The effects of salt on nitrogen-fixation and ammonium assimilation inAnabaena variabilis

Isolation and characterization of high temperature tolerant mutant from the cyanobacterium Anabaena doliolum

The mutant strain of the diazotrophic cyanobacterium Anabaena doliolum able to tolerate high temperature was isolated by induced mutation techniques using ethyl methane sulphonate. This mutant strain exhibited higher temperature tolerance than the wild type. The wild type was able tolerate temperature up to 40 °C whereas the mutant was able to grow at an elevated temperature of 48 °C. This mutant exhibited higher growth rate, heterocyst frequency, and nitrogen fixation. Mutant strains exhibited comparable levels of chlorophyll, phycocyanin, PS II activity, and O₂ evolution as compared to unexposed control. Results also showed that the mutant accumulated low levels of peroxides and lipid peroxidation products with enhanced activity of antioxidant enzymes. The FAME analysis revealed quantitative and qualitative changes in the profile of fatty acids in the mutant strain. Maximum number of saturated fatty acids was observed in the mutant strain followed by control whereas the wild type exposed to elevated temperature showed least diversity of fatty acids. Enhanced level of antioxidant enzymes coupled with efficient modulation of fatty acid profile could therefore enhance the mutant to resist the high temperature stress. The results could be exploited further to decipher molecular mechanisms underlying the temperature tolerance and enhancing the utility of A. doliolum as efficient biofertilizer for rice paddy keeping in view of the future climatic change scenario.

Complete genome sequence of Anabaena variabilis ATCC 29413

Anabaena variabilis ATCC 29413 is a filamentous, heterocyst-forming cyanobacterium that has served as a model organism, with an extensive literature extending over 40 years. The strain has three distinct nitrogenases that function under different environmental conditions and is capable of photoautotrophic growth in the light and true heterotrophic growth in the dark using fructose as both carbon and energy source. While this strain was first isolated in 1964 in Mississippi and named Anabaena flos-aquae MSU A-37, it clusters phylogenetically with cyanobacteria of the genus Nostoc. The strain is a moderate thermophile, growing well at approximately 40(°) C. Here we provide some additional characteristics of the strain, and an analysis of the complete genome sequence.

Response to NaCl of nitrate assimilation and nitrogenase activity of the cyanobacterium Anabaena sp. PCC 7120 and its mutants

The presence of NaCl in the nutrient solution promoted nitrate uptake in parent Anabaena sp. PCC 7120, mutants SP7 (defective in nitrate reductase activity) and SP17 (partially defective in nitrate reductase activity), but not in the mutant SP9 (defective in nitrate transport and reduction). Nitrate reductase activity of the parent and mutant SP17 increased with increasing concentration of nitrate in saline medium, while mutants SP7 and SP9 did not respond to the altered salinity. Although Na+ was not required for nitrate reductase activity, its presence in the nutrient solution enhanced nitrate reduction. Complete removal of Na+ from the nutrient solution markedly reduced nitrogenase activity in all the strains, while raising the concentration of NaCl to 50 mmol l-1 or above, was equally toxic to nitrogenase activity. External NaCl at 200 mmol l-1 brought down the nitrogenase activity to the same residual level as observed without Na+.

Environmental factors influencing the nitrogen fixation activity of free-living terrestrial cyanobacteria from a high arctic area, Spitsbergen

The influence of environmental factors on the nitrogen fixation activity of free-living, terrestrial cyanobacteria from a high arctic area were investigated using experimental manipulations with two different types of field samples, including macroscopic sheets of Nostoc commune and soil samples with a cyanobacterial crust from a Puccinellia salt marsh. In addition, a cultured Anabaena sp. previously isolated from the salt marsh was examined. Nitrogen fixation activity was measured using the acetylene reduction method. The nitrogen fixation mainly took place in the light, but even after 12 h incubation in darkness, low activities were maintained. Phosphorus fertilization stimulated the nitrogen fixation activity, and the highest activities were obtained with about 300 μM phosphate, both in the field samples and the cultured Anabaena sp. Ammonium (28 mM) immediately inhibited the nitrogen fixation activity of the cultured Anabaena sp, whereas 14 mM urea and 540 μM glutamate led to a weaker and slower inhibition of the nitrogen fixation activity, showing that the cultured Anabaena sp. was able to assimilate these combined nitrogen sources. Nitrate did not have any inhibitory effect on nitrogen fixation activity, either in the field samples or in the cultured Anabaena sp. Both the field samples and the cultured Anabaena sp. showed tolerance against sodium chloride concentrations corresponding to the concentration in seawater. The temperature optimum of the nitrogen fixation activity of the cultured Anabaena sp. was about 20°C. Key words: nitrogen fixation, cyanobacteria, Nostoc commune, Anabaena sp., high arctic.