Thus, starch synthesis inhibition increases antioxidant nonstarch carbohydrates and unsaturated fatty acids, indicating that non-starch carbohydrate synthesis competes with starch and lipid synthesis for carbon precursors. This study elucidates the metabolic relationship between carbohydrates and lipids in microalgae and expands the applications of starchless microalgal mutants via non-starch carbohydrate and unsaturated fatty acid production.