen fixation. Our findings may offer an economical and environmentally friendly means of improving the biological nitrogen fixation as well as solving the insufficiency of nitrogen fertilizer.
TL;DR: The obtained data suggest that Eucalyptus may benefit from biological nitrogen fixation, with many abundant genera being closely related to nitrogen-fixing bacteria.
Abstract: Eucalyptus plantations offer a cost-effective and renewable source of raw material. There is substantial interest in improving forestry production, especially through sustainable strategies such as the use of plant growth-promoting bacteria. However, little is known about Eucalyptus microbiology. In this study, the endophytic bacterial community was assessed in Eucalyptus urograndis roots using culture-dependent and culture-independent techniques with plants grown under different conditions. Three phyla accounted for approximately 95% of the community, with Actinobacteria corresponding to approximately 59%. This contrasts with previous studies in which Actinobacteria accounted for only 5 to 10%. Our data also revealed a high diversity of bacteria, with 359 different genera but a high level of dominance. Six genera, Mycobacterium, Bradyrhizobium, Streptomyces, Bacillus, Actinospica, and Burkholderia, accounted for more than 50% of the classified sequences. We observed a significant influence of the treatments on some genera, causing changes in the bacterial community structure. The obtained data also suggest that Eucalyptus may benefit from biological nitrogen fixation, with many abundant genera being closely related to nitrogen-fixing bacteria. Using N-depleted media, we also cultured 95 bacterial isolates, of which 24 tested positive for the nifH gene and were able to maintain growth without any N source in the medium.
TL;DR: In this article, in situ measurements of N2 fixation throughout the main growing period in high arctic tundra, Greenland, in climate change treatments, shading and warming, and control.
Abstract: Biological nitrogen (N2) fixation is one of the main sources of available N for pristine ecosystems such as subarctic and arctic tundra. Although this has been acknowledged more than a decade ago, few attempts have been undertaken to identify the foremost driver of N2 fixation in the high Arctic. Here, we report results from in situ measurements of N2 fixation throughout the main growing period (June– August) in high arctic tundra, Greenland, in climate change treatments, shading and warming, and control. Nitrogen fixation was also measured in cores that received additional water prior to the measurements. The climate change field treatments did not lead to significant changes in any measured parameters; however, N2 fixation was promoted by adding water, and moisture was the most important factor influencing N2 fixation in all climate change field treatments. Maximum N2 fixation rates were measured below 14°C soil temperature, which is much lower than the theoretical and previously reported temperature optimum for the nitrogenase enzyme. Diazotroph (N2 fixing bacteria) communities are adapted to low temperatures in high arctic settings, and increased temperature in a future climate may lead to decreased N2 fixation rates, or to a shift in diazotroph communities.
TL;DR: Stylophora pistillata from Heron Island on the Great Barrier Reef showed that the rates of net photosynthesis, steady state quantum yields of photosystem II (PSII) fluorescence (∆Fv/Fm′) and calcification varied based on irradiance as expected, and shallow corals were enriched in genes involved in nitrogen metabolism, and N2 fixation specifically.
Abstract: Diazotrophs, both Bacteria and Archaea, capable of fixing nitrogen (N2), are present in the tissues and mucous, of corals and can supplement the coral holobiont nitrogen budget with fixed nitrogen (N) in the form of ammonia (NH3). Stylophora pistillata from Heron Island on the Great Barrier Reef collected at 5 and 15 m, and experimentally manipulated in the laboratory, showed that the rates of net photosynthesis, steady state quantum yields of photosystem II (PSII) fluorescence (∆Fv/Fm′) and calcification varied based on irradiance as expected. Rates of N2 fixation were, however, invariant across treatments while the amount of fixed N contributing to Symbiodinium spp. N demand is irradiance dependent. Additionally, both the Symbiodinium and diazotrophic communities are significantly different based on depth, and novel Cluster V nifH gene phylotypes, which are not known to fix nitrogen, were recovered. A functional analysis using PICRUSt also showed that shallow corals were enriched in genes involved in nitrogen metabolism, and N2 fixation specifically. Corals have evolved a number of strategies to derive nitrogen from organic (e.g., heterotrophic feeding) and inorganic sources (e.g., N2 fixation) to maintain critical pathways such as protein synthesis to succeed ecologically in nitrogen-limited habitats.