The aim of this study was to investigate the effects of Mg2+ (0-4 mM), and Cu2+ (0-5 mu M) concentrations on biomass and photosynthetic pigment-metabolites, and photosystem II activity in Arthrospira platensis Gomont 1892. The highest biomass was determined on control condition including both Mg2+ (0.8 mM) and Cu2+ (0.5 mu M) as 49.1 +/- 1.6 mg mL(-1). Although the levels of phycocyanin, allophycocyanin and chlorophyll a were varied due to Mg2+ concentration, phycocyanin and allophycocyanin were the major fractions. The highest chlorophyll a level was detected at 0.5 mu M Cu2+. The major carotenoid was beta-carotene, and its highest value was determined in the absence of Cu2+ as 2400 +/- 60 mu g mL(-1). In addition, excessive and insufficient amounts of Mg2+ and Cu2+ caused a decrease in photosystem II activities of A. platensis. This work provides a good example for the production of carotenoids for use in various industries, such as food, cosmetic and pharmaceutical products.