Modified Natural Seawater as Growth Medium for Halotolerant Cyanobacterium Aphanothece halophytica to Increase Lipid Content for Biodiesel Production

Biodiesel derived from cyanobacterial oils becomes attractive as an efficient renewable energy. The present study aims to optimize growth and lipid production of halotolerant unicellular cyanobacterium Aphanothece halophytica cultivated in natural seawater. In this study, A . halophytica was able to grow in natural seawater when supplemented with low concentration of NaNO ₃ , whereas no growth occurred without supplementation. The specific growth rate of 0.230 day ^− 1 and cell concentration of 25.17 x 10 ⁶ cells mL ^− 1 were achieved in A. halophytica cultivated in natural seawater supplemented with 17.6 mM NaNO ₃ and Turk Island salt solution (suitable natural seawater; SNSW) for 14 days. This growth rate was comparable to that of cells grown in normal BG11 plus Turk Island salt solution. The lipid content and fatty acid profiles of A. halophytica varied with changes in NaCl concentrations. The highest lipid content of 50.47% and lipid productivity of 48.33 mg L ^− 1 day ^− 1 were obtained in cultures supplemented with 1.89 mmol C-atom L ^− 1 glucose and 0.75 M NaCl. The optimal medium pH and cultivation temperature for lipid production was 7.5 and 25–35°C, respectively. When cultivating A. halophytica in optimized SNSW with various NaCl concentrations, the highest contents of linoleic and linolenic acids, and the lowest contents of palmitic, stearic, and oleic acids were observed with 0.75 M NaCl. In contrast, cultures grown in optimized SNSW with 0.5 M NaCl showed fatty acid methyl ester profiles rich in monounsaturated fatty acids, which are favorable for high-quality biodiesel production.