Synthesis of nitrogenase by Paenibacillus sabinae T27 in presence of high levels of ammonia during anaerobic fermentation

Enrichment of nitrogen-fixing hydrogen-oxidizing bacteria community for efficient microbial protein production in airlift reactor

Autotrophic nitrogen-fixing hydrogen-oxidizing bacteria (NF-HOB) have the capability to directly utilize carbon dioxide and dinitrogen for the sustainable production of microbial protein (MP), offering a promising alternative for food and feed applications. However, the low production rate of MP remains a major bottleneck for the practical implementation of NF-HOB. This study enriched a highly active and stable NF-HOB functional community for enhanced MP production. Utilizing an airlift reactor and implementing a two-stage gas supply strategy achieved a maximum cell dry weight (CDW) of 4.8 g/L and a biomass yield of 0.25 g CDW/L/day, surpassing previously reported values. The produced MP exhibited an essential amino acid profile superior to soybean meal and comparable to fish meal, with a notable accumulation of branched-chain amino acids (BCAAs). These findings provide new insights into NF-HOB enrichment strategies and further highlight their potential as a sustainable platform for MP production.

Effects of nitrogen supply on hydrogen-oxidizing bacterial enrichment to produce microbial protein: Comparing nitrogen fixation and ammonium assimilation

To investigate the effects of nitrogen source supply on microbial protein (MP) production by hydrogen-oxidizing bacteria (HOB) under continuous feed gas provision, a sequencing batch culture comparison (N2 fixation versus ammonium assimilation) was performed. The results confirmed that even under basic cultivation conditions, N2-fixing HOB (NF-HOB) communities showed higher levels of CO2 and N2 fixation (190.45 mg/L Δ CODt and 11.75 mg/L Δ TNbiomass) than previously known, with the highest biomass yield being 0.153 g CDW/g COD-H2. Rich ammonium stimulated MP synthesis and the biomass accumulation of communities (increased by 7.4 ～ 14.3 times), presumably through the enhancement of H2 and CO2 absorption. The micro mechanism may involve encouraging the enrichment of species like Xanthobacter and Acinetobacter then raising the abundance of nitrogenase and glutamate synthase to facilitate the nitrogen assimilation. This would provide NF-HOB with ideas for optimizing their MP synthesis activity.

Occurrence, identification and characterization of diazotrophic bacteria from aerial roots of Rhynchostylis retusa (L.) Blume for plant growth-promoting activity

In this study, the diversity of diazotrophic bacteria of orchid Rhynchostylis retusa (L.) Blume and its potential application in plant growth promotion were evaluated. About 183 nitrogen-fixing bacteria were isolated to screen various plant growth-promoting traits viz. phosphate solubilization,IAA, siderophore, HCN, biofilm and ammonia production. Based on 16S rRNA gene sequencing analysis Achromobacter, Arthrobacter, Acinetobacter, Bacillus, Brevibacterium, Curtobacterium, Erwinia, Kosakonia, Lysinibacillus, Klebseilla, Microbacterium, Mixta, Pantoea, Pseudomonas and Stenotrophomonas isolates were selected and showed positive results for PGP traits. Overall, genus Pantoea, Brevibacterium, Achromobacter, Arthrobacter, Klebsiella, Mixta, Bacillus, and Pseudomonas had the most pronounced PGP characteristics and acetylene reduction among the screened isolates. BOX PCR fingerprinting analysis showed variation in polymorphic banding patterns among diazotrophic strains. PCR amplification of nifH gene and the presence of 37 kDa nitrogenase reductase enzyme band in western blot indicated presence of nitrogenase activity. Our study showed that orchid R. retusa diazotroph interaction helps orchid plant to fix nitrogen, essential nutrients, and control pathogen entry. To the best of our knowledge, this is the first report on characterization of diazotrophic bacterial community from aerial roots of R. retusa.

Assembly of nitrogenase biosynthetic pathway in Saccharomyces cerevisiae by using polyprotein strategy

Nitrogenase in some bacteria and archaea catalyzes conversion of N₂ to ammonia. To reconstitute a nitrogenase biosynthetic pathway in a eukaryotic host is still a challenge, since synthesis of nitrogenase requires a large number of nif (nitrogen fixation) genes. Viral 2A peptide mediated "cleavage" of polyprotein is one of strategies for multigene co-expression. Here, we show that cleavage efficiency of NifB-2A-NifH polyprotein linked by four different 2A peptides (P2A, T2A, E2A, and F2A) in Saccharomyces cerevisiae ranges from ~50% to ~90%. The presence of a 2A tail in NifB, NifH, and NifD does not affect their activity. Western blotting shows that 9 Nif proteins (NifB, NifH, NifD, NifK, NifE, NifN, NifX, HesA, and NifV) from Paenibacillus polymyxa that are fused into two polyproteins via 2A peptides are co-expressed in S. cerevisiae. Expressed NifH from Klebsiella oxytoca NifU and NifS and P. polymyxa NifH fusion linked via 2A in S. cerevisiae exhibits Fe protein activity.

Functional analysis of multiple nifB genes of Paenibacillus strains in synthesis of Mo-, Fe- and V-nitrogenases

Background

Biological nitrogen fixation is catalyzed by Mo-, V- and Fe-nitrogenases that are encoded by nif, vnf and anf genes, respectively. NifB is the key protein in synthesis of the cofactors of all nitrogenases. Most diazotrophic Paenibacillus strains have only one nifB gene located in a compact nif gene cluster (nifBHDKENX(orf1)hesAnifV). But some Paenibacillus strains have multiple nifB genes and their functions are not known.

Results

A total of 138 nifB genes are found in the 116 diazotrophic Paenibacillus strains. Phylogeny analysis shows that these nifB genes fall into 4 classes: nifBI class including the genes (named as nifB1 genes) that are the first gene within the compact nif gene cluster, nifBII class including the genes (named as nifB2 genes) that are adjacent to anf or vnf genes, nifBIII class whose members are designated as nifB3 genes and nifBIV class whose members are named as nifB4 genes are scattered on genomes. Functional analysis by complementation of the ∆nifB mutant of P. polymyxa which has only one nifB gene has shown that both nifB1 and nifB2 are active in synthesis of Mo-nitrogenase, while nifB3 and nifB4 genes are not. Deletion analysis also has revealed that nifB1 of Paenibacillus sabinae T27 is involved in synthesis of Mo-nitrogenase, while nifB3 and nifB4 genes are not. Complementation of the P. polymyxa ∆nifBHDK mutant with the four reconstituted operons: nifB1anfHDGK, nifB2anfHDGK, nifB1vnfHDGK and nifB2vnfHDGK, has shown both that nifB1 and nifB2 were able to support synthesis of Fe- or V-nitrogenases. Transcriptional results obtained in the original Paenibacillus strains are consistent with the complementation results.

Conclusions

The multiple nifB genes of the diazotrophic Paenibacillus strains are divided into 4 classes. The nifB1 located in a compact nif gene cluster (nifBHDKENX(orf1)hesAnifV) and the nifB2 genes being adjacent to nif or anf or vnf genes are active in synthesis of Mo-, Fe and V-nitrogenases, but nifB3 and nifB4 are not. The reconstituted anf system comprising 8 genes (nifBanfHDGK and nifXhesAnifV) and vnf system comprising 10 genes (nifBvnfHDGKEN and nifXhesAnifV) support synthesis of Fe-nitrogenase and V-nitrogenase in Paenibacillus background, respectively.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01629-9.