A symbiotic yeast to enhance heterotrophic and mixotrophic cultivation ofChlorella pyrenoidosausing sucrose as the carbon source

Heterotrophic or mixotrophic culture of microalgae is feasible alternative approach to avoid light limitation in autotrophic culture. However, only a few kinds of organic carbon sources are available for algal culture. Disaccharides, such as sucrose, are difficult to be utilized by microalgae under both heterotrophic and mixotrophic conditions. In this study, a symbiotic yeast was accidentally found in a contaminated algal suspension. The symbiotic yeast was isolated and identified asCryptococcussp. This yeast was able to extracellularly hydrolyze sucrose and accumulated monosaccharides in the medium. It can enhance algal growth using sucrose as the carbon source at both heterotrophic and mixotrophic modes when mix-cultured withChlorella pyrenoidosa. The highest algal cell density of 118.8 x 10(6)and 151.2 x 10(6)cells/mL was achieved with a final algal percentage of 83.5 and 93.2% at heterotrophic and mixotrophic culture, respectively. Furthermore, the protein and lipid content was significantly enhanced by mix-cultureC. pyrenoidosawithCryptococcusYZU-1. The fatty acid accumulated in this co-culture system was suitable for the production of biodiesel. This symbiotic yeast solved the problem thatC. pyrenoidosacannot heterotrophically or mixotrophically utilize sucrose. A high algae density was obtained and the protein and lipid accumulation were also significantly enhanced. This study provided a novel approach for production of protein or lipid-rich biomass using sucrose or sucrose-rich wastes as the carbon source.