The high cost and environmental impact of traditional microalgal harvesting methods limit commercialization of microalgal biomass. Fungal bioflocculation of microalgae is a promising low-cost, eco-friendly method but the range of fungal and microalgal species tested to date is narrow. Here, eight non-pathogenic, filamentous fungi were screened for their ability to self-pelletize and flocculate Euglena gracilis (ca.50 µm motile microalga) in suspension. Self-pelletization was tested under various rotational speeds, and species which formed pellets (Ø > 0.5 cm) were selected for harvesting tests. Filaments of each species were combined with E. gracilis at various ratios based on dry weight. Harvesting efficiency was determined by measuring the change in cell counts over time, and settling of the flocs was evaluated by batch settling tests. Three fungal species, Ganoderma lucidum, Pleurotus ostreatus, and Penicillium restrictum, were able to reliably flocculate and harvest 62–75% of the microalgae while leaving it unharmed. The results demonstrated that self-pelletization, harvesting, and settling were dependent on the fungal species. The fungi to algae ratio also had significant but contrasting effects on harvesting and settling. In balancing the needs to both harvest and settle the biomass, the optimal fungi to algae ratio was 1:2. The application of fungal filaments to microalgae in suspension produced readily settling flocs and was less time-consuming than other commonly used methods. This method is especially attractive for harvesting microalgal biomass for low-value products where speed, low cost, and cell integrity is vital.
Food production produces large amounts of nutrient-rich waste streams which are disposed of - a wasted opportunity given nutrients such as phosphorus and nitrogen are in short supply. A more circular approach would be to reuse these waste streams. Fungi and yeasts are possible candidates as they require lots of organic carbon (which is especially high in food production waste streams) for growth, producing value-added biomass. Here, we determined the growth of seven fungi and three yeasts in five different food production waste streams. Initial tests were done to find the most appropriate waste stream for growth. All species were then cultured in the waste stream best suited for growth. Oxygen uptake was measured to gauge metabolic activity and as a proxy for growth rate. Pelletization’s effect on metabolic rates was tested on the most pellet-forming species, by adding agar to inhibit pellet formation. The most promising waste stream for yeast/fungal growth was cheese whey (Whey). Pellet inhibition (i.e., filamentous growth) resulted in increased metabolism in the confectionary bakery waste stream but decreased metabolism in Whey. The best growing species, Geotrichum candidum, has potential commercial value as a producer of enzymes, biochemicals and lipids and could provide added value while improving the circularity of water and nutrients in food production.
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