Abstract: The interactions of bicarbonate (HCO3¯) and different nitrogen sources in the non-diazotrophic cyanobacterium Spirulina platensis in terms of pigments content, photosynthetic O2 evolution, Ca++-dependent ATPase activity and cytochrome c oxidase activity were examined in the present investigation. The cyanobacterium was acclimated to 300 mol m-3 HCO3¯-concentrations in the growth medium containing no nitrogen. The addition of different nitrogen sources in the growth medium resulted in further acclimatization up to 400 mol m-3 HCO3¯. This finding revealed that nitrogen sources incorporated into necessary metabolites, nitrogen storage material and photosynthetic pigments as well. The combined transport of HCO3¯and nitrogen sources ultimately resulted in gradual increase in the examined parameters in order of NO3¯<NH4+<urea. Nitrate and ammonium assimilating enzymes i.e., Nitrate Reductase (NR) and Glutamine Synthetase (GS) were also examined in graded concentration of HCO3¯ and the findings suggested that both the enzymes showed higher activity in the medium containing higher HCO3¯ concentration this suggested that the activity of these enzymes closely linked with carbon fixation. Further, we examined the Ca++-dependent ATPase activity and cytochrome c oxidase activity in the presence of graded concentration of HCO3¯. The enzyme was more active in the presence of high HCO3¯ concentration; the co-existence of NADPH further enhances the enzyme activities. The use of such cultural practices would help in mass production of biopigments on industrial scale and would paved the way for the development of new technology for CO2 fixation using the solar light as the energy source.