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

The conversion factor between acetylene reduction and N-15 incorporation in free-living cyanobacteria was determined in different high arctic habitats in the area of Ny-Angstrom lesund (78.5 degrees N, 11.6 degrees E), Spitsb ergen, in the summer of 1994. The experiments were carried out under consta nt conditions, 19 degrees C and 200 mu E . m(-2). s(-1). The nitrogen-fixat ion activities, measured as N-15-incorporation, were in the range 4.01-6.54 mg N-2 fixed . gdw(-1). day(-1) (dw, dry weight) in sheets of Nostoc commu ne 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 produc ed . 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 conversi on 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 con version factors near the theoretical factor of 4, when determined at 14.0 a nd 17.3 degrees C. It was concluded that the acetylene reduction activity o f free-living cyanobacteria in high arctic habitats results in underestimat es of the real nitrogen-fixation activity in these environments.