Energy crisis as a result of population growth and industrial development is a global challenge which the limited sources of fossil fuels is not able to respond the rising demand for energy. Moreover, environmental pollution and greenhouse gas emission together with fluctuation and instability of fossil fuels prices encouraged the researchers for finding an alternative source of renewable, safe and environmental-friendly energy. In recent years, Lipid constituent of microalgae has been attracted as the most promising source of alternative fuels to replace the fossil fuels. Many factors affect the growth behavior of algae but the concentration of carbon and nitrogen sources in the medium is the most important factors in algal cultivation. In this study, a marine microalgae species Isochrysis galbana was used. The impact of culture condition, type and concentration of carbon source and concentration of nitrogen source on the biomass production and lipid content of biomass were investigated. Then in another experiment, I. galbana was cultured in sequence two phase process under different amount of nitrogen in order to increase both biomass and lipid productivity. In the phototrophic culture, in the presence of nitrogen concentration of 0.4 g.L -1 , the maximum cell concentration and the specific growth rate were obtained 5.12×10 6 cell.ml -1 and 0.25 d -1 , respectively. In the mixotrophic culture in the presence of 50 mM glycerol concentration as organic carbon source the maximum cell concentration and lipid production were obtained 4.68×10 6 cell.ml -1 and 0.061 g, respectively. In the heterotrophic culture using 25mM glycerol as organic carbon source the maximum cell concentration and lipid production were obtained 29.3×10 6 cell.ml -1 and 0.049 g, respectively. In another experiment, when glycerol replace with carbonate the maximum cell concentration and the specific growth rate were obtained 5.88×10 6 cell.ml -1 and 0.22 d -1 , respectively, in the presence of the carbonate concentration of 1 g.L -1 . After estimation of the optimum carbonate concentration, in the next step, I. galbana was cultured using different amounts of nitrogen. In the first phase to achieve high biomass productivity the microalgae was cultured in a nitrogen replete medium before the end of lag phase period and after that in second phase the culture was performed under 0.5 and 0.05 dilution in nitrogen concentration for three days. The maximum cell concentration and specific growth rate were obtained 0.525.88×10 6 cell.ml -1 and 0.35 d -1 respectively using the nitrate concentration of 0.4 g/l. In two phase culture, 46.71% enhancement in the lipid content was observed when the nitrogen concentration in the first phase and the dilution rate were 0.1 g/l and 0.05, respectively. Keywords: biodiesel, lipid, Isochrysis galbana , carbonate
