Growth and Some Metabolic Features of Cyanobacterium Fischerella Sp. FS18 in Different Combined Nitrogen Sources.

In the present research the growth and metabolism of cyanobacteriumfischerella sp. fs18, isolated from rice fields of gillan province (iran), were evaluated in different nitrogen sources. this strain is a good candidate as biological fertilizer due to high efficiency to n2 fixation. in this way, cells in logarithmic phase of stock were treated with different nitrogen sources (n-free, 1 & 2 mm nh4 + and 5 and 10 mm no3-). results indicated that maximum growth rate belonged to no3- treatment. dry weights of fischerella sp. fs18 in n-free medium and ammonium were in decreasing order of preference; and nh4 + 2 mmwas drastically toxic for this strain. the highest chlorophyll concentration was detected in n-free medium. no3 - 5 mm and nh4 + 2 mm had the lower contents of this pigment. photosynthesis rate varied the same as chlorophyll. phycocyanin and phycoerythrin had the highest rate in nitrate. the highest light-saturated photosynthetic rate and also steepest initial slope clearly belonged to nitrogen free-medium. all types of combined nitrogen sources had inhibitory effect on nitrogenase activity, regardless of the concentrations used. our results indicated that fischerella sp. fs18 is a potent diazotrophic species with increasing growth in the presence of nitrate; and ammonium 2mm seems to be toxic for it.