Effects of pH on cell growth, lipid production and CO₂ addition of microalgae Chlorella sorokiniana

Microalgae have emerged as one of the most promising alternative energy feedstocks. Some advantages include the simple cellular structure, short production cycle, high lipid content, and fast growth. However, high production costs due to high CO₂ usage and low lipid productivity have been some of the major challenges impeding the commercial production of algal biodiesel. Here, cell growth and lipid content of Chlorella sorokiniana DOE1412 were first evaluated at different pH in flask cultivation. Culture pH was manipulated by CO₂ addition. The optimal pH for DOE1412 is approximately 6.0 when only accounting for cell growth and lipid production and not considering the CO₂ efficiency. A flat panel airlift photobioreactor (PBR) was used for scale-up cultivation at five different pH levels (6.5, 7, 7.5, 8 and 8.5). Data of pH values and CO₂ addition was collected by a data logger. Biomass productivity increased with decreasing pH. By taking into account not only the cell growth and lipid production but also CO₂ addition, the lowest value of CO₂ addition was achieved at pH 8 (2.01 g CO₂/g biomass). The fatty acid profiles and biodiesel properties, such as iodine value (IV), saponification value (SV), cetane number (CN), degree of unsaturation (DU), long-chain saturated factor (LCSF), and cold filter plugging point (CFPP), were determined as a function of pH. CN of biodiesel produced at pH 6.5, 7 and 7.5 satisfied the US standard ASTM D6751; among them, the pH 6.5 products met the European standard EN 14214. Finally, protein content in microalgal biomass increased with increasing pH, while C/N ratio in cells decreased.