Diazotrophic Community Research Articles

Filtration via Conventional Glass Fiber Filters in 15N2 Tracer Assays Fails to Capture All Nitrogen-Fixing Prokaryotes

Biological dinitrogen fixation (BNF) represents a major input of reduced nitrogen (N) to the oceans. Accurate direct measurements of BNF rates are crucial for reliably determining the biogeochemical significance of diazotrophy at local and global scales. Traditionally, borosilicate glass fiber filters (GF/F, Whatman) with a nominal pore size of 0.7 µm are used to collect suspended particles by filtration after incubations with added 15N2 tracer. We carried out BNF experiments in the Baltic Sea, Danish coastal waters, and the Pacific Ocean comparing the retentive characteristics of precombusted GF/F filters with newer Advantec glass fiber filters which have a smaller nominal pore size of 0.3 µm. Where BNF was detected, rates were nearly always higher, and sometimes even exclusively detectable, when using Advantec filters. In the majority of samples across tested habitats, significantly more cells were lost to GF/F filtrate (average = 51 %, range = 10 – 70 % of cells) than to Advantec filtrate (average = 40 %, range = 10 – 54 %). Using Illumina sequencing of nitrogenase (nifH) gene amplicons, we show that diazotroph communities can markedly differ between bulk water and filtrates from GF/F and Advantec filtrations, suggesting that different diazotrophs can pass through the filter types. In order to reduce the potential underestimations of BNF due to filtration loss of diazotrophs, we recommend using Advantec filters or alternatively silver membranes with 0.2 µm pore size, especially in waters expected to be inhabited by relatively small, unicellular diazotrophs.

Diazotroph abundance and community composition in an acidic soil in response to aluminum-tolerant and aluminum-sensitive maize (Zea mays L.) cultivars under two nitrogen fertilizer forms

In acidic soil, plant aluminum (Al) tolerance and nitrogen (N) fertilizer form play the important roles in influencing plant growth. Diazotrophs as plant growth promoting rhizobacteria can contribute to plant available N, but the response to plant growth differences resulting from plant Al tolerance or N fertilizer form in acidic soil is poorly understood. Here, we investigated this response. Three maize cultivars with different Al-tolerance levels were grown in an acidic soil that was added with nitrate (NO3−-N) or ammonium (NH4+-N) fertilizers. After 42 days, maize biomasses and root morphologies as well as the physicochemical properties of bulk and rhizosphere soils were determined. Based on nifH gene, soil diazotroph abundance was determined by quantitative RT-PCR and community composition was assayed using high-throughput sequencing. For the same maize cultivar, application of NO3−-N fertilizer relative to NH4+-N fertilizer increased maize biomass, root length, root surface area, and rhizosphere pH, but not significantly affect rhizosphere nifH copy number and community composition. The nifH copy numbers did not show significant differences among rhizosphere samples of three maize cultivars, but community composition was obviously different between Al-tolerant and Al-sensitive cultivars. In acidic soil, maize cultivars depending on Al tolerance altered rhizosphere diazotrophic community composition, but this effect seemed to be not susceptible to N fertilizer forms.

N-fertilizer-driven association between the arbuscular mycorrhizal fungal community and diazotrophic community impacts wheat yield

Nitrogen is a macronutrient which plays an important role in the net primary productivity of terrestrial ecosystems, and is critical to understand how nitrogen addition affects above- and below-ground organisms. However, direct and indirect interactions between microbial species response to nitrogen addition for wheat growth are still unclear. In this study, arbuscular mycorrhizal fungi (AMF) and diazotrophs (nifH gene) were selected as two groups of key functional microbes in wheat soil to investigate the comprehensive impact of diversity and community structures between them on wheat yield under a nitrogen gradient. Nitrogen addition resulted in a significant shift in AMF and nifH phylogenetic diversity and community structure. The interactions among AMF and nifH species were diminished in high N-treated soils compared to those in low N-treated soils. Structural equation model analysis showed that AMF community structure affected wheat yield directly, however, which was influenced by AMF diversity, diazotrophic diversity and community structure indirectly. The comprehensive effects of AMF and diazotrophs on wheat productivity indicated the importance of soil functional microbes as drivers for farmland ecosystem and plant growth. These results could be useful in understanding soil nutrient cycling and may help improve the mechanistic understanding of the tripartite interaction among nitrogen addition, microbes and wheat productivity.

