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In this present investigation an attempt was made to study the Bicarbonate tolerance of selected micro algal strains (Scenedesmus sp. Botryococcus sp. and Euglena sp.). The three microalgal strains were collected from the Presidency College, Chennai, India. These strains were grown in different molar concentrations of NaHCO3 in two different medium BBM and BG11 without any changes in other substances of media. The Optical density (OD) was measured at regular interval time of 24hrs at 680nm employing UV-Vis Spectrophotometer. Growth rate and Biochemical composition were investigated in this study. All the three microalgal strains were grown well in the 0.1 molar concentration of NaHCO3 in both BBM and BG11 media but the best growth rate was showed in BG11 media when compared to BBM. The highest growth rate was seen in Euglena sp. and lipid content was found high in Botryococcus sp. at 0.1 molar concentration of NaHCO3. Introduction Microalgae are aquatic unicellular organisms in both fresh and marine water having unique potential to synthesize valuable natural products (Plaza et al., 2009; Plaza et al., 2010). In recent years, lipid storage in microalgal cells turns the focus on biodiesel production, Triglycerides present in storage lipid used as a source for biodiesel and as a sustainable alternative for petroleum fuels (Mata et al,. 2010). Microalgae promises to be a suitable and efficient energy resource that can convert atmospheric carbon dioxide into biomass, fatty acid and lipids. Microalgae are low cost effective to culture, characterized by rapid growth and high biomass production, which utilizes bicarbonate as the external source of carbon for photosynthesis (Munoz and Beer, 1994). Few algae are capable of uptake of carbon dioxide directly (Raven, 1991) while others convert bicarbonate to carbon dioxide either inside the Plasmalemma (Dixon et al., 1987) or externally allowing only bicarbonate to diffuse into the cell (Badger et al., 1980). Microalgae can fix CO2 from different sources which can be categorized as CO2 from atmosphere, CO2 from industrial flue gases, and fixed CO2 in the form of soluble carbonates (NaHCO3/Na2CO3). Carbon dioxide (CO2) could be directly fed into microalgae culture for biofixation but sometimes it is difficult to obtain a stable and consistent supply of CO2 unless the location of microalgae cultivation system is very close to a factory or a power plant. The major objective of this study was to optimize the growth rate of three different microalgal strains in different molar concentrations of NaHCO3 to compare BBM and BG11 media, and analysis of biochemical composition from obtained microalgal biomass. Materials and Methods Three green microalgae, Scenedesmus sp., Botryococcus sp., and Euglena sp., were selected for this experiments. The strains were obtained from Culture Collection of Presidency College, Chennai, India. The aforementioned strains were explored in terms of Growth and biochemical study in different molar concentration of NaHCO3 in two different medium (BBM and BG11). BBM and BG11 culture media were selected and prepared for the growth of microalgae strains, in which BG11 consists of carbonate source in it. Then different molar concentrations (0.1, 0.5, 1.0, 1.5, and 2.0) of NaHCO3 salt was weighed and allowed to dissolve in BBM and BG11 media for carbon source and Control were prepared having normal BBM and BG11 without any strain inoculation. The pH was adjusted to 7.5 in each of the flasks using pH tutor meter (Eutech instruments). Light condition For our analysis, white inflorescence lamps (Crompton 40W, Cool day light 6500 K) at an intensity of 2000 lux in a 12:12 light and dark regime were employed and the temperature was adjusted to 25°C for all the flasks. Analytical method The Optical Density of the three microalgal cultures Scenedesmus sp. Botryococcus sp. and Euglena sp. were measured at regular interval time of 24 hrs by absorbance at 680 nm using spectrophotometer (Hitachi U-2900) till 15th day. At the end of the experiment, all the culture flasks were centrifuged and filtered. The pellet obtained biomass were oven dried, weighed and stored for further biochemical analysis. Biochemical composition and analysis on microalgal biomass Total Protein, Carbohydrate and Lipid was analyzed from biomass, based on Lowry et al., 1951, Dubois et al., 1956 and Ronald, 2001 methods respectively. Results Effect of Bicarbonate tolerance and utilization Three microalgae strains Scenedesmus sp. Botryococcus sp. and Euglena sp. were investigated against different molar concentrations of NaHCO3 in their media. Primary and suitable media for microalgae BG11 and BBM were selected and analyzed for this study. The three microalgal strains Scenedesmus sp. Botryococcus sp. and Euglena sp. showed simultaneous growth initially on both the media, but the rate was deviated after further growth based on the utilization and tolerance of sodium bicarbonate on each organism. The biomass and growth rate was higher at the 12th and 13th day after inoculation. Scenedesmus sp. showed slightly deviated growth rate in both the media but the growth rate was higher at 0.1 M concentration of NaHCO3 in both BBM and BG11 media when compared to other strains. Botryococcus sp. gained growth rate at the same molar concentration as in