Fixation Activity Research Articles

Atomically dispersed Palladium-Ethylene Glycol- Bismuth oxybromide for photocatalytic nitrogen fixation: Insight of molecular bridge mechanism

Performance of single-atom catalysis largely depends on the interaction between the metal and the supporter. Herein, ethylene glycol (EG) was used as a molecular bridge connecting Palladium (Pd) and bismuth oxybromide (BiOBr) to form atomically dispersed Pd catalyst (Pd-EG-BiOBr) for photocatalytic nitrogen fixation under ambient conditions. Compared with 0.20 wt% Pd-BiOBr, 0.20 wt% Pd-EG-BiOBr greatly promoted the photocatalytic nitrogen fixation activity, affording an ammonia formation rate of 124.63 μmol·h−1. The molecular bridge mechanism during catalyst formation and photocatalysis is speculated based on Transmission electron microscopy, In-situ Fourier transform infrared spectra, Electron spin resonance spectra, UV–vis diffuse reflectance spectra, Photoluminescence spectra and Density Functional Theory calculations. The results show that EG not only induces the formation of atomically dispersed Pd, but also enhances the electron density of Pd and activation capacity of nitrogen molecules. This work opens a new door to applications of atomically dispersed Pd supported catalysts for high efficiency photocatalytic nitrogen fixation.

Defect density modulation of La2TiO5: An effective method to suppress electron-hole recombination and improve photocatalytic nitrogen fixation

Highly active and efficient photocatalysts are crucial for the exploration of ammonia synthesis because of the serious problem of energy deficiency. La2TiO5 (LTO) perovskite materials have great advantages in the field of photocatalytic nitrogen fixation because of the broadly diversified properties. The rational design of surface defect is a valid method to modulate photoinduced charge traps and create defect energy levels, especially it is an effective way to suppress the photoinduced charge recombination. Herein, LTO was obtained by a simple sol-gel method and was further reduced by NaBH4 to introduce oxygen defect on its surface. UV–vis spectra proved that the surface defects could reduce the band gap value of samples, which is beneficial for improving photocatalytic nitrogen fixation activity. For the best photocatalytic samples with good cycle stability, the nitrogen fixation rate is 158.13 μmol·g−1·h−1. The mechanism of photocatalytic nitrogen fixation was proposed by the PL, XPS, and PEC results, which provided possibilities for exploring more promising perovskite catalysts in the field of nitrogen fixation.

Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

Coral reefs experience phase shifts from coral- to algae-dominated benthic communities, which could affect the interplay between processes introducing and removing bioavailable nitrogen. However, the magnitude of such processes, i.e., dinitrogen (N2) fixation and denitrification levels, and their responses to phase shifts remain unknown in coral reefs. We assessed both processes for the dominant species of six benthic categories (hard corals, soft corals, turf algae, coral rubble, biogenic rock, and reef sands) accounting for > 98% of the benthic cover of a central Red Sea coral reef. Rates were extrapolated to the relative benthic cover of the studied organisms in co-occurring coral- and algae-dominated areas of the same reef. In general, benthic categories with high N2 fixation exhibited low denitrification activity. Extrapolated to the respective reef area, turf algae and coral rubble accounted for > 90% of overall N2 fixation, whereas corals contributed to more than half of reef denitrification. Total N2 fixation was twice as high in algae- compared to coral-dominated areas, whereas denitrification levels were similar. We conclude that algae-dominated reefs promote new nitrogen input through enhanced N2 fixation and comparatively low denitrification. The subsequent increased nitrogen availability could support net productivity, resulting in a positive feedback loop that increases the competitive advantage of algae over corals in reefs that experienced a phase shift.