Diazotroph diversity and nitrogen fixation in the coral Stylophora pistillata from the Great Barrier Reef

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.

Temporal dynamics of total and free-living nitrogen-fixing bacterial community abundance and structure in soil with and without history of arsenic contamination during a rice growing season.

Despite the fact that the nitrogen (N) fixers act as the key regulator of ecosystem process, a detailed study of their abundance, diversity, and dynamics in arsenic (As)-contaminated rice fields is missing so far. DNA extracted from soil followed by 16S rRNA and nifH gene-based real-time qPCR, clone library analysis, and DNA sequencing were used to examine the status of the total and diazotrophic communities in two agricultural fields with and without arsenic contamination history during one rice cultivation season. In general, higher nifH and 16S rRNA gene copy numbers were observed in rice growing soils with lesser As than that with higher As. Elevated levels of 16S rRNA and nifH genes in soil is directly associated with total and nitrogen fixers abundance in the agricultural land without As contamination history through the cultivation period, but the copy number of 16S rRNA gene was decreased, and the nifH gene remained unchanged in the As-contaminated land. Additionally, Canonical Correspondence Analysis (CCA) indicated the possible suppression of nifH gene abundance by soil pH, phosphate, and As content. Increased abundance of total and Acidobacterial lineages in low As-containing soil and the detection of several uncultured groups among nifH gene sequence in higher frequency indicated the presence of novel nifH bearing bacterial groups. Conversely, the abundance of copiotrophic Proteobacterial lineages gradually increased in soil with higher As. Herein, our study demonstrated that the dynamics of free-living nitrogen-fixing bacterial communities were perturbed due to As contamination in agricultural land.

Nitrogen fixation in two coastal upwelling regions of the Taiwan Strait

Recent studies have demonstrated that dinitrogen fixation can be important in nutrient-rich coastal upwelling regions. During a cruise to the Taiwan Strait in summer 2015, we found that the nitrogen fixation rate in surface waters ranged from below detection limits to 7.51 nmol N L−1 d−1. Higher rates accompanied by low N:P ratios (1–10.4:1) associated with low temperatures occurred in the surface water where the Pingtan and the Dongshan upwelling regions met (the NE area). In contrast, insignificant rates were observed in the southwest area of the Dongshan upwelling region (the SW area) with sufficient N and deficient P, and therefore high N:P ratios (e.g., >43 at station C2) due largely to the influence of the Pearl River plume. Diatom-associated symbionts (het-1; 104–106 copies L−1) that are efficient in organic matter export were found to dominate the other diazotrophic groups that were surveyed, which may represent a direct relationship between new nitrogen input and export in the upwelling regions. Our results suggest a hydrographical influence on the diazotroph community and N2 fixation in coastal upwelling regions.

Influence of nitrogen and phosphorus additions on N2-fixation activity, abundance, and composition of diazotrophic communities in a Chinese fir plantation

Although biological nitrogen (N) fixation (BNF) is an important N input process in subtropical forest ecosystems, how the diazotrophic communities related to this process respond to N and phosphorus (P) inputs is largely unknown. We investigated the effects of exogenous N and/or P inputs on N2-fixation activity, diazotrophic abundance and community composition using a continuous application of fertilizers over 5years experiment in a Chinese fir plantation. The fertilization regimes included control (CK), P treatment (P), low N addition treatment (N1), high N addition treatment (N2), low N and P addition treatment (N1P) and high N with P addition treatment (N2P). N2-fixation activity was determined using the acetylene reduction assay (ARA). Quantitative PCR and Illumina Miseq sequencing of nifH gene were performed to analyze diazotrophic abundance and community composition, respectively. Our results showed that P addition increased N2-fixation activity and nifH gene abundance by 189.07nmol C2H4 and 1.02×107copiesg−1 dry soil, respectively, while were reduced by 1.19nmol C2H4 and 2.04×106copiesg−1 dry soil when N was added. The application of P with low N (N1P) effectively alleviated the inhibitory effect of N input on N2-fixation activity. N-related treatments resulted in significant decreases in operational taxonomic unit (OTU) richness and shifts in diazotrophic community structure. N2-fixation activity and nifH gene abundance were strongly and positively correlated with soil pH and negatively correlated with mineral N (NH4+-N and NO3−-N) contents, while mineral N concentrations rather than soil pH appeared to be the main factor altering diazotrophic community structure. These results revealed that P addition played a positive role in regulating biological nitrogen fixation in subtropical forest ecosystems.

