Optimizing microalgae cultivation in tofu wastewater for sustainable resource recovery: the impact of salicylic acid on growth and astaxanthin production

This study focuses on the utilization of tofu wastewater for cultivation of mixed microalgae culture, namely Chlorella vulgaris and Nannochloropsis oculata, in a 55-L open raceway pond. Salicylic acid (SA) was added with concentration of 0, 20, and 200 µM on day 5 to induce astaxanthin production as value-added compound from microalgae biomass. The results indicated that the mixed culture of C. vulgaris and N. oculata, supplemented with 20 µM SA, exhibited optimal growth, characterized by a specific growth rate of 0.66/day, biomass gain of 0.83 g/L, biomass productivity of 0.12 g/L day, and a chlorophyll-a level of 6.38 mg/L. Moreover, Nannochloropsis oculata dominated the microalgae population by the end of cultivation period. Values of pH during cultivation increased from 9.08 to 10.22 due to photosynthetic activity of microalgae cells. The addition of 20 µM SA also yielded the highest astaxanthin level at 0.30 mg/g (w/w), indicating rapid production of astaxanthin within 7 days of cultivation period in tofu wastewater. According to this study, the use of tofu wastewater as a culture medium with SA addition is expected to increase sustainable and cost-effective production of microalgae biomass, which is a vital resource for a variety of important chemicals such as astaxanthin.