Algal-fungal interactions and biomass production in wastewater treatment: Current status and future perspectives

To further the development of green wastewater biotreatments, an in-depth analysis of algal-fungal interactions and high-value biomass production is necessary, as interactions in these cultures are complex. Microalgae can release a remarkable diversity of extracellular metabolites, ranging from alkaloids to fatty acids as well as abundant amino acids and peptides. Those metabolites are associated with algaecidal, antiprotozoal, insecticidal/larvicidal, and antimicrobial activities. Acyl-homoserine lactones (AHLs) can regulate algal-fungal relationships, as they are signaling molecules that significantly influence the metabolism and growth of algae based on the organic matter in their surrounding environment. Fungi can use both organic and inorganic nitrogen (N), but are not as effective as algae; because of this, algal-fungal co-culture is more capable than algal mono-culture in removing N from wastewater. Moreover, the phosphorus removal performance of fungi is comparable to or even superior to that of algae. Algal-fungal symbiotic systems have a high biogas purification capability through the use of photosynthesis. In addition, numerous value-added compounds can be obtained from the co-culture biomass, including fatty acids, polysaccharides, triglycerides, pigments, and vitamins, used as specialty chemicals and bulk commodities in various industrial sectors. Fungi can also act as flocculation agents in a manner that is harmless to algal biomass, allowing recycling of culture medium and easier harvesting, further lowering the overall costs. Compared with mono-culture, considerable increases in biomass and lipid production are observed in co-cultivated algal-fungal pellets. This review aims to discuss metabolic research hotspots in microalgal-fungal consortia, discuss quorum sensing in these cultures, and describe state-of-the-art applications in wastewater treatment and biomass production technologies. Future perspectives on microalgal-fungal consortia in wastewater treatment are also proposed. The results of this review would present a reference for current algal-fungal mechanism research and further large-scale harvesting.