Highly heterogeneous diazotroph communities in the Kuroshio Current and the Tokara Strait, Japan.

In this study, we used 454-pyrosequencing to report the highly diverse diazotroph communities in the Kuroshio and its adjacent waters along a transect across the Tokara Strait, Japan. Terrestrial input from the islands resulted in a highly heterogeneous diazotroph community within a relatively small geographic region, which was presumably caused by the remarkably different responses of UCYN-A2, UCYN-C and Trichodesmium to the steep environmental gradient. On the other hand, most major cyanobacterial OTUs found in this study were also detected in an unpublished dataset from the upstream Kuroshio, which suggests transportation of diazotrophs by the Kuroshio in large geographic scale. A significant amount of UCYN-C was found in the Kuroshio and offshore stations, suggesting the importance of this potentially overlooked group in the western North Pacific Ocean (WNPO). Moreover, a novel sublineage of UCYN-B was defined, which was predominant in an oligotrophic water sample; and it was also found to be widely distributed in oceanic waters. In addition, the apparent increase in relative abundance of UCYN-A2 from offshore to near-shore water provides evidence for the earlier and under-debating view that UCYN-A2 prefers coastal conditions. Our report provides new knowledge for understanding the phylogeny and ecology of unicellular cyanobacterial diazotrophs in WNPO.

Diversity and Structure of Diazotrophic Communities in Mangrove Rhizosphere, Revealed by High-Throughput Sequencing.

Diazotrophic communities make an essential contribution to the productivity through providing new nitrogen. However, knowledge of the roles that both mangrove tree species and geochemical parameters play in shaping mangove rhizosphere diazotrophic communities is still elusive. Here, a comprehensive examination of the diversity and structure of microbial communities in the rhizospheres of three mangrove species, Rhizophora apiculata, Avicennia marina, and Ceriops tagal, was undertaken using high-throughput sequencing of the 16S rRNA and nifH genes. Our results revealed a great diversity of both the total microbial composition and the diazotrophic composition specifically in the mangrove rhizosphere. Deltaproteobacteria and Gammaproteobacteria were both ubiquitous and dominant, comprising an average of 45.87 and 86.66% of total microbial and diazotrophic communities, respectively. Sulfate-reducing bacteria belonging to the Desulfobacteraceae and Desulfovibrionaceae were the dominant diazotrophs. Community statistical analyses suggested that both mangrove tree species and additional environmental variables played important roles in shaping total microbial and potential diazotroph communities in mangrove rhizospheres. In contrast to the total microbial community investigated by analysis of 16S rRNA gene sequences, most of the dominant diazotrophic groups identified by nifH gene sequences were significantly different among mangrove species. The dominant diazotrophs of the family Desulfobacteraceae were positively correlated with total phosphorus, but negatively correlated with the nitrogen to phosphorus ratio. The Pseudomonadaceae were positively correlated with the concentration of available potassium, suggesting that diazotrophs potentially play an important role in biogeochemical cycles, such as those of nitrogen, phosphorus, sulfur, and potassium, in the mangrove ecosystem.

