Conversion factor between acetylene reduction and nitrogen fixation in free-living cyanobacteria from high arctic habitats

The conversion factor between acetylene reduction and 15 N incorporation in free-living cyanobacteria was determined in different high arctic habitats in the area of Ny-Ålesund (78.5°N, 11.6°E), Spitsbergen, in the summer of 1994. The experiments were carried out under constant conditions, 19°C and 200 µE·m -2 ·s -1 . The nitrogen-fixation activities, measured as 15 N-incorporation, were in the range 4.01-6.54 mg N 2 fixed·gdw -1 ·day -1 (dw, dry weight) in sheets of Nostoc commune and 778-1206 mg N 2 fixed·m -2 ·day -1 in the cyanobacterial crusts. The acetylene reduction activities were in the range 0.72-1.91 mg ethylene produced·gdw -1 ·day -1 of N. commune and 12.8-63.7 mg ethylene produced·m -2 ·day -1 in the cyanobacterial crusts. The conversion factor of N. commune ranged from 0.11 to 0.48 for ethylene produced to nitrogen fixed, whereas the cyanobacterial crusts covering the soil surface gave conversion factors in the range 0.022-0.073 for ethylene produced to nitrogen fixed. An Anabaena sp., isolated from one of the habitats investigated, gave conversion factors near the theoretical factor of 4, when determined at 14.0 and 17.3°C. It was concluded that the acetylene reduction activity of free-living cyanobacteria in high arctic habitats results in underestimates of the real nitrogen-fixation activity in these environments.Key words: nitrogen fixation, acetylene reduction, conversion factor, cyanobacteria, Nostoc commune, high arctic.