Microbial Phytases as Functional Feed Additives in Aquaculture: Impact, Challenges, Recent Developments and Future Opportunities

In many developing nations throughout the world, including those in Africa, aquaculture is a veritable source of animal protein consumption. However, the high price of fish meal (FM), which raises the cost of fish feed due to its scarcity and high market prices, may impact the sector’s long-term viability. Replacement of FM with plant proteins (PP) reduces feed cost; however, plants have a poor essential amino acids (AA) profile and