Optimization of seawater-based triacylglycerol accumulation in a freshwater green alga, Chlorella kessleri, through simultaneous imposition of lowered-temperature and enhanced-light intensity

Triacylglycerols (TG), the major components of crop oils used for the production of food oils and biodiesel fuel, accumulate in algal species under single environmental stress conditions including nitrogen-starvation. In a freshwater green alga, Chlorella kessleri, its TG content reached 24.7% in dry cell weight in 72 h, on cultivation in 3-fold diluted seawater (1/3 SW), through the synergetic stimulating effects of hyperosmosis and nutrient-limitation. These observations would indicate the importance of combinatory stresses for strong induction of TG accumulation [Hirai et al. 2016]. We here searched for novel stresses in C. kessleri to elevate the seawater-based TG accumulation level through their additional actions. A higher light intensity (moderate light, ML) caused a small, but definite increase of TG, whereas a lowered growth temperature (LT) led to a large, but temporal increase of it until 48 h, followed by severe degradation. The decrease of TG in LT cells was abolished on simultaneous imposition of ML. C. kessleri cells were then cultured in 1/3 SW under combinatory conditions of ML/LT, the TG content consequently increasing to as high as 48.5% in dry cells. These results indicated the validity of combinatory stresses for strong induction of algal TG accumulation. As to the fatty acid composition of TG, C. kessleri was intermediate between palm and the other three major oil crops including rapeseed. The rapid and abundant accumulation of TG with fatty acid composition suitable for industrial use demonstrated the great potential of development of the industrial algal TG production system with the use of the combinatory stresses in C. kessleri. TG of C. kessleri was also characteristic to contain C18 acids as representing 81 mol% of its constituent fatty acids or 90% of fatty acids at its sn-2 position, which suggested a peculiar TG synthetic pathway in this oleaginous alga.