Preliminary Study Of Microalgae Harvesting Technology Based On Bioflocculation Effect Of Edible Fungi

Microalgae,broadly defined as prokaryotic cyanobacteria and eukaryotic unicellular algae,are photosynthetic microorganisms capable of rapid growth using light energy and inorganic nutrients for a wide range of applications in biofuels,feed,food,medicine,and carbon capture.However,several natural properties of microalgae,including low concentration suspension distribution during growth,tiny cell size(<30μm diameter)and electrostatic repulsion between cells,make it challenging to separate microalgae from the medium.Filamentous fungi can grow in spherical pellets and assist in immobilizing microalgal cells using entangled hyphae,which is an effective form for harvesting.Cultivating filamentous fungi and interacting with microalgae to form fungal algal cell clusters(flocs)is a new way to harvest microalgae.Most of the existing fungi used in microalgal bioflocculation studies in the literature are inedible or have harmful components.Thus the downstream application scenarios of the harvested microalgae are greatly limited and not suitable for human or animal consumption,etc.In this study,Ganoderma lucidum and Chlorella sp.were selected as research objects to explore the microalgae harvesting technology based on the bioflocculation effect of edible fungi.To optimize the combination mode and culture conditions of microalgae and fungi,the filamentous fungus Ganoderma lucidum was used to harvest microalgae efficiently,and the harvested microalgae-fungus biomass was further used as raw materials for food,medicine and feed.A method for quantification of non-sporulating fungi was investigated.Unlike the counting and quantification methods for sporulating fungi,non-sporulating fungi need to be explored for quantification to meet the need for reproducibility and operability and to facilitate further studies.The optimization of conditions and mechanistic study of edible fungi flocculating microalgae was explored to find ways to improve flocculation efficiency.In this paper,using edible fungi to flocculate harvested microalgae can not only reduce the flocculation cost and improve the recovery efficiency,but more importantly,the products are non-toxic and harmless,and can be used in food,medicine,health products,feed,etc.,with richer application scenarios and higher industrialization value.The specific research results are as follows.(1)The edible fungus Ganoderma lucidum was selected as the fungus for flocculating microalgae.During the experiment,no spores were observed under the microscope,so the preparation of liquid strains was proposed,and the reaction of fungal biomass led to the optimal preparation time of 7 d for the primary strain culture and 7 d for the secondary strain culture.Because the dry weight of the liquid strain was difficult to measure precisely,it was found that the linear correlation between the fungal oxygen consumption and dry weight reached an R~2 of 0.99 or more,which can be used as a substitute indicator for the rapid determination of biomass.(2)When the speed of liquid incubation was decreased from 100 rpm to 50 rpm,the morphology of the fungi changed from spheres to irregular shapes and loose mycelial structures,however,the bioflocculation efficiency increased significantly from 13%(100 rpm,3 h)to 22%(50 rpm,3 h).Increasing the incubation time from3 d to 6 d also showed no significant change in morphology,while the flocculation efficiency further increased to nearly 40%.Thus,morphological changes may be responsible for the significant differences in harvesting efficiency.The highest harvesting efficiency of 39.7%was obtained during fungal-assisted bioflocculation with a fungal addition of 40 mg/m L,a temperature of 23°C and a speed of 100 rpm.After the flocculation of fungi and microalgae,the total lipid content decreased,the polysaccharide content increased and the protein content was average,suggesting that the flocculation process might involve the transformation of lipid metabolites into polysaccharides.Through economic feasibility analysis,it was found that the key constraint of edible mushroom-assisted microalgae harvesting is the production cost of the fungi themselves,and the rational use of this technology requires further exploration of the application and added value of microalgae-fungal biomass mixtures.(3)The zeta potential,scanning electron microscopy(FE-SEM),Fourier infrared(FT-IR),and three-dimensional fluorescence(3D-EEM)measurements of fungi,microalgae,and microalgae-fungus biomass showed that the fine structure of fungi formed under different culture conditions was significantly different,which may be the key to determine the difference in flocculation efficiency;The alkaline environment of the microalgal medium may have enhanced the fungal aggregates and microalgal cells between interaction,either by enhancing the dispersion of fungi or by widening the potential difference between fungal and microalgal particles;That metabolite enzymes,polysaccharides and organic acids secreted by filamentous fungi play a key role in the interaction of algal fungi;and that the content of humic compounds increases significantly during flocculation,and that small amounts of soluble microbial products(e.g.polysaccharides,proteins,etc.)are released.