Enhanced inorganic carbon uptake by Chlorella sp IMMTCC-2 under autotrophic conditions for lipid production and CO2 sequestration

To achieve sustainable production of biofuel from microalgae, a well-optimized and sustained biomass production is prerequisite. The major factor determining the higher productivity of algae is the availability and uptake of CO2 for biomass growth. In this study, an improved CO2 sequestration method leading to improved biomass yields has been investigated. The ability of OH- ions in fixing dissolved CO2 in form of HCO (3) (-) in algal growth medium was studied using a Chlorella sp. and scaled-up in a photobioreactor. It was observed that a critical concentration of 0.005 M OH- is required for HCO (3) (-) formation and utilization by algae. HCO (3) (-) uptake was enhanced by 70.8% (in presence of 0.01 M NaOH) with a sixfold increase in growth rate compared with only CO2 system. In mineral carbon systems such as NaHCO3 and Na2CO3, increase in HCO (3) (-) uptake was enhanced by 65.4% and 63.4%, respectively. The maximum rate of CO2 fixation of 6.6 mg L-1 h(-1) was obtained with 0.01 M NaOH which was 1.5 times compared with mineral carbon sources. The biomass from scale-up experiment contained 16.3% lipid (by weight) of which 75% is unsaturated fatty acids (in total lipids). This supports the idea that fixing the dissolved CO2 in the form of bicarbonate using alkali helps in increased biomass productivity rather than CO2 itself, forms a precursor for biodiesel, and increases CO2 sequestration in a cyclic process.