Evaluation of growth, nitrogen fixation and P-solubilizing ability of diazotrophic cyanobacteria under mineral phosphorus sources

Five cyanobacterial strains, viz. Anabaena variabilis, Nostoc muscorum, Tolypothrix tenuis, Aulosira fertilissima and Westiellopsis prolifica were utilized extracellular insoluble tricalcium phosphate (TCP) and Mussorie rock phosphate (MRP) when provided as sole P source by replacing the conventional source of P (K2HPO4) in BG-11 medium. These strains exhibited a differential response to insoluble forms of phosphorus both in terms of growth and nitrogen fixation. A. variabilis, N. muscorum and W. prolifica showed better growth in presence of TCP while T. tenuis and A. fertilissima proliferated better in presence of MRP. N. muscorum recorded maximum growth in terms of total chlorophyll content (6.21 mu g/ml) in presence of TCP (20 mg P/l) followed by T. tenuis (6.10 mu g/ml) in presence of MRP (20 mg P/l) after 21 days of incubation. Nitrogen fixation measured as acetylene reduction (AR) activity showed significant variation among different strains when grown at varied concentrations of MP or MRP and time of incubation. N. muscorum showed maximum AR activity of 6.17 mu mole C2H4/mg chl/h in presence of MRP (20 mg P/l) followed by A. variabilis which recorded 5.46 mu mole C2H4/mg chl/h but in presence of TCP (20 mg P/l) after 28 days of incubation. In general, in all the strains tested, AR activity was more in presence of higher concentration of TCP/ MRP than their lower concentrations (10 mg P/l). In terms of P-solubilization, A. variabilis was found to be the best, showing maximum amount of available P, i.e. 0.224 mu g/ml and TCP (20 mg P/l) was the best utilized source of mineral P compared to MRP.