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Abstract Background Current efforts on the optimization of the two-stage cultivation using stress-induced lipid accumulation have mostly focused only on the lipid induction stage. Although recent studies have shown that stress-induced lipid accumulation is affected by the physiological status of the cells harvested at the preceding cultivation stage, this issue has hardly been examined hitherto. Such a study needs to be carried out in a systematic way in order to induce lipid accumulation in a consistent and predictable manner with regard for variances seen at the cultivation stage. Results After a photoautotrophic cultivation of Chlorella sorokiniana HS1 in a modified BG11, harvested cells were re-suspended in the fresh medium, and then NaCl was added as the sole stress inducer with light illumination to induce additional accumulation of lipid. Effects of culture temperature on the lipid accumulation were analyzed by the Kruskal–Wallis test. From the microscopic observation, we had observed a definite increase in lipid body induced by the stress since the cell entered a stationary phase. A multiple linear regression model was developed so as to identify significant parameters to be included for the estimation of lipid induction. As a result, several key parameters at the end of cultivation, such as cell weight, total lipid content, chlorophyll a in a cell, and Fv/Fm, were identified as the important proxy variables for the cell’s physiological status, and the modeling accuracy was achieved by 87.6%. In particular, the variables related to Fv/Fm were shown to have the largest influence, accounting for 65.7% of the total variance, and the Fv/Fm had an optimal point of maximum induction at below its average. Clustering analysis using the K-means algorithm indicated that the algae which are 0.15 pg cell−1 or less in chlorophyll concentration, regardless of other conditions, had achieved high induction results. Conclusion Experimental results showed that it usually achieves high lipid induction after the cells naturally end their division and begin to synthesize lipid. The amount of lipid induction could be estimated by the selected proxy variables, and the estimation method can be adapted according to practical situations such as those with limited measurements.

Increased lipid accumulation of algal cells as a response to environmental stress factors attracted much attention of researchers to incorporate this stress response into industrial algal cultivation process with the aim of enhancing algal lipid productivity. This study applies high-salinity stress condition to a two-phase process in which microalgal cells are initially grown in freshwater medium until late exponential phase and subsequently subjected to high-salinity condition that induces excessive lipid accumulation. Our initial experiment revealed that the concentrated culture of Chlorella sorokiniana HS1 exhibited the intense fluorescence of Nile red at the NaCl concentration of 60 g/L along with 1 g/L of supplemental bicarbonate after 48 h of induction period without significantly compromising cultural integrity. These conditions were further verified with the algal culture grown for 7 days in a 1 L bottle reactor that reached late exponential phase; a 12% increment in the lipid content of harvested biomass was observed upon inducing high lipid accumulation in the concentrated algal culture at the density of 5.0 g DW/L. Although an increase in the sum of carbohydrate and lipid contents of harvested biomass indicated that the external carbon source supplemented during the induction period increased overall carbon assimilation, a decrease in carbohydrate content suggested the potential reallocation of cellular carbon that promoted lipid droplet formation under high-salinity stress. These results thus emphasize that the two-phase process can be successfully implemented to enhance algal lipid productivity by incorporating high-salinity stress conditions into the pre-concentrated sedimentation ponds of industrial algal production system. [ABSTRACT FROM AUTHOR]