Fungi and Actinobacteria: Alternative Probiotics for Sustainable Aquaculture

Aquatic animals are continually being exposed to stressors under farming conditions, increasing risks to gut health that result in dysbiosis. Since restrictions are imposed on the frequent use of antibiotics in aquaculture, there is emerging demand for economically viable, environmentally safe, and sustainable alternatives for the intensive production of aquaculture species. The application of beneficial microorganisms as probiotics has been suggested and widely practiced in recent times. Bacteria and fungi are ubiquitous microorganisms that can grow in various environments where organic substrates are available. Being rich in nutrients, a fish’s aquatic environment and gastrointestinal tract confer a favorable culture milieu for the microorganisms. However, the colonization and probiotic potential of fungi and mycelial bacteria resembling fungi (actinobacteria), either in the culture environment or within the gastrointestinal tracts of fish, have received less emphasis. Apart from bacilli and lactic acid bacteria, as the most conventionally used probiotics in aquaculture, numerous studies have focused on other promising alternatives. Diverse species of yeasts and molds belonging to the kingdom ‘Fungi’ have been characterized for their prospective roles in nutrition, immunomodulation, and disease prevention in fish. Bioactive compounds such as manno-oligosaccharides and β-glucans are recognized as fungal postbiotics that improve innate immunity and disease resistance in fish. Actinobacteria are known to possess different hydrolytic enzymes and novel secondary metabolites representing their probiotic attributes. The application of these groups in water quality amelioration has also been explored. Thus, this paper presents an overview of the present status of knowledge pertaining to the effects of yeasts (Candida, Cryptococcus, Debaryomyces, Geotrichum, Leucosporidium, Pichia, Rhodosporidium, Rhodotorula, Saccharomyces, Sporidiobolus, Sporobolomyces, Trichosporon and Yarrowialipolytica), molds (Aspergillus spp.) and actinobacteria (Streptomyces) as probiotics in finfish aquaculture, as well as their occurrence within the gastrointestinal tracts of finfish. Furthermore, probiotic mechanisms, selection criteria, and future perspectives on using fungi and actinobacteria as promising probiotics are discussed.