High-cell-density fermentation of microalgae can be coupled with photoautotrophic culture mode to produce a variety of algal bio-products including oils. However, fermentation, a costly and energy-intensive process, will become economically feasible only if ultra-high cell density can be achieved.

Recently, in a study published in Biotechnology and Bioengineering, the researchers from Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences, Biotechnology Investment Co. under State Development & Investment Corp.,Ltd., and Jinan University, invented an ultra-high cell density heterotrophic cultivation technology of the unicellular green microalga Scenedesmus acuminatus.

By optimizing cultural conditions and employing a sophisticated substrate feed control strategy involving stepwise constant feeding and simultaneously maintaining a low-concentration glucose, the researchers obtained an ultra-high-cell-density of 286 and 283.5 g L^-1 in 7.5 L bench-scale and 1,000 L pilot-scale fermenters, respectively.

Besides, they found that heterotrophically-grown S. acuminatus cells are more productive in terms of both biomass and lipid accumulation when they are inoculated in outdoor pilot-scale photobioreactors for lipid production as compared to the cells originally grown under photoautotrophic conditions.

Further techno-economic analysis based on the pilot-scale data indicated the cost of heterotrophic culture of microalgae for biomass production is comparable with that of the open pond system and much lower than that of tubular PBR, if the biomass yield was higher than 200 g L^-1.

This study demonstrated the economic viability of heterotrophic cultivation on large-scale microalgal inocula production, and showed great commercial application potential of this ultra-high cell density cultivation technology in microalgae resources development.

This work was supported by grants from State Development & Investment Corporation of China, Natural Science Foundation of Hubei Province and the Chinese Academy of Sciences.

Scale-up of heterotrophic culture of S. acuminatus in 1, 000 L fermentor with the optimal cultural conditions. a: Time courses of DO and stirring speed. b: Change of biomass and glucose concentrations with time. (Image by IHB)