Naturally induced humoral response against Plasmodium vivax reticulocyte binding protein 2P1

BackgroundPlasmodium vivax is the most prevalent malaria parasite in many countries. A better understanding of human immunity to this parasite can provide new insights for vaccine development. Plasmodium vivax Reticulocyte Binding Proteins (RBPs) are key parasite proteins that interact with human proteins during erythrocyte invasion and are targets of the human immune response. The aim of this study is to characterize the human antibody response to RBP2P1, the most recently described member of the RBP family.MethodsThe levels of total IgG and IgM against RBP2P1 were measured using plasmas from 68 P. vivax malaria patients and 525 villagers in a malarious village of western Thailand. The latter group comprises asymptomatic carriers and healthy uninfected individuals. Subsets of plasma samples were evaluated for anti-RBP2P1 IgG subtypes and complement-fixing activity.ResultsAs age increased, it was found that the level of anti-RBP2P1 IgG increased while the level of IgM decreased. The main anti-RBP2P1 IgG subtypes were IgG1 and IgG3. The IgG3-seropositive rate was higher in asymptomatic carriers than in patients. The higher level of IgG3 was correlated with higher in vitro RBP2P1-mediated complement fixing activity.ConclusionsIn natural infection, the primary IgG response to RBP2P1 was IgG1 and IgG3. The predominance of these cytophilic subtypes and the elevated level of IgG3 correlating with complement fixing activity, suggest a possible role of anti-RBP2P1 antibodies in immunity against P. vivax.

Different sexual impacts of dioecious Populus euphratica on microbial communities and nitrogen cycle processes in natural forests

Plant-soil microbe interactions are determined by plant characters. Sexual dimorphism in root development, nitrogen (N) assimilation and resource allocation have been studied in different environments. However, how dioecious plants affect soil microbial communities in natural forests, particularly in low precipitation regions, is still poorly known. In this study, natural Populus euphratica forests were investigated in three arid regions. We hypothesized that males and females impose sex-specific impacts on physiochemical traits of soil, microbial communities and N-cycling processes. We discovered only little sex effect on most physiochemical traits, and bacterial and fungal communities in top soil (0–20 cm) in the three studied forests. However, the sex effect was greater in deep soil. Compared with fungi, the structure and composition of bacterial communities were affected more by plant sex in the rhizosphere and bulk soil. Sex indirectly affected N-cycling processes through a negative impact on the soil water content. Expressions of AOA, AOB, nifH, nirS and nirK in the rhizosphere soil were significantly affected by sex, forest site and their interactions. Proteobacteria, Actinobacteria and Firmicutes in the rhizosphere and bulk soils of P. euphratica males showed more significant effects on ammoxidation, N fixation, denitrification and protease activities when compared to females. The results suggest that sexual differences in shaping bacterial communities and affecting N-cycling processes are greater when the soil becomes drier. Thus, low precipitation causes intense sex differences in the nitrogen uptake and use efficiency. Our study highlights the importance of sexual effects on shaping specific microbial communities and N-cycling processes.

An engineered, non-diazotrophic cyanobacterium and its application in bioelectrochemical nitrogen fixation

Summary The reduction of chemically inert nitrogen to ammonia is a critical step in the global nitrogen cycle. Microbial nitrogen fixation is a promising way to realize nitrogen reduction and ammonia production at mild conditions. Here, we report an engineered, non-diazotrophic Synechococcus elongatus PCC 7942 strain with nitrogen fixation activity that is constructed by integrating a modified nitrogenase gene cluster into the genome. The engineered S. elongatus PCC 7942 strain is employed in a bioelectrochemical nitrogen-fixation (e-BNF) system for ammonia production. Because the e-BNF system supplies adequate external electrons for the turnover of nitrogenase, the nitrogen fixation activity of the engineered S. elongatus PCC 7942 strain is significantly improved. After 48 h of reaction, the e-BNF system accumulates 173 μM of NH3, which is 21 times higher than that generated from solely photosynthesis-driven nitrogen fixation, with faradaic efficiency of 6.85%. This work may provide new insight into biological nitrogen-fixation systems and ammonium production.

Nitrate Transport and Distribution in Soybean Plants With Dual-Root Systems.

