Non-heterocystous nitrogen fixing strains of cyanobacteria were screened by their ability to grow in nitrogen deficient media. The selected nitrogen fixing cyanobacterial cells were then cultured in BG11 media supplemented with [15N]-labeled sodium nitrate. Under these growth conditions any organic [14N] found in the cyanobacterial cells would simply come from nitrogen fixation because [15N] was the only available source of nitrogen in the medium. Amino acids extracted after different time periods (after 15, 30, 40, 50 and 60 days of inoculation) were used for the determination of the 14N/15N ratio using GC-MS. Results from the present study support the conclusion that at stationary phase of growth cyanobacterial nitrogen fixation was no longer supplying a significant amount of nitrogen. This approach not only provided a detailed method for the evaluation of the nitrogen fixing potential of the cyanobacteria in culture, but also suggests novel approaches for the assessment of the ability of the strains to provide nitrogen enrichment to plants under co-cultivation conditions.

Sumaira Mazhar, Jerry D. Cohen and Shahida Hasnain