Metabolically active fungus is not always required for fungal-assisted microalgae immobilization

Fungal-assisted immobilization of microalgae involves capturing microalgae cells within a fungal hyphal matrix. The prevailing consensus in the literature suggests that fungal metabolic activity is essential for successful immobilization, which limits the potential applications of inactive fungal pellets. However, evidence from fungal-assisted immobilization of other cell types suggests that inactive fungal pellets may display successful immobilization under certain conditions. Therefore, this study was designed to address whether metabolic activity of fungi is a strict requirement for immobilizing microalgae cells utilizing filamentous fungus Aspergillus awamori to immobilize microalgae cells of Haematococcus pluvialis. The results demonstrated that heat-deactivated (HD) fungal pellets effectively immobilized microalgae and achieved a maximum immobilization efficiency of 62.3 % within 35 h but only under low agitation conditions (75 rpm). In contrast, high agitation power (150 rpm) resulted in a significantly lower immobilization efficiency of only 9.5 %. An investigation of changes in surface charge, bonds, and morphology revealed that a balance between physical entrapment and shear forces were likely key factors driving effective immobilization with heat-deactivated fungal pellets. The findings suggest that while the mechanism of immobilization can involve both physical and biological components, biologically inactive fungal sorbents may be conducive to a wider range of material handling and bioprocessing applications than previously recognized. This study highlights the need for further investigations into the mechanisms of cellular immobilization by metabolically inactive fungi and the economic and environmental implications of this consideration in biomanufacturing systems.