Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Phenotypic and genetic diversity in Sinorhizobium meliloti and S. medicae from drought and salt affected regions of Morocco

Abstract: Background: Sinorhizobium meliloti and S. medicae are symbiotic nitrogen fixing bacteria in root nodules of forage legume alfalfa (Medicago sativa L.). In Morocco, alfalfa is usually grown in marginal soils of arid and semi-arid regions frequently affected by drought, extremes of temperature and soil pH, soil salinity and heavy metals, which affect biological nitrogen fixing ability of rhizobia and productivity of the host. This study examines phenotypic diversity for tolerance to the above stresses and genotypic diversity at Repetitive Extragenic Pallindromic DNA regions of Sinorhizobium nodulating alfalfa, sampled from marginal soils of arid and semi-arid regions of Morocco. Results: RsaI digestion of PCR amplified 16S rDNA of the 157 sampled isolates, assigned 136 isolates as S. meliloti and the rest as S. medicae. Further phenotyping of these alfalfa rhizobia for tolerance to the environmental stresses revealed a large degree of variation: 55.41%, 82.16%, 57.96% and 3.18% of the total isolates were tolerant to NaCl (>513 mM), water stress (-1.5 MPa), high temperature (40°C) and low pH (3.5), respectively. Sixty-seven isolates of S. meliloti and thirteen isolates of S. medicae that were tolerant to salinity were also tolerant to water stress. Most of the isolates of the two species showed tolerance to heavy metals (Cd, Mn and Zn) and antibiotics (chloramphenicol, spectinomycin, streptomycin and tetracycline). The phenotypic clusters observed by the cluster analysis clearly showed adaptations of the S. meliloti and S. medicae strains to the multiple stresses. Genotyping with rep-PCR revealed higher genetic diversity within these phenotypic clusters and classified all the 157 isolates into 148 genotypes. No relationship between genotypic profiles and the phenotypes was observed. The Analysis of Molecular Variance revealed that largest proportion of significant (P < 0.01) genetic variation was distributed within regions (89%) than among regions (11%). Conclusion: High degree of phenotypic and genotypic diversity is present in S. meliloti and S. medicae populations from marginal soils affected by salt and drought, in arid and semi-arid regions of Morocco. Some of the tolerant strains have a potential for exploitation in salt and drought affected areas for biological nitrogen fixation in alfalfa.

Two seasons’ study on nifH gene expression and nitrogen fixation by diazotrophic endophytes in sugarcane (Saccharum spp. hybrids): expression of nifH genes similar to those of rhizobia

Abstract: To explore the presence and expression of the nifH gene of diazotrophic endophytes in sugarcane (Saccharum spp. hybrids) cv. NiF8, we conducted pot experiments for two seasons in a glasshouse from 10 June to 17 September (high temperature and long days) in 2005 and 1 September to 9 December (low temperature and short days) in 2006. The expression of nifH genes in the stems and roots of sugarcane plants at 50 (or 59) and 100 days after transplanting was investigated by reverse transcription (RT)–PCR and by examining the nifH nucleotide sequence diversity. N2 fixation was assessed by the 15N-dilution method. The nifH RNA sequences in the stems and roots of the first experiment were similar to those of Bradyrhizobium sp. and Azorhizobium caulinodans. In the second experiment, nifH expression from these bacteria was not detected in the stems. nifH gene expression in the stems was in accordance with 18–24% N derived from air in the high-temperature season and negligible in the low-temperature season. Both the proliferation of N2 fixers and expression of the nifH gene indicated that bacteria similar to rhizobia may contribute to N2 fixation in sugarcane under our experimental conditions.

Cyanobacteria and black mangroves in northwestern Mexico: colonization, and diurnal and seasonal nitrogen fixation on aerial roots

Abstract: Nitrogen fixation and colonization by associative cyanobacteria in the aerial roots (pneumatophores) of black mangrove trees was evaluated in situ at Balandra lagoon, Baja California Sur, Mexico, for 18 consecutive months. Year-round vertical zonation of cyanobacterial colonization was determined along the pneumatophores. The bottom part close to the sediment was colonized mainly by nonheterocystous, filamentous cyanobacteria resembling Lyngbya sp. and Oscillatoria sp. The central zone was colonized mainly by filaments resembling Microcoleus sp. and the upper part was colonized by coccoidal cyanobacteria within defined colonies resembling Aphanothece sp. mixed with undefined filamentous cyanobacteria. Two of the cyanobacteria (Microcoleus sp. and Anabaena sp.) isolated from the pneumatophore were diazotrophs. Massive biofilm production along the pneumatophores was evident throughout the observation period. The surrounding sediment was seasonally dominated by heterocystous Anabaena sp. Glass and dead-wood surfaces incubated for 18 months in the pneumatophore vicinity showed no zonation in the colonization pattern, although they were heavily colonized. In situ N2 fixation showed seasonal and diurnal fluctuations. N2 fixation was low during winter, increased in early summer, and reached its peak in midsummer. N2 fixation in the summer showed diurnal peaks: one in the morning until midday and the second in the late afternoon. N2 fixation was at its lowest levels near midnight. Light and water temperature are probably primary environmental factors governing N2 fixation on the pneumatophores.