(683a) Understanding the Microalgae Fungal Attraction for a Novel Co-Pelletization Algae Harvesting Process

Algae harvesting using pelletization viathe filamentous fungi represents an innovative approach to address both the cost and sustainability issues in algae biofuel production and also has potential with direct commercial applications. This new process can be applied to microalgae cultures in both autotrophic and heterotrophic conditions to allow microalgae cells and filamentous fungi attach to each other. The mechanism of co-pelletization based algae harvesting is still not fully understood, and the knowledge on the mechanism of co-pelletization has the potential to significantly decrease the processing cost for generating microalgae biofuel or other microalgae based bio-products. The different surface charges of fungi and microalgae could be the main reason for the attraction between fungus and microalgae. The electrostatic properties of the surfaces of both cell types (individually) need to be characterized in order to understand the electrostatic relation between the microalgae and fungal cells. The two (interconnected) concepts that will be important to this study are the zeta-potential and available surface charge (sometimes referred to as the charge demand). The former gauges the strength of the electric field (at the slip plane) emanating from the cell surface while the latter describes the amount of charge available on the surface.

It is widely known that microalgae, Chlorella vulgaris carry a negative charge in their cells and because of this characteristic, flocculation is broadly employed to harvest microalgae by applying a positively charged chemical compound. Based on the idea above, it is hypothesized that under some conditions in the co-culture, the filamentous fungus Aspergillus niger could carry a positive charge in its hyphae so that attraction with microalgal cells occurred due to their opposite charges. In order to support the hypothesis that the surface charges played an important role in the co-pelletization, zeta potential measurements were carried out for the fungal pellets and the microalgal cells under two different conditions that were believed to have a significant effect on the surface charge, namely pH and ionic strength. Four different pH values were applied to the medium (i.e. 3, 5, 7 and 10) in this study. In addition, calcium (Ca²⁺) and magnesium (Mg²⁺) were used to apply ionic strength conditions because of their positive charge. Zeta-potential measurements indicated that (i) both microalgae and fungi have low zeta-potential values regardless of the pH on the bulk (i.e. <-10 mV) (ii) fungi can have a positive electric charge at low pH (ie. pH=3). These values suggest that it might be possible that the degree of repulsion and dispersion between these organisms is low which facilitates the attraction between them.