Highly efficient bioflocculation of microalgae using Mucor circinelloides

Harvesting of microalgae is the major challenge in cost-efficient large-scale microalgal biomass production due to their low concentration and small cell size in the culture medium. The present paper aimed to study the efficiency of the filamentous fungus Mucor circinelloides spores suspensions to harvest the green unicellular microalga Chlorella pyrenoidosa grown in synthetic medium. Results showed that the optimal co-culture conditions were pH=6.0, 1.25 g·L^-1 glucose and 1∶250 fungi to microalgae ratio with harvest efficiency of 91.08%. In addition, the mentioned optimal conditions could be applied for actual sewage with harvest efficiency of 92.33%. Polysaccharide concentrations measured before and after 48 h of cultivation showed that the polysaccharide of C. pyrenoidosa cultured alone was increased by 0.047 g·L^-1, while co-cultured mixture showed increase in polysaccharides by 0.019 g·L^-1. The recorded decrease in polysaccharides concentration in the co-culture might be attributed to using of excreted polysaccharides by M. circinelloides to grow, confirming the symbiotic association of both organisms. Furthermore, with decreasing the pH, C. pyrenoidosa Zeta potential was stable, while it was increased from -37.7 mV to -9.87 mV in M. circinelloides, which indicated that charge neutralization was the mechanism of flocculation between algae and fungi.