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The growth characteristics, biochemical composition and ultrastructure of a novel starchless mutant of Chlorella pyrenoidosa, designated as STL-PI, are compared to the same characteristics of its parental strain, C. pyrenoidosa 82T. The STL-PI mutant had a 22 ± 5% higher growth rate, and 24.5 ± 4.2% more protein than the parental strain, 82T. Furthermore, the STL-PI mutant accumulated 20.4% more polyunsaturated fatty acids and 18% less saturated fatty acids than the parental 82T. When the parental 82T was cultured in a nitrogen-free media, their starch content increased from 6.8 ± 2.8% to 22.5 ± 3.1%. In contrast, the STL-PI mutant produced no starch, regardless of the growth conditions. Instead, the mutant cells responded to nitrogen limitation by further increasing their lipid content from 25.2 ± 1.2% to 38.0 ± 2.3% per dry weight. Transmission electron micrography revealed that nitrogen limitation typically stimulates the formation of starch granules in the chloroplast of 82T cells. Yet no starch granules were observed in the STL-PI cells. Instead, only the formation of large lipid globules was observed in the mutant cells. These results demonstrate that the starchless mutant STL-P1 possesses novel physiological and phytochemical properties distinct from the 82T cells: their cells were deficient in starch synthesis and showed higher growth rates and productivity than 82T cells. [ABSTRACT FROM AUTHOR]