Growth and reproduction of marine algae are primarily controlled by light, temperature, nutrients, water movement, and salinity. Light quality and quantity supplied by sun- or artificial light is one of the most important parameters for phototrophic organisms, as it is required for photosynthesis and the regulation of several cellular processes.Salt stress is a major abiotic environmental factor that limits plant growth and productivity. Since this environmental factors affect on the growth, production and biochemical compositions in microalgae, therefore, the combined effects of salinity and photoperiod on the growth, specific growth rate, doubling time ( Dt ), biomass, chlorophyll and carotenoids ofsea algae Tetraselmis tetrathele were investigated. The experiment was carried out as completely randomized design at three photoperiods (3L:3D, 4L:4D and 12L:12D) and five salinity levels (20, 25, 30, 35 and 40 g/L) for 23 days. Results showed that the increasing of photoperiod and salinity was significantly (p 0.05) increased density, specific growth rate and the biomass. The maximum densities at T. tetrathele were 3.6×10 7 ±520 (mean±SE), 5.1×10 7 ±5000, 4.8×10 7 ±2036, 6.2×10 7 ±2428 and 5.7×10 7 ±2155 cells/ml in combination of 12L:12Dlight/dark photoperiod at 20, 25, 30, 35 and 40 g/L sea water, respectively. However, the densities of in 3L:3D light/dark photoperiod at 20, 25, 30, 35 and 40 g/L sea water were 5.7×10 7 ±4918, 3.01×10 7 ±1376, 8.7×10 7 ±2809, 11.2×10 7 ±2602 and 4.6×10 7 ±1749 cells/ml, respectively. the densities of 3.7×10 7 ±10180, 3.4×10 7 ±629, 7.8×10 7 ±901, 5.7×10 7 ±3031 and 3.1×10 7 ±1952cells/ml were obtained at 4L:4D light/dark photoperiod at 20, 25, 30, 35 and 40 g/L sea water respectively. In addition, the mean specific growth rate and doubling time were ranged 0.029-0.44 day and 1.24-32.9 days, respectively. Maximum chlorophyll a and b were 22.3±0.11 mg/L and 35.4±0.43 mg/L, respectively, at 3L:3D and 25 g/L. The Maximum biomass and total carotenoids was 1.4±0.58 mg/L and 4.8±0.23 mg/L at 4L:4D and 40 g/L, respectively.The results indicated that increasing salinity and photoperiod in the cultivation of microalgae T. tetrathele improve growth, biomass and reproduction. Keywords :Microalgae, Salinity, Photoperiod, Growth, Biomass,Chlorophyll, Tetraselmis tetrathele