Soil pH is a major driver of soil diazotrophic community assembly in Qinghai-Tibet alpine meadows

The biological nitrogen input is essential for nutrient enrichment in natural grassland ecosystems. This process is carried out by diazotrophic microbes. However, the patterns and assembly processes of diazotrophic communities in grassland soil are not fully understood, especially in alpine meadows of high-altitude Qinghai-Tibet Plateau (QTP). In this study, the diversity patterns and community assembly processes of soil diazotrophic communities were investigated in QTP alpine meadows. The results showed that different diazotrophic taxa responded differentially to the increase of soil pH, and α-diversity increased linearly with pH values. Diazotrophic community turnover was enhanced by the increased variation in soil pH, moreover, it also showed strong response to mean annual temperature. Null model analysis showed that deterministic processes were most important in shaping the phylogenetic community structure and assembly of diazotrophs. Diazotrophic communities of each sampling site tended to be phylogenetically clustered, while communities at high pH soils (pH 7.00 to 8.00) showed more clustering than those of low pH (pH 5.02 to 7.00). The soil pH is a main driver in structuring soil diazotrophic community assembly by regulating the relative importance of deterministic and stochastic processes in QTP alpine meadows.

Fertilization with inorganic and organic nutrients changes diazotroph community composition and N-fixation rates

Nitrogen fixation by free-living diazotrophs from the atmosphere is an important pathway for nitrogen input into the soil. However, there is little information regarding soil diazotrophic community composition and diversity under long-term fertilization in rice paddy ecosystems. Using the 15N2-tracing method and nifH gene as a molecular marker, we investigated the abundance, structure, and activity of soil diazotrophic community in soil at a 30-year-old filed experimental site treated with four different fertilizer management practices: control (non-fertilization), chemical NPK fertilizers, NPK plus rice straw (NPK+RS), or NPK plus chicken manure (NPK+OM). Among all the treatments, the NPK+OM treatment significantly improved the soil nutritional status. Fertilization increased both bacteria and nifH gene abundances, with the highest values (p < 0.05) found in the NPK+OM treatment. The potential nitrogen fixation rate ranged from 14.6 to 118 μg kg−1 day−1, and the highest rates (p < 0.05) were also observed in the NPK+OM treatment. Long-term chemical NPK fertilization decreased the diversity of diazotrophic community, whereas NPK+RS and NPK+OM treatments maintained the diversity of diazotrophic community. Long-term fertilization changed diazotrophic community as compared to non-fertilization, but there were no significant differences among fertilized treatments. Most nifH sequences were closely linked to Alphaproteobacteria, which was dominated by the genera Bradyrhizobium. Relatively higher Cyanobacteria abundances were observed in the unfertilized soil as compared with fertilized soil. Our results suggest that long-term fertilization increased the abundance of diazotrophs and changed their community structure, and combined use of chicken manure and chemical NPK fertilizers can significantly improve the activity of diazotrophic community.

Bacterial and diazotrophic diversities of endophytes in Dendrobium catenatum determined through barcoded pyrosequencing.

As an epiphyte orchid, Dendrobium catenatum relies on microorganisms for requisite nutrients. Metagenome pyrosequencing based on 16S rRNA and nifH genes was used to characterize the bacterial and diazotrophic communities associated with D. catenatum collected from 5 districts in China. Based on Meta-16S rRNA sequencing, 22 bacterial phyla and 699 genera were identified, distributed as 125 genera from 8 phyla and 319 genera from 10 phyla shared by all the planting bases and all the tissues, respectively. The predominant Proteobacteria varied from 71.81% (GZ) to 96.08% (YN), and Delftia (10.39–38.42%), Burkholderia (2.71–15.98%), Escherichia/Shigella (4.90–25.12%), Pseudomonas (2.68–30.72%) and Sphingomonas (1.83–2.05%) dominated in four planting bases. Pseudomonas (17.94–22.06%), Escherichia/Shigella (6.59–11.59%), Delftia (9.65–22.14%) and Burkholderia (3.12–11.05%) dominated in all the tissues. According to Meta-nifH sequencing, 4 phyla and 45 genera were identified, while 17 genera and 24 genera from 4 phyla were shared by all the planting bases and all the tissues, respectively. Burkholderia and Bradyrhizobium were the most popular in the planting bases, followed by Methylovirgula and Mesorhizobium. Mesorhizobium was the most popular in different tissues, followed by Beijerinckia, Xanthobacter, and Burkholderia. Among the genera, 39 were completely overlapped with the results based on the 16S rRNA gene. In conclusion, abundant bacteria and diazotrophs were identified in common in different tissues of D. catenatum from five planting bases, which might play a great role in the supply of nutrients such as nitrogen. The exact abundance of phylum and genus on the different tissues from different planting bases need deeper sequencing with more samples.

