Interference of starch accumulation in microalgal cell growth measurement

Starch accumulation in microalgae is a subject of growing interest in the energy sector since this polysaccharide can be used as feedstock to produce ethanol and biogas. Microalgal cell growth dynamics are commonly monitored by measuring the optical density (OD) of the culture, which then is converted to either biomass dry weight or cell concentration with the aid of standard curves. However, starch accumulation causes changes in the cell morphology, which may impair the use of OD for the measurement of cell growth. Therefore, this study investigated the correlation of OD to cell concentration and dry weight during the cultivation of Chlorella sorokiniana, a starch-accumulating green microalga. A linear correlation between OD and cell concentration was observed only during the exponential and the decelerating growth phases. The OD increase during the stationary phase was related to starch accumulation rather than to cell growth. Similarly, a lack of correlation between OD and biomass dry weight occurred during the stationary growth phase, as the increase of dry weight, due to the pronounced starch accumulation, was greater than the OD changes. Thus, this study shows that the use of OD to measure microalgal cell growth in starch-accumulating cultures can be misleading. The direct measurement of cell concentration and dry weight is advisable for monitoring the full cell growth cycle and accurately calculating biomass productivity.