Direct utilization of starch for heterotrophic cultivation of Chlorella pyrenoidosa by co-culture with immobilized Saccharomycopsis fibuligera

Heterotrophic culturing of microalgae is limited by the high cost of the medium due to the addition of an extra carbon source. Starch is a natural, abundant, cheap, and renewable glucose polymer. Algal cells are incapable of hydrolyzing starch under heterotrophic conditions. To solve this issue, a co-culture system was developed using a mixed culture of Chlorella pyrenoidosa and immobilized Saccharomycopsis fibuligera. The latter is an amylolytic yeast that secretes amylase and hydrolyzes starch into glucose and other oligosaccharides after being immobilized. The co-cultured C. pyrenoidosa efficiently utilized the accumulated available sugar for its growth, and the highest algal biomass concentration reached 3.6 g/L, higher than that achieved by C. pyrenoidosa grown on an equivalent amount of glucose. The lipid content of these algal cells cultured in medium with starch was as high as that achieved when cultured in a medium with glucose. In addition, pure algal cells were able to be well separated from this system as yeast cells were immobilized on beads and could be easily separated by simple filtration. This strategy provides a new approach that allows microalgae to heterotrophically utilize starch, which will help reduce the carbon source cost in heterotrophic cultivation of microalgae.