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 mu M 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 mu M 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 degrees C.