Analysis of nifH-RNA reveals phylotypes related to Geobacter and Cyanobacteria as important functional components of theN2 -fixing community depending on depth and agriculturaluse of soil.

In this survey, a total of 80 787 reads and 28 171 unique NifH protein sequences were retrieved from soil RNA. This dataset extends our knowledge about the structure and diversity of the functional diazotrophic communities in agricultural soils from Argentinean Pampas. Operational taxonomic unit (OTU)‐based analyses showed that nifH phylotypes related to Geobacter and Anaeromyxobacter (44.8%), Rhizobiales (29%), Cyanobacteria (16.7%), and Verrucomicrobiales (8%) are key microbial components of N2 fixation in soils associated with no‐till management and soil depth. In addition, quantification of nifH gene copies related to Geobacter and Cyanobacteria revealed that these groups are abundant in soils under maize–soybean rotation and soybean monoculture, respectively. The correlation of physicochemical soil parameters with the diazotrophic diversity and composition showed that soil stability and organic carbon might contribute to the functional signatures of particular nifH phylotypes in fields under no‐till management. Because crop production relies on soil‐borne microorganism's activities, such as free N2 fixation, the information provided by our study on the diazotrophic population dynamics, associated with the edaphic properties and land‐use practices, represents a major contribution to gain insight into soil biology, in which functionally active components are identified.

Contrasting bacterial communities in two indigenous Chionochloa (Poaceae) grassland soils in New Zealand.

The cultivation of grasslands can modify both bacterial community structure and impact on nutrient cycling as well as the productivity and diversity of plant communities. In this study, two pristine New Zealand grassland sites dominated by indigenous tall tussocks (Chionochloa pallens or C. teretifolia) were examined to investigate the extent and predictability of variation of the bacterial community. The contribution of free-living bacteria to biological nitrogen fixation is predicted to be ecologically significant in these soils; therefore, the diazotrophic community was also examined. The C. teretifolia site had N-poor and poorly-drained peaty soils, and the C. pallens had N-rich and well-drained fertile soils. These soils also differ in the proportion of organic carbon (C), Olsen phosphorus (P) and soil pH. The nutrient-rich soils showed increased relative abundances of some copiotrophic bacterial taxa (including members of the Proteobacteria, Bacteroidetes and Firmicutes phyla). Other copiotrophs, Actinobacteria and the oliogotrophic Acidobacteria showed increased relative abundance in nutrient-poor soils. Greater diversity based on 16S rRNA gene sequences and the Tax4Fun prediction of enhanced spore formation associated with nutrient-rich soils could indicate increased resilience of the bacterial community. The two sites had distinct diazotrophic communities with higher diversity in C. teretifolia soils that had less available nitrate and ammonium, potentially indicating increased resilience of the diazotroph community at this site. The C. teretifolia soils had more 16S rRNA gene and nifH copies per g soil than the nutrient rich site. However, the proportion of the bacterial community that was diazotrophic was similar in the two soils. We suggest that edaphic and vegetation factors are contributing to major differences in the composition and diversity of total bacterial and diazotrophic communities at these sites. We predict the differences in the communities at the two sites will result in different responses to environmental change.