Nitrate absorbed by soybean (Glycine max L. Merr.) roots from the soil can promote plant growth, while nitrate transported to nodules inhibits nodulation and nodule nitrogen fixation activity. The aim of this study was to provide new insights into the inhibition of nodule nitrogen (N) fixation by characterizing the transport and distribution of nitrate in soybean plants. In this research, pot culture experiments were conducted using a dual root system of soybeans. In the first experiment, the distribution of 15N derived from nitrate was observed. In the second experiment, nitrate was supplied–withdrawal–resupplied to one side of dual-root system for nine consecutive days, and the other side was supplied with N-free solution. Nitrate contents in leaves, stems, petioles, the basal root of pealed skin and woody part at the grafting site were measured. Nitrate transport and distribution in soybean were analyzed combining the results of two experiments. The results showed that nitrate supplied to the N-supply side of the dual-root system was transported to the shoots immediately through the basal root pealed skin (the main transport route was via the phloem) and woody part (transport was chiefly related to the xylem). There was a transient storage of nitrate in the stems. After the distribution of nitrate, a proportion of the nitrate absorbed by the roots on the N-supply side was translocated to the roots and nodules on the N-free side with a combination of the basal root pealed skin and woody part. In conclusion, the basal root pealed skin and woody part are the main transport routes for nitrate up and down in soybean plants. Nitrate absorbed by roots can be transported to the shoots and then retranslocated to the roots again. The transport flux of nitrate to the N-free side was regulated by transient storage of nitrate in the stems.

Biological Nitrogen Fixation and Denitrification in Rhizosphere of Potato Plants in Response to the Fertilization and Inoculation

The study aim was to evaluate the potential nitrogen fixation and denitrification in the rhizosphere soil of potato plants, crop yield and output quality in response to the different fertilization systems and the inoculation with Azospirillum brasilense 410. Field stationary experiment was conducted between 2016 and 2019 with potato in a crop rotation system on leached chernozem soil. Farmyard manure, 40 t/ha, applied prior to potatoes planting promotes nitrogen fixation (0.8–2.0 times compared to control). However, it has also affected denitrification (in 1.4–2.2 times higher compared to control). The lowest rate of mineral fertilizers used in the experiment, N40P40K40, was shown as most environmentally feasible. Under its use the increase of soil nitrogenase activity and low denitrification levels were observed. Same trends were also noted for the medium fertilizer rate, N80P80K80. The highest doses of mineral fertilizers, N120P120K120, substantially affected the denitrification process and reduced the nitrogen fixation activity (in 1.9–2.2 times). The combination of manure with the medium fertilizers rate has also resulted in high denitrification levels, while the soil nitrogen fixation activity has restored only at flowering stage. Crop inoculation with A. brasilense combined with the manure application, has not affected studied processes. However, crop inoculation after the green manure intercropping has shown the growth of nitrogenase activity. Used on the mineral fertilizers background inoculation has activated nitrogen fixation and has ensured the decrease of denitrification levels, subject to the fertilization background. High fertilizer rates have hampered the inoculation efficiency. Inoculation has promoted crop yields on unfertilized and mineral backgrounds or following green manure. Crop inoculation following organic and the organo-mineral backgrounds had no significant effect, probably due to the competition for A. brasilense from microorganisms that have created a competitive environment for A. brasilense. Despite its environmental expediency, inoculation combined with the low fertilizer doses underperforms the action of inoculation combined with the medium fertilizer rates showing the latter as the compromise between the environmental requirements and crop productivity. The use of inoculation has promoted the accumulation of starch and ascorbic acid and has contributed to the reduction of nitrate contents in the tubers of inoculated plants.

