Total phenolic content and <i>in vitro </i>evaluation of antioxidant activity of microbial extract of defatted biomass of mutant <i>Pseudochlorella pringsheimii</i> EMM2

Microalgal biomass is considered to be a renewable source for organic antioxidants. The present study aimed to evaluate the defatted biomass (DB) of the mutated green microalga Pseudochlorella pringsheimii EMM2 for increasing total phenolic content (TPC) extraction and antioxidant activity via microbial fermentation. Three distinct bacterial strains, Bacillus amyloliquefaciens (Accession number: KT276356), B. stearothermophilus (Accession number: KT282130), and B. subtilis (Accession number: KT282131) were obtained from sago industrial wastewater, utilized the hydrolysate from the DB of P. pringsheimii EMM2 as the exclusive medium for fermentation in this work. Among these bacterial strains, B. stearothermophilus was chosen for further investigation due to its effectiveness in enhancing the extraction of TPC and antioxidant activity of the hydrolysate of DB through fermentation. The fermentation conditions were optimized using a classical method, identifying the optimum physical parameters as a pH of 7.0, a temperature of 55 °C, an agitation speed of 150 rpm, and a fermentation time of 40 hours for maximizing the antioxidant activity of the hydrolysate of DB. Under these optimal conditions, a 6.11-fold increase in DPPH radical scavenging activity was observed in the hydrolysate. Consequently, this study demonstrated that the DB of the mutant microalga P. pringsheimii EMM2 is a valuable source for TPC production through fermentation with B. stearothermophilus.