Impact of fertilization regimes on diazotroph community compositions and N2-fixation activity in paddy soil

Abstract Nutrient status in soil is crucial for the growth and activity of resident microorganisms, which in turn regulate nitrogen transformation in the terrestrial biosphere. Biological nitrogen fixation (BNF) is an important N input in agricultural ecosystems, but the influence of fertilization on the structural and functional behavior of diazotrophs is not clear. In this study, we assessed the nutrient limitations on the abundance and N2-fixation activity of diazotrophs in a long term (20 years) fertilization experiment. Although both phosphorus (P) deficiency and potassium (K) deficiency resulted in significant decreases in nifH gene expression and N2-fixation activity, P deficiency exhibited more restrictive effects. Assessments of community structures based upon transcripts also indicated that the active diazotroph populations were more sensitive to P deficiency than K deficiency, and some diazotroph groups were detected in the P deficiency treatment. Long-term rice straw addition significantly increased diazotroph abundance, but in contrast, sharply reduced nifH gene expression and N2-fixation activity. The close relationship between N2-fixation activity and nifH gene expression rather than its copy number suggests that the nifH gene transcript level is a suitable indicator for predicting the N2-fixation activity of N2-fixing microorganisms in paddy soil of various fertility status.

Impact of 25 years of inorganic fertilization on diazotrophic abundance and community structure in an acidic soil in southern China

Although diazotrophs are important in the nitrogen (N)-cycle and contribute to the pool of plant available N, the population response to long-term inorganic fertilization is largely unknown. Here, we investigated the diazotrophic populations in both the bulk and rhizosphere soils of maize grown in an acidic farmland soil that experienced 25 years of inorganic fertilization. The fertilization regimes included unfertilized control, N fertilizer alone, N fertilizer with quicklime, phosphorus (P) and potassium (K) fertilizers, N + P + K fertilizers, and N + P + K fertilizers with quicklime. Quantitative PCR and high-throughput pyrosequencing of the nifH gene were used to analyze diazotrophic abundance and community composition. All of the fertilizer treatments improved soil nutrient availability, but those without quicklime caused soil acidification. Maize biomasses and nifH copy numbers were significantly lower under N and N + P + K treatments but increased under P + K fertilization. Quicklime applications effectively alleviated the inhibitory effect of N input. Fertilization led to decreases in operational taxonomic unit richness and shifts in diazotrophic community composition. Soil pH and nutrient availability had a cooperative effect on diazotrophic abundance, while soil nutrient availability appeared to be the main factor shaping diazotrophic community structure. Rhizosphere effects increased the nifH gene copy number but did not obviously change the diazotrophic community composition on the current research scale. Overall, the long-term inorganic fertilization affected both diazotrophic abundance and community composition, and the fertilizer treatment had a greater influence than quicklime remediation or crop cultivation on community composition.

Diversity and Activity of Diazotrophs in Great Barrier Reef Surface Waters.

Discrepancies between bioavailable nitrogen (N) concentrations and phytoplankton growth rates in the oligotrophic waters of the Great Barrier Reef (GBR) suggest that undetermined N sources must play a significant role in supporting primary productivity. One such source could be biological dinitrogen (N2) fixation through the activity of “diazotrophic” bacterioplankton. Here, we investigated N2 fixation and diazotroph community composition over 10° S of latitude within GBR surface waters. Qualitative N2 fixation rates were found to be variable across the GBR but were relatively high in coastal, inner and outer GBR waters, reaching 68 nmol L-1 d-1. Diazotroph assemblages, identified by amplicon sequencing of the nifH gene, were dominated by the cyanobacterium Trichodesmium erythraeum, γ-proteobacteria from the Gamma A clade, and δ-proteobacterial phylotypes related to sulfate-reducing genera. However, diazotroph communities exhibited significant spatial heterogeneity, correlated with shifts in dissolved inorganic nutrient concentrations. Specifically, heterotrophic diazotrophs generally increased in relative abundance with increasing concentrations of phosphate and N, while Trichodesmium was proportionally more abundant when concentrations of these nutrients were low. This study provides the first in-depth characterization of diazotroph community composition and N2 fixation dynamics within the oligotrophic, N-limited surface waters of the GBR. Our observations highlight the need to re-evaluate N cycling dynamics within oligotrophic coral reef systems, to include diverse N2 fixing assemblages as a potentially significant source of dissolved N within the water column.

