Enhancement of Chlorella vulgaris cell density: Shake flask and bench-top photobioreactor studies to identify and control limiting factors

Low cell density is a major bottleneck in any microalgal bioprocess that prevents the large scale exploitation of this potential bioresource from commercialization of commodities like biofuels. Control of factors limiting growth is the key to enhancing cell density. Factors limiting photoautotrophic growth of C. vulgaris were identified and controlled to a possible extent. Limiting CO2-transfer rate, light attenuation, scarcity of nutrients, and high pH compounded to retard growth gradually in the basal medium. Analysis of the maximum feasible CO2 mass-transfer rate and CO2 fixation rates enabled the assessment of CO2-limited growth without on-line estimation of dissolved CO2. Growth (1.4x10(8) cells mL(-1), 12.6 g dry wt L-1) was extensively enhanced when limiting factors were staved in a customized 250mL stirred-tank photobioreactor. Scaling the culture 8 times with constant k (L) a (volumetric mass-transfer coefficient) and Re (i) (impeller Reynolds number) resulted in reduction of biomass titer by 80% because of light attenuation.