Harvesting the Microalga Chlorella sorokiniana by Fungal-Assisted Pelletization

Currently, the co-cultivation of fungal and microalgal cells is considered a potential technology to minimize harvesting costs and energy inputs in algae-to-biofuel systems. In this study, the cocultivation of five selected filamentous fungi and microalgae Chlorella sorokiniana to form cell pellets was evaluated under different conditions, including organic carbon source, with and without an alternative carbon source, e.g., wheat straw, and initial fungal/microalgae cells ratio for mixotrophic growing. In addition, cellulase activity and zeta potential measurements were assessed to gain a better understanding of the pelletization process. Among five species tested, Aspergillus flavus was the most efficient for pelletizing in nutrient supplemented BG-11, despite the alkaline pH (7.5-8.3) under mixotrophic conditions, aiding in harvesting up to 96% of the microalgal cells (4.2E + 07 cells mL(-1) initial algal concentration) within 48 h. and a supplementation of 5, 10, and 1 g L-1 of fructose, NaNO3, and KH2PO4, respectively. The harvesting efficiency was assessed over time based on biomass (optical density at 750 nm), residual sugar composition, pH, and cellulase activity (2.3 U mL(-1) maximum). The A. flavus isolate studied here shows promise for use in efficient and economically-viable harvesting of microalgae for biofuel production and for the valuable extracellular fungal metabolites.