Photosynthetic carbon partitioning into starch and neutral lipid was investigated in the oleaginous green microalga Pseudochlorococcum sp. When grown under low light and nitrogen-replete conditions, the algal cells possessed a basal level of starch. When grown under high light and nitrogen-limited conditions, starch synthesis was transiently up-regulated. After nitrogen depletion, starch content decreased while neutral lipid rapidly increased to 52.1% of cell dry weight, with a maximum neutral lipid productivity of 0.35 g L(-1) D(-1). These results suggest that Pseudochlorococcum used starch as a primary carbon and energy storage product. As nitrogen was depleted for an extended period of time, cells shift the carbon partitioning into neutral lipid as a secondary storage product. Partial inhibition of starch synthesis and degradation enzymes resulted in a decrease in neutral lipid content, indicating that conversion of starch to neutral lipid may contribute to overall neutral lipid accumulation. Biotechnological application of Pseudochlorococcum sp. as a production strain for biofuel was assessed. (C) 2010 Elsevier Ltd. All rights reserved.