a new report of n fixation by two species of cyanobacteria

cyanobacteria were collected and identified in five paddy fields with different soil textures of golestan province in iran. it was determined that nostoc ellipsosporum and nostoc muscorum were dominant in all sites among 20 species. to assess the ability of atmospheric nitrogen fixation by the species, their pure cultures were provided by growing on bg-11 solid culture medium. subsequently, each species was grown on allen liquid culture medium for 21 days and the growth curves were obtained by measuring thallus chlorophyll amount at 1.5 day intervals. the amounts of n fixed by each species were measured by the indophenol method and by measuring ethylene amounts produced via the acetylene reduction process using gas chromatography. the results showed significantly higher rates in n fixation, cellular doubling time and growth rate in n.ellipsosporum with larger heterocysts than n.muscorum. therefore, n.ellipsosporum with a higher ability of n and ethylene production is suggested as the superior biological fertilizer.