Higher free-living N2 fixation at rock-soil interfaces than topsoils during vegetation recovery in karst soils

The widely distributed karst ecosystem in China has experienced rapid deterioration by deforestation and intensive cultivation in the last 50 years. Under the “Grain for Green” program, disturbed croplands were progressively abandoned to restore soil functions and ecosystem services, observed as discrete vegetation recovery phases: cultivated land, abandoned land and secondary forest, and compared to natural primary forest. Karst soils are naturally poor in nitrogen and cessation of nitrogen fertilizer application after abandonment presents an opportunity to explore the reemergence of indigenous soil N2 fixation mechanisms. In this study, we investigated how vegetation recovery affects key soil properties and their relationships with free-living N2 fixation (FLNF) activity and nifH gene abundance, microbial community structure and network interactions at different soil depths from the surface to the rock-soil interface at the bottom of the soil profile. Soil FLNF activity was increased slightly but not significantly after abandonment and was greatest in primary forest soils. High levels of FLNF activity in primary forest soils was facilitated directly by larger total organic carbon to total nitrogen ratios and an altered free-living diazotrophic community composition that was mainly modified by soil moisture content and pH. Higher FLNF was identified at rock-soil interfaces compared to topsoils, which was mainly driven by altered diazotrophic community composition stimulated by increases in soil pH and dissolved organic carbon to available inorganic nitrogen ratios. Diazotrophic network interactions also contributed to variations in FLNF activity, with relatively stable and competitive, while less niche-separated, diazotrophic communities supporting high FLNF activity in primary forest soils, while complex and mutualistic interactions facilitated the high FLNF at rock-soil interfaces. These results suggest that there is considerable and persistent FLNF in degraded karst landscapes at rock-soil interfaces which enables the provision of a vital biological source of nitrogen to overcome the nutrient limitations of karst rock weathering at the bottom of the soil profile.

Investigation of Newly Isolated Methylobacterium sp. as Potential Biofertilizer

Methylobacterium sp. are pink-pigmented facultative methylotrophs (PPFMs) that can promote plant’s growth due to several physiological characteristics such as the ability to produce phytohormone namely indole acetic acid (IAA) and effective roles in phosphate solubilization and nitrogen fixation process. Members of this genus are ubiquitous in the environment and characterized by its ability to utilize single C compound such as methanol as its energy source. These characteristics made Methylobacterium sp. as potential candidates for biofertilizer. In this study, Methylobacterium sp. were isolated from the leaves of paddy and palm oil tree using Ammonia Mineral Salt (AMS) agar supplemented with methanol. Nine isolates were selected based on the appearance of pink colonies on AMS agar. Several analyses were conducted to evaluate plant growth-promoting activities of the selected isolates. The production of IAA was quantified spectrophotometrically using Salkowski’s reagent, and solubilization of inorganic phosphate was determined using vanadate molybdate assay. The results showed that, all the isolates are capable to produce IAA and solubilize inorganic phosphate. As for nitrogen fixation, the lack/poor growth of the isolates on two nitrogen-free media, namely Burks’s and Jensen media manifested the absence or weak nitrogen fixing activity. Findings from these qualitative and quantitative analyses are vital and it will serve as preliminary data for the future exploration of Methylobacterium sp. as biofertilizer.

Preliminary analysis of Endophytic Plant Growth Promoting (PGP) Methylobacterium sp. Isolated from Palm Oil (Elaeis guineensis) Leaves

The genus Methylobacterium is known for its pink-pigmented facultative methylotrophs (PPFMs). They are able to utilize methanol emitted from plants and in return produce plant growth promoting substances. The genus Methylobacterium can be found in diverse habitats and they are known particularly for their close association with plants. The aims of this study are to isolate and characterize endophythic Methylobacterium sp. from palm oil leaves. Isolation of Methylobacterium sp was conducted using Ammonia Mineral Salts (AMS) agar supplemented with methanol. Three isolates were selected namely ENPM1, ENPM2 and ENPM3. Production of IAA by the selected isolates was evaluated under different concentration of L-tryptophan as an inducer. It was observed that the amount of(IAA) produced increased when higher concentration of tryptophan was used and ENPM1 recorded the highest amount of IAA production (1.930 μg/mL) at the 1000 μg/mL concentration of L-tryptophan. The ability to solubilized inorganic phosphate was investigated using Solubilisation Index (SI) assay on phosphate solubilisation media (PSM). The largest clear zone formation on PSM agar was shown by ENPM3 with the SI 1.5. As for nitrogen fixation, the growth of the selected isolates on two nitrogen free media namely Burk’s and Jensen media indicates potential nitrogen fixing activity.

