The disrupted loci in the two isolated starchless mutants are distinct and each mutant has a unique phenotype . In the sta6 mutant, the small, catalytic subunit of ADP-glucose pyrophosphorylase (AGPase-SS) is disrupted , and this mutant accumulates less than 1% of the starch observed in wild-type (WT) cells under conditions of nitrogen deprivation. The sta7 mutant contains a disrupted isoamylase gene and also has severely attenuated levels of starch, but it accumulates a soluble glycogen-like product . In this study, we conducted an examination of the unique physiological acclimations that are utilized by these mutants to adapt to the loss of starch synthesis. As the genetic lesions in these two mutants are distinct and block starch synthesis via two very different mechanisms, we investigated the physiological consequences of starch inhibition in both of these mutants from a holistic bioenergy perspective, which included photosynthetic parameters and the overall yields of lipids and carbohydrates, the two primary bioenergy carriers in C. reinhardtii. Specifically, we examined whether the inability to synthesize starch would result in the accumulation of additional lipid, alter cellular growth or cell size, affect acetate utilization, and/or influence photosynthetic O2 evolution. Our data indicate that both the sta6 (BAFJ5) and sta7 (sta7-10) mutants accumulate more lipid than the CC124 control during nitrogen deprivation. However, the additional lipid does not completely offset the loss of starch synthesis from a complete energetic perspective. Increased lipid accumulation during nitrogen stress has also been reported for a variety of starch mutants in recent papers . A significant feature in both of the starchless mutants studied here is that O2 evolution and acetate utilization are diminished during nitrogen stress, which is undesirable from an overall bioenergy perspective. Remarkably, complementation of sta7-10 with genomic DNA encoding the wild-type isoamylase gene resulted in cells that were larger than those of the sta6, sta7-10, and CC124 strains, exhibited the highest total lipid levels during nitrogen deprivation, and overaccumulated starch even in nutrient-replete medium.