Analysis of the nifHDK operon and structure of the NifH protein from the unicellular, diazotrophic cyanobacterium, Cyanothece strain sp. ATCC 51142(1)

What determines symbiotic nitrogen fixation efficiency in rhizobium: recent insights into Rhizobium leguminosarum

Symbiotic nitrogen fixation (SNF) by rhizobium, a Gram-negative soil bacterium, is an essential component in the nitrogen cycle and is a sustainable green way to maintain soil fertility without chemical energy consumption. SNF, which results from the processes of nodulation, rhizobial infection, bacteroid differentiation and nitrogen-fixing reaction, requires the expression of various genes from both symbionts with adaptation to the changing environment. To achieve successful nitrogen fixation, rhizobia and their hosts cooperate closely for precise regulation of symbiotic genes, metabolic processes and internal environment homeostasis. Many researches have progressed to reveal the ample information about regulatory aspects of SNF during recent decades, but the major bottlenecks regarding improvement of nitrogen-fixing efficiency has proven to be complex. In this mini-review, we summarize recent advances that have contributed to understanding the rhizobial regulatory aspects that determine SNF efficiency, focusing on the coordinated regulatory mechanism of symbiotic genes, oxygen, carbon metabolism, amino acid metabolism, combined nitrogen, non-coding RNAs and internal environment homeostasis. Unraveling regulatory determinants of SNF in the nitrogen-fixing protagonist rhizobium is expected to promote an improvement of nitrogen-fixing efficiency in crop production.

Chlorpyrifos inhibits nitrogen fixation in rice-vegetated soil containing Pseudomonas stutzeri A1501

Chlorpyrifos, a common organophosphorus pesticide, is widely used for agricultural pest control and can inhibit nitrogen-fixing bacteria biomass in paddy. In this study, the additions of chlorpyrifos (1 and 8 mg kg^-1) to soil, with or without Pseudomonas stutzeri A1501, resulted in a significant decrease in nitrogen fixation, despite insignificant effects on the abundances of P. stutzeri A1501 and bacteria in soil. Toxic effect of chlorpyrifos on P. stutzeri A1501 nitrogenase activity in medium was also observed, accompanied by a significant reduction in the expression of nitrogen-fixing related genes (nifA and nifH). Furthermore, rhizosphere colonization and biofilm formation by P. stutzeri A1501 were repressed by chlorpyrifos, leading to decreased nitrogenase activity in the rhizosphere. Biofilm formation in medium was inhibited by bacterial hyperkinesis and reduction of extracellular polymeric substance, including exopolysaccharides and proteins. Together, these findings showed that chlorpyrifos-induced production of reactive oxygen species (ROS) which was directly responsible for reduced nitrogenase activity in the medium, soil, and rhizosphere by inhibiting the expressions of nitrogen-fixing related genes. Furthermore, the inhibition of biofilm formation by chlorpyrifos or ROS likely aggravated the reduction in rhizospherere nitrogenase activity. These findings provide potentially valuable insights into the toxicity of chlorpyrifos on nitrogen-fixing bacteria and its mechanisms. Furthermore, for sustainable rice production, it is necessary to evaluate whether other pesticides affect nitrogen fixation and select pesticides that do not inhibit nitrogen fixation.

Better living through cyanothece - unicellular diazotrophic cyanobacteria with highly versatile metabolic systems

Cyanothece sp. ATCC 51142 is a unicellular, diazotrophic cyanobacterium with a versatile metabolism and very pronounced diurnal rhythms. Since nitrogen fixation is exquisitely sensitive to oxygen, Cyanotheceutilizes temporal regulation to accommodate these incompatible processes in a single cell. When grown under 12 h light-dark (LD) periods, it performs photosynthesis during the day and N(2) fixation and respiration at night. Genome sequences of Cyanothece sp. ATCC 51142 and that of five other Cyanothece species have been completed and have produced some surprises. Analysis at both the transcriptomic and the proteomic levels in Cyanothece sp. ATCC 51142 has demonstrated the relationship of the metabolic synchrony with gene expression and has given us insights into diurnal and circadian regulation throughout a daily cycle. We are particularly interested in the regulation of metabolic processes, such as H(2) evolution, and the way in which these organisms respond to environmental cues, such as light, the lack of combined nitrogen, and changing O(2) levels. Cyanothece strains produce copious amounts of H(2) under different types of physiological conditions. Nitrogenase produces far more H(2) than the hydrogenase, in part because the nitrogenase levels are extremely high under N(2)-fixing conditions. With Cyanothece 51142 cultures grown in NO(3)-free media, either photoautotrophically or mixotrophically with glycerol, we have obtained H(2) production rates over 150 mumol/mg Chl/h.

Determination of nitrogen-fixing phylotypes in Lyngbya sp. and Microcoleus chthonoplastes cyanobacterial mats from Guerrero Negro, Baja California, Mexico

In many environments, biological nitrogen fixation can alleviate nitrogen limitation. The high rates of N(2) fixation often observed in cyanobacterial mats suggest that N(2) fixation may be an important source of N. In this study, organisms expressing nifH were identified in a Lyngbya sp.- and two Microcoleus chthonoplastes-dominated cyanobacterial mats. The pattern of nitrogenase activity was determined for the Lyngbya sp. mat and a Microcoleus chthonoplastes mat sampled directly in Guerrero Negro, Mexico. Their maximum rates were 23 and 15 micro mol of C(2)H(4) m(-2) h(-1), respectively. The second Microcoleus mat, which was maintained in a greenhouse facility, had a maximum rate of 40 micro mol of C(2)H(4) m(-2) h(-1). The overall diel pattern of nitrogenase activity in the three mats was similar, with the highest rates of activity occurring during the dark period. Analysis of nifH transcripts by reverse transcription-PCR revealed that several different organisms were expressing nifH during the dark period. nifH phylotypes recovered from these mats were similar to sequences from the unicellular cyanobacterial genera Halothece, Myxosarcina, and Synechocystis, the filamentous cyanobacterial genera Plectonema and Phormidium, and several bacterial nifH groups. The results of this study indicate that several different organisms, some of which were not previously known to fix nitrogen, are likely to be responsible for the observed dark-period nitrogenase activity in these cyanobacterial mats.