Study on Interactive Effects of Different Levels of Lead and Mercury on Nitrogen Fixation of Some Diazotrophs

Researchers have studied the effects of addition of metal elements in combination with nitrogen fixing organisms as inoculants on the plants (growth) predominantly in legumes, however there is a major gap because responses and effects of these proposed micronutrients on the nitrogen fixation activity of these microbes both free living and symbiotic remains sketchy at best. Therefore, the effect of supplementation of lead and Mercury (bioaugmentation) on the nitrogen fixation potential of two (2) diazotrophs was evaluated in this study.
 Aims: To evaluate the interactive effects of different levels of Lead and Mercury on Nitrogen fixation of both Rhizobium and Xanthobactersppin-vitro.
 Place and Duration of Study: Sample organisms where collected from Groundnut rhizospheric soil of a farm in Cross River state, Nigeria. The microorganism isolation and nitrogen fixation analysis was further carried out at MacCliff General services Laboratory, Owerri, Nigeria for a duration of 3 months.
 Study Design: The interactive plots serve to show the effect of one variable (lead) on the value of mercury (the other) and is derived by selecting high and low values for lead (Pb) and entering them into the equation along with the range of values for Mercury (Hg). The values of independent variables (lead and mercury levels) used in the plots were selected by observing the highest concentration (+1) and lowest concentration (-1) values which are able to support nitrogen fixation independently in Rhizobium and Xanthobacter.
 Methodology: The soil samples were collected from groundnut rhizosphere at a 20 cm depth using sterile soil corer (sterilized with 95% ethanol) and matured Groundnut plants were uprooted with care. From these samples, both Rhizobium and Xanthobacterspp were isolated. The isolated organisms were re-vitalized in Jensen’s nitrogen free broth and standardized to 0.5 McFarland standards. To determine nitrogen fixation, the broth cultures were examined for nitrate nitrogen (NO3-N) and amino nitrogen (Amino-N) levels after ten days of the experiment under continuous airflow using the Jensen’s nitrogen free broth containing the metal salts, Mercury (II) chloride HgCl2 and Lead (II) acetate trihydratePb (CH3COO)2.3H2O). Nitrate nitrogen and amino Nitrogen was obtained using cataldo and ninhydrins methods respectively. The data obtained was made in triplicates and reported as mean values. Interactive effect plots and statistical analysis were done using Minitab 17 software at 5% level of significance (p<0.05).
 Results: The main effect plots illustrate that to maximize nitrogen fixation in Xanthobacterspthrough the utilization of the selected metals as micronutrient, we should use lead at 6.25 mg/L and mercury at 25mg/L yielding 0.508 mg/L for nitrogen fixation response. The plot also suggests that if lead metals are used at a higher concentration than stated nitrogen fixation will decline. On the interaction plots, the slopes indicate that an interference or antagonistic interaction effect (crossed lines) exist between lead and mercury in the nitrogen fixation activity of Xanthobacter. The R-squared adjusted value suggests that 70.87% of the variations in nitrogen fixation response is explained by the interaction of lead and mercury, hence the model likely fits the data. However, the P-value was not significant at 0.102. For Rhizobium sp. mercury also has a higher fixation magnitude than Lead but relatively at 0.554 mg/L. However, the interaction plot showed parallel lines indicating that there was no interaction effect. Therefore, one can say that the relationship between lead and nitrogen fixation does not depend on the concentration of mercury and vice versa. The model was also statistically insignificant at 0.981.
 Conclusion: Interactive effect only occurred in the nitrogen fixation of Xanthobactersp. This raises a need for further study combination of metal elements which could be utilized to stimulate nitrogen, phosphorus and potassium production in Diazotrophs both in the field and in-vitro.

