Abstract
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.
Highlights
Microalgal biomass is a prominent source for a wide variety of high-value products such as food and beverages, supplements, cosmetics, and nutraceuticals (Biorizon Biotech 2018)
Liquid suspensions were made by cutting ~ 2 cm2 from the outer edge of the fungal mass, suspending in 40 mL of Yeast Malt Broth (YMB) (NRRL Medium No 6 w/o agar) in 250 mL culture flasks and incubating at 24 °C until they were in the exponential growth phase
P. ostreatus was chosen as the reference organism for microalgal harvesting method development due to its fast growth rate (Egede et al 2016; Bellettini et al 2019), and proven ability to form pellets and harvest algae (Luo et al 2019)
Summary
Microalgal biomass is a prominent source for a wide variety of high-value products such as food and beverages, supplements, cosmetics, and nutraceuticals (Biorizon Biotech 2018). Traditional microalgal harvesting methods fall into one of three categories of separation methods: mechanical, physical, or chemical (Barros et al 2015; Fasaei et al 2018). The most efficient of these methods are the mechanical separation methods of centrifugation and filtration which can harvest up to 99% of the microalgae. These methods have high operating costs (Barros et al 2015; Fasaei et al 2018; Najjar and Abu-Shamleh 2020). The commonly used physical methods, flotation and sedimentation, are not as efficient as centrifugation or filtration so are usually combined with chemical flocculation employing metal salts or
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