Although production of biodiesels from microalgae is proved to be technically feasible, a commercially viable system has yet to emerge. High‐cell‐density fermentation of microalgae can be coupled with photoautotrophic cultivation to produce oils. In this study, by optimizing culturing conditions and employing a sophisticated substrate feed control strategy, ultrahigh‐cell‐density of 286 and 283.5 g/L was achieved for the unicellular alga Scenedesmus acuminatus grown in 7.5‐L bench‐scale and 1,000‐L pilot‐scale fermenters, respectively. The outdoor scale‐up experiments indicated that heterotrophically grown S. acuminatus cells are more productive in terms of both biomass and lipid accumulation when they are inoculated in photobioreactors for lipid production as compared to the cells originally grown under photoautotrophic conditions. Technoeconomic analysis based on the pilot‐scale data indicated that the cost of heterotrophic cultivation 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. This study demonstrated the economic viability of heterotrophic cultivation on large‐scale microalgal inocula production, but ultrahigh‐productivity fermentation is a prerequisite. Moreover, the advantages of the combined heterotrophic and photoautotrophic cultivation of microalgae for biofuels production were also verified in the pilot‐scale.
Heterotrophic cultivation of Chlorella has achieved commercial success, but the application of Chlorella biomass is still limited due to the high cost of biomass production. In this study, an effective and industrially scalable heterotrphic cultivation technology has been developed for a production strain Chlorella sorokiniana GT‐1. Under the optimized culturing conditions, the ultrahigh biomass concentration of 271 and 247 g L−1 was achieved in 7.5 L bench‐scale and 1000 L pilot‐scale fermenters, respectively. Technoeconomic (TE) analysis indicated that the production cost of C. sorokiniana GT‐1 could be reduced to $1601.27 per ton of biomass if the biomass concentration reached 200 g L−1, which is 24.2% lower than that of the reported highest Chlorella biomass production through fermentation with the same TE model. Under the same growth conditions, the maximum biomass concentration of a low‐starch mutant SLM2 was reduced to 93 g L−1, which was 54% lower than that of the wild type, indicating the capabilities of C. sorokiniana GT‐1 cells in accumulating large amounts of starch are essential for achieving the ultrahigh‐cell‐density under the heterotrophic conditions. In addition, the ultrahigh‐cell‐density growth potential of C. sorokiniana GT‐1 cells was inferred to be related to the intrinsic biological characteristics including the tolerance to low dissolved oxygen and a moderate doubling time under the heterotrophic conditions as well. The breakthrough in cultivation technology is promising for Chlorella industry and would expand its applications in food and feed.
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