OPTIMAL GROWTH CONDITIONS AND ANTIOXIDATIVE ACTIVITIES OF CYLINDROTHECA CLOSTERIUM (BACILLARIOPHYCEAE)1

We isolated the unialgal strain of Cylindotheca closterium (Ehrenb.) Reimann et J. C. Lewin and produced an axenic strain using an antibiotic cocktail of enriched f/2 artificial seawater medium. The optimal growth conditions were estimated under 27 different combinations of temperature, salinity, and nutrients, and mass culture was performed based on the best specific growth conditions. Its antioxidant activities were determined from the extracts of methanol, water, and enzymes (proteases and carbohydrases). The maximum specific growth rate (μ_max) varied from 0.63 to 0.97 · d^?1. The maximum cell density was 7.20 × 10⁴ cells · mL^?1, while the μ_max was 0.82 · d^?1 in culture conditions of 20°C, 30 psu (practical salinity unit), and “F” nutrient concentrations on day 10 of the culture period. The scavenging rates for 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) free radical were 72.5% and 69.4% from Viscozyme and methanol extracts, respectively. The enzymatic extracts of C. closterium prepared by the hydrolyses of Amyloglucosidase (AMG) and Viscozyme showed 45.8% and 45.5% nitric‐oxide‐scavenging rates, slightly lower than the activity of alpha‐tocopherol (α‐tocopherol) but similar to butylated hydroxytoluene (BHT). The extract from methanol and water showed 44.8% and 44.4% scavenging rates, statistically similar with BHT. The metal‐chelating activities of the Kojizyme, Alcalase, methanol, Viscozyme, and Neutrase extracts were 67.1, 53.9, 53.2, 52.1, and 50.2 %, respectively, five to six times higher than the commercial antioxidants. The AMG, Viscozyme, and Neutrase extracts showed a remarkable linoleic acid peroxidation inhibition, which was higher than BHT and statistically similar with α‐tocopherol.