Enhancement of biomass productivity and biochemical composition of alkaliphilic microalgae by mixotrophic cultivation using cheese whey for biofuel production

Abstract

The growth of microalgae under alkaline conditions ensures an ample supply of CO₂ from the atmosphere, with a low risk of crashing due to contamination and predators. The present study investigated the mixotrophic cultivation of two alkaliphilic microalgae (Tetradesmus obliquus and Cyanothece sp.) using cheese whey as an organic carbon source. The variation in cheese whey concentration (0.5-4.5% (v/v)), culture pH (7-11), and NaNO₃ concentrations (0-2 gL^-1) was evaluated using central composite design in response to biomass productivity and the contents of lipids, total proteins, and soluble carbohydrates. Both investigated microalgae effectively utilized cheese whey as an organic carbon source. The optimum conditions for simultaneously maximizing biomass and lipid productivity in T. obliquus were 3.5% (v/v) whey, pH 10.0, and 0.5 g L^-1 NaNO₃. Under these conditions, the biomass, lipid, soluble carbohydrate, and protein productivities were 48.69, 20.64, 7.02, and 10.97 mg L^-1 day^-1, respectively. Meanwhile, Cyanothece produced 52.78, 11.42, 4.31, and 7.89 mg L^-1 day^-1 of biomass, lipid, carbohydrate, and protein, respectively, at 4.5% (v/v) whey, pH 9.0, and 1.0 g L^-1 NaNO₃. The lipids produced under these conditions were rich in saturated fatty acids (FAs) and monounsaturated FAs, with no polyunsaturated FAs in both microalgae. Moreover, several biodiesel characteristics were estimated, and results fell within the ranges specified by international standards. These findings indicate that the mixotrophic cultivation of alkaliphilic microalgae could open new avenues for promoting microalgae productivity through low-cost biofuel production.