Microalgae are rich in natural antioxidants, with occurrence and quality depending primarily on the species of microalgae and the conditions of cultivation. Light and photoperiod affect microalgae accumulation of antioxidants. This study examined the growth, biomass, and antioxidant responses of microalgae Chlorella vulgaris (UMT-M1), Isochrysis galbana (CB), and Tetraselmis chuii (CT) grown in 30 ppt F/2-enriched seawater and exposed to 12:12 and 24:0 hours light/dark cycles at 24°C ± 2°C. Overall cell density, wet biomass, chlorophyll a, chlorophyll b, carotenoids, and ascorbic acid content of UMT-M1, CB, and CT increased upon continuous light exposure for 8 days (p < 0.05). Constant light exposure induced α-tocopherol accumulation in UMT-M1 and CT microalgae but not CB. Results indicated that as a response to continuous light treatment, UMT-M1, CB, and CT may exert natural antioxidant protection mechanism.