Growth and Production of Lipids in Raphidocelis subcapitata Immobilized in Sodium Alginate Beads

The growth and production of lipids in the green microalga Raphidocelis subcapitata immobilized in alginate gel are studied. The beads are made from alginate (2% w/v) and CaCl2 (1% w/v). The dry weight, the concentration of cells, and the lipid content are determined after dissolution of the beads in a sodium phosphate buffer. The results show that variations in biomass do not reflect variations in the number of cells in R. subcapitata. Cells divide more rapidly (G(c) = 3.45 +/- 0.3 days) than biomass is produced (G(m) = 4.1 +/- 0.4 days) during the exponential growth phase. Therefore, the average mass of the immobilized cells decreases until it reaches its minimum at the end of the exponential phase. Thus, during the stationary phase, cell division ceases while biomass production continues, and the average mass of the immobilized cells increases. In the present study, it is shown that this increase is due to the accumulation of lipids following the depletion of nitrates and phosphates in the culture medium. A lipid content of 24.7 +/- 2.5% (dcw) and a lipid productivity of LP = 29.8 +/- 3.0 mg/L/day are recorded at the end of culture. These results suggest that immobilized R. subcapitata has promising potential for biodiesel production.