Nitrogen Fixation and Diazotroph Community in the Subarctic Sea of Japan and Sea of Okhotsk

AbstractNitrogen‐fixing microorganisms (diazotrophs) significantly influence marine productivity by converting N2 gas into bioavailable nitrogen. Recent studies revealed that nitrogen fixation occurs in both warm oligotroph and colder and/or N‐rich water; however, little is known about the spatial variability of nitrogen fixation activity and diazotroph diversity in cold waters. In this study, we examined the nitrogen fixation activity and diazotroph community structure in the subarctic waters around Hokkaido in northern Japan, including the Sea of Japan, the Sea of Okhotsk, and the North Pacific Ocean. Nitrogen fixation activity was detected at temperatures ranging from −1.1 to 15.6°C and was significantly related to high salinity and high temperature, which characterize the Tsushima Warm Current. The highest recorded nitrogen fixation activity (5.42 nmol N L−1 d−1) was comparable to that of subtropical regions. Diazotrophs usually reported in subtropical regions dominated the diazotroph communities and were likely advected from the upstream area of the Tsushima Warm Current—a tributary of the Kuroshio Current. In particular, the nifH abundance of the most dominant diazotroph—the symbiotic cyanobacterium UCYN‐A1—showed a significant positive relationship with nitrogen fixation rate. We also detected a diazotroph community and low but active nitrogen fixation in the N‐rich waters sourced from the Sea of Okhotsk, which infers the influence of both advected and indigenous diazotrophs on the regional nitrogen fixation. Although further research is needed, our study points toward the widespread distribution of diazotrophy in the Sea of Okhotsk.

Impact of Lysinibacillus macroides, a potential plant growth promoting rhizobacteria on growth, yield and nutritional value of tomato Plant (Solanum lycopersicum L. f1 hybrid Sachriya)

Plant growth promoting bacteria enhance the growth in plants by solubilizing insoluble minerals, producing phytohormones and by secreting enzymes that resist pathogen attack. The present study was aimed at identifying the potential of Lysinibacillus macroides isolated from pea plant possessing rich microbial rhizobiome diversity in promoting the growth of tomato plant (Solanum lycopersicum L). Potential of L. macroides in the promotion of S. lycopersicum L. growth by increased shoot length, terminal leaf length and breadth was assessed. Anatomical sectioning of stem and root revealed no varied cellular pattern indicating that the supplemented bioculture is not toxic to S. lycopersicum. Plantlets treated with L. macroides along with organic compost showed an increased total phenol content (17.58±0.4 mg/g) compared to control samples (12.44±0.41 mg/g). Carbohydrate content was noticed to be around 1.3 folds higher in the L. macroides plus compost mixture supplemented slots compared to control sample. Significant increase in shoot length was evident in the L. macroides plus compost supplied slots (23.4±2.7 cm). Plant growth promoting properties might be due to the nitrogen fixing activity of the bacteria which enrich the soil composition along with the nutrients supplied by the organic compost. Rich microbial rhizobiome diversity in pea plant and the usage of L. macroides from a non-conventional source improves the diversity of the available PGPR for agricultural practices. Further research is needed to detect the mechanism of growth promotion and to explore the plant microbe interaction pathway.

