- The responses to PAR intensity and nitrogen deficiency have been investigated in the Δ5-desaturase-deficient mutant (P127) of the microalga Parietochloris incisa (Reisigl) Shin Watan. (Chlorophyta, Trebouxiophyceae). The mutant accumulates dihomo-γ-linolenic acid (DGLA, C20:3 ω6) instead of arachidonic acid (C20:4 ω6) characteristic of the wildtype. The growth, fatty acid and pigment composition, and light absorption by P127 cell suspensions were studied for the first time during cultivation on complete and N-free BG-11 medium at 35, 130, and 270 μE · m−2 · s−1. On complete medium under high irradiance, an increase in biomass was observed, and total fatty acid (TFA) and DGLA contents were higher than in N-starving cultures. A distinct irradiance-dependent rise in carotenoid-to-chl ratio was recorded in P127 due to an increase in carotenoids (on complete medium) or by a decline in chl (on N-free medium). Cultivation under high and medium irradiances caused a decline in light-harvesting xanthophylls and an increase in β-carotene, localized predominantly in cytoplasmic oil bodies (OB). The P127 mutant, similar to wildtype, responded to the stresses by coordinated induction of fatty acid and carotenoid syntheses, but displayed the same magnitude of the response as was observed in wildtype under 30% lower irradiance. The changes in optical properties of the P127 cultures tightly correlated with their pigment composition, and hence with fatty acid content, making it possible to develop a nondestructive technique for the assay of TFA and DGLA. The peculiarities of the stress responses in the wildtype and the mutant are discussed.