Basin scale variability of active diazotrophs and nitrogen fixation in the North Pacific, from the tropics to the subarctic Bering Sea

AbstractNitrogen‐fixing microorganisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later noncyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here we examined diazotroph community structure and activity along a 7500 km south‐north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ‐24774A11 and highly active cyanobacterial phylotypes (&gt;50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN‐A2, which was abundant near the surface (1.2×105 nifH gene copies L−1). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

The diversity and co-occurrence patterns of diazotrophs in the steppes of Inner Mongolia

Soil diazotrophs possess the function of fixing atmospheric N2 into biologically available ammonium in terrestrial ecosystems. However, there are limited studies into how diazotrophic communities change when grassland environments differ. In this study, we elucidated the diversity and interactions of diazotrophic communities in three steppe types of Inner Mongolia, China, including non-desert steppe (MT), desert steppe (D) and sandy steppe (S). We expected to improve the predictions of diazotrophic responses to grassland degradation at time gradients based on their discrepancies among steppe types along spatial gradients. Steppe types affected the alpha- and beta-diversity of diazotrophic communities, which were mainly controlled by plant biomass and soil pH, respectively. Diazotrophic communities were strongly aggregated with their respective steppe environments, but dissimilar under different types according to Bray-Curtis distance. In addition, network analysis showed that the network structure of diazotrophs from the D was relatively simpler than the other two types (MT and S). More exclusionary relationships in diazotrophic communities occurred in the D and S steppes than in the MT steppe. The species shared across the steppes changed their interactions with other species to adapt to different habitats. Moreover, different steppes had distinct key species to underpin the whole network. The disturbances or extinctions of these key species may lead to the instability of the diazotrophic co-occurrence network. Overall, we speculate that if grassland degradation happens, the change of grassland microenvironment may mediate the differences in the diversity and co-occurrence patterns of diazotrophs, allowing them to adapt to environmental changes.

Corrigendum.

Molecular EcologyVolume 26, Issue 8 p. 2405-2405 CorrigendumFree Access Corrigendum This article corrects the following: Biogeographic patterns of soil diazotrophic communities across six forests in the North America Qichao Tu, Ye Deng, Qingyun Yan, Lina Shen, Lu Lin, Zhili He, Liyou Wu, Joy D. Van Nostrand, Vanessa Buzzard, Sean T. Michaletz, Brian J. Enquist, Michael D. Weiser, Michael Kaspari, Robert B. Waide, James H. Brown, Jizhong Zhou, Volume 25Issue 12Molecular Ecology pages: 2937-2948 First Published online: May 21, 2016 First published: 17 April 2017 https://doi.org/10.1111/mec.13925AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Qichao Tu, Ye Deng, Qingyun Yan, Lina Shen, Lu Lin, Zhili He, Liyou Wu, Joy D. Van Nostrand, Vanessa Buzzard, Sean T. Michaletz, Brian J. Enquist, Michael D. Weiser, Michael Kaspari, Robert B. Waide, James H. Brown and Jizhong ZhouRegrettably, a grammatical error appeared in the title of the published paper by Tu et al. (2016).The correct title is ‘Biogeographic patterns of soil diazotrophic communities across six forests in North America’. As it was still in review at the time Tu, Q., et al. (2016), Biogeographic patterns of soil diazotrophic communities across six forests in the North America. Mol Ecol, 25: 2937-2948. doi:10.1111/mec.13651 was published, the reference for Zhou et al. (2016) was inadvertently omitted from the list of references. We include it here:Zhou J, Deng Y, Shen L et al. (2016) Temperature mediates continental-scale diversity of microbes in forest soils. Nature Communications. DOI: 10.1038/ncomms12083 Reference Tu Q, Deng Y, Yan Q et al. (2016) Biogeographic patterns of soil diazotrophic communities across six forests in the North America. Molecular Ecology, 25, 2937– 2948. Volume26, Issue8April 2017Pages 2405-2405 ReferencesRelatedInformation