RETRACTED: Mixed-ligand coordination polymers: Catalytic CO2 fixation and treatment activity on the intestinal polyps by inducing the intestinal epithelial cells apoptosis

Abstract By virtue of a dual-ligand strategy, two transition metal coordination polymers of {[Ni2(NDC)2(L1)(H2O)2(H2O)]·(DMF)2(H2O)}n (1, L1 = 3,3′-azobis pyridine) and [Mn(NDC)(L2)(H2O)]n (2, L2 = 1,4-bis(1H-benzo[d]imidazol-1-yl)butane) have been synthesized via the corresponding metal salts reacting with the 1,4-naphthalene dicarboxylic acid (H2NDC) in the existence of distinct nitrogen-donor co-ligands. Given the permanently microporous nature, moderate CO2 sorption capacity as well as high density of the open metals positions, the activated 1 revealed the chemical fixation of the cyclic carbonates and carbon dioxide or an efficient activity of catalysis in the existence of the cocatalyst under the mild and solvent-free conditions. To evaluate the activity of the compound against intestinal polyps, following experiments were conducted. The Cell Counting Kit-8 (CCK-8) method was implemented to evaluate the inhibitory effects of these two compounds against the intestinal epithelial cells. Next, the western blotting was conducted and the apoptotic protein Bcl-2 expression levels was determined as well.

The metallic 1T-WS2 as cocatalysts for promoting photocatalytic N2 fixation performance of Bi5O7Br nanosheets

Recently, widespread attention has been devoted to the typical layered BiOCl or BiOBr because of the suitable nanostructure and band structure. However, owing to the fast carrier recombination, the photocatalytic performance of BiOX materials is not so satisfactory. Loading 1T phase WS2 nanosheets (NSs) onto Bi5O7Br NSs can improve the photocatalytic N2 fixation activity. Among these, the obtained 1T-WS2@Bi5O7Br composites with optimum 5% 1T-WS2 NSs display a significantly improved photocatalytic N2 fixation rate (8.43mmolL−1h−1g−1), 2.51 times higher than pure Bi5O7Br (3.36mmolL−1h−1g−1). And the outstanding stability of 1T-WS2@Bi5O7Br-5 composites is also achieved. Exactly, the photoexcited electrons from Bi5O7Br NSs are quickly transferred to conductive 1T phase WS2 as electron acceptors, which can promote the separation of carriers. In addition, 1T-WS2 NSs can provide abundant active sites on the basal and edge planes, which can promote the efficiency of photocatalytic N2 fixation. This work offers a novel solution to improve the photocatalytic performance of Bi5O7Br NSs.

Promotion of biological nitrogen fixation activity of an anaerobic consortium using humin as an extracellular electron mediator

Nitrogen fertiliser is manufactured using the industrial Haber–Bosch process, although it is extremely energy-consuming. One sustainable alternative technology is the electrochemical promotion of biological nitrogen fixation (BNF). This study reports the promotion of BNF activity of anaerobic microbial consortia by humin, a solid-phase humic substance, at any pH, functioning as an extracellular electron mediator, to levels of 5.7–11.8 times under nitrogen-deficient conditions. This was evidenced by increased acetylene reduction activity and total nitrogen content of the consortia. Various humins from different origins promoted anaerobic BNF activity, although the degree of promotion differed. The promotion effected by humin differed from the effects of chemical reducing agents and the effects of supplemental micronutrients and vitamins. The promotion of anaerobic BNF activity by only reduced humin without any other electron donor suggested that humin did not serve as organic carbon source but as extracellular electron mediator, for electron donation to the nitrogen-fixing microorganisms. The next generation sequencing (NGS) of partial 16S rRNA genes showed the predominance of Clostridiales (Firmicutes) in the consortia. These findings suggest the effectiveness of humin as a solid-phase extracellular electron mediator for the promotion of anaerobic BNF activity, potentially to serve for the basis for a sustainable technology.

The Composition and Primary Metabolic Potential of Microbial Communities Inhabiting the Surface Water in the Equatorial Eastern Indian Ocean.

