Two-stage cultivation of the marine microalga Chlorella salina for starch and carbohydrate production

Growing concern and awareness towards environmental issues, such as plastic pollution, have encouraged scientific focus on microalgae as a potential feedstock for thermoplastic starch production. Given their unique ability to utilize wastewater nutrients, microalgae are suitable to be used in wastewater treatment. In first-stage cultivation, Chlorella salina was grown in a Conway medium at 30 °C and exposed to red light under a photoperiod of 24:0 h light–dark cycle to maximize biomass production. The microalgal biomass harvested from the first stage was used as the inoculum for second-stage cultivation. The effects of photoperiod, CO2 concentration, and nutrient limitation were investigated. Cultivation using wastewater was compared with the synthetic medium for starch and carbohydrate accumulation. C. salina cultivated under 12:12 h light–dark cycle significantly accumulated the highest starch and carbohydrate with respective concentrations of 16.769 and 70.850 mg L−1. Under 5% (v/v) CO2, C. salina significantly accumulated starch and carbohydrate with respective concentrations of 13.699 and 58.910 mg L−1. The combination of nitrogen and sulfur limitation significantly triggered the highest starch (30.505 mg L−1) and carbohydrate (145.994 mg L−1) accumulation. Under optimized 5% v/v CO2 supply, the concentrations of starch and carbohydrate accumulated by C. salina using wastewater were 24.971 and 110.756 mg L−1, respectively, which were approximately twice higher compared with the synthetic medium. The usage of wastewater aerated with 5% v/v CO2 would be a more sustainable and economical strategy for high starch and carbohydrate accumulation in C. salina compared with the combination of nitrogen with sulfur limitation.