Simplifying Fungal-Assisted Microalgal Harvesting: Bioflocculation Without pH Adjustment, Temperature Control and or Agitation

Microalgae are increasingly recognized as versatile platforms for producing valuable products for human and animal health, biofuels, and bio-inputs. However, biomass harvesting remains a major economic, energy, and environmental bottleneck for microalgal biorefineries. Here, we introduce a simplified fungal-assisted bioflocculation approach that enables microalgal biomass recovery at room temperature without pH adjustment or agitation. A wild Desmodesmus heteracanthus was cultivated in BG11 medium and treated urban wastewater, and harvested using pre-cultured pellets of three Aspergillus species at fungi–microalgae ratios of 10%, 15%, and 20%, without modifying culture conditions. D. heteracanthus showed substantial biomass accumulation in BG11 medium, reaching a 1,353% increase, whereas growth in treated wastewater was limited to 152%, likely due to nutrient constraints and competition with indigenous microorganisms. Fungal inoculum ratio had limited influence on harvesting efficiency, indicating that lower fungal inputs can achieve comparable biomass recovery. In previous tests at a 10% ratio, A. brasiliensis showed the highest performance, removing 94% of biomass, decreasing to 84.7% and 75.5% of microalgal biomass from BG11 and wastewater cultures, respectively, when the tests scaled up. By eliminating pH adjustment and agitation, the simplified process is proposed to reduce the energy and chemical demand for harvesting compared with conventional approaches. These findings demonstrate that fungal bioflocculation can provide an effective, low-energy, and chemically independent alternative for microalgal harvesting, supporting more economically and environmentally sustainable biorefineries within a circular bioeconomy.