Simple SummaryMarine microbes are regarded as the most diverse organisms in the biosphere and drive biogeochemical cycles through their metabolism. It is essential to understand the structure and metabolic function of microbial communities. The Indian Ocean is the third largest ocean in the world, and it possesses unique hydrographical properties. So far, assessments of microbial diversity and metabolism need to be improved in the Indian Ocean. Therefore, we carried out a series of investigations in the equatorial eastern Indian Ocean in order to clarify the local microbial communities and detect the genetic potential for microbial functions. The obtained results suggested Cyanobacteria was the dominant microbial group, and predicted the Calvin cycle and the assimilatory nitrate and nitrite reduction played important role in the pathway of carbon fixation and nitrogen metabolism respectively. This study provides insights into microbial community structures as well as the metabolic potential that may be active in the local environment, and lays the groundwork for understanding the roles of microbes in energy and resource cycling in this habitat.Currently, there is scant information about the biodiversity and functional diversity of microbes in the eastern Indian Ocean (EIO). Here, we used a combination of high-throughput sequencing of 16S rRNA genes and a metagenomic approach to investigate the microbial population structure and its metabolic function in the equatorial EIO. Our results show that Cyanobacterial Prochlorococcus made up the majority of the population. Interestingly, there were fewer contributions from clades SAR11 (Alphaproteobacteria) and SAR86 (Gammaproteobacteria) to microbial communities than contributions from Prochlorococcus. Based on functional gene analysis, functional genes rbcL, narB, and nasA were relatively abundant among the relevant genes. The abundance of Prochlorococcus implies its typically ecological adaptation in the local ecosystem. The microbial metabolic potential shows that in addition to the main carbon fixation pathway Calvin cycle, the rTCA cycle and the 3-HP/4-HB cycle have potential alternative carbon fixation contributions to local ecosystems. For the nitrogen cycle, the assimilatory nitrate and nitrite reduction pathway is potentially the crucial form of nitrogen utilization; unexpectedly, nitrogen fixation activity was relatively weak. This study extends our knowledge of the roles of microbes in energy and resource cycling in the EIO and provides a foundation for revealing profound biogeochemical processes driven by the microbial community in the ocean.

Toxicity and environmental impact of multi-walled carbon nanotubes to nitrogen-fixing bacterium Azotobacter chroococcum

With the large-scale production and wide applications of carbon nanotubes (CNTs), they are inevitably entering the environment and bringing unknown threats to environmental organisms. Nitrogen-fixing bacteria dominate 80% of the nitrogen fixation in nitrogen cycle. Herein, we investigated the toxicity and environmental impact of multi-walled CNTs (MWCNTs) to nitrogen-fixing bacterium Azotobacter chroococcum. MWCNTs slightly inhibited the growth of A. chroococcum and induced small increase of death rate at high concentrations. MWCNTs did not penetrate the cell walls of A. chroococcum and no cytoplasm loss was observed. MWCNTs were only observed surrounding the A. chroococcum cells. According to oxidative stress assays, MWCNTs induced meaningful oxidative damage at 0.5 mg/mL after 6 d exposure, but the oxidative damage diminished at 14 d. MWCNTs did not affect the nitrogen fixation activity of A. chroococcum and the soil nitrogen contents. Our findings provided the fundamental data for the environmental safety of CNTs to nitrogen-fixing bacteria.

Transcriptional analysis of nitrogen fixation in Paenibacillus durus during growth in nitrogen-enriched medium.

Paenibacillus durus strain ATCC 35681T is a Gram-positive diazotroph that displayed capability of fixing nitrogen even in the presence of nitrate or ammonium. However, the nitrogen fixation activity was detected only at day 1 of growth when cultured in liquid nitrogen-enriched medium. The transcripts of all the nifH homologues were present throughout the 9-day study. When grown in nitrogen-depleted medium, nitrogenase activities occurred from day 1 until day 6 and the nifH transcripts were also present during the course of the study albeit at different levels. In both studies, the absence of nitrogen fixation activity regardless of the presence of the nifH transcripts raised the possibility of a post-transcriptional or post-translational regulation of the system. A putative SigA box sequence was found upstream of the transcription start site of nifB1, the first gene in the major nitrogen fixation cluster. The upstream region of nifB2 showed a promoter recognizable by SigE, a sigma factor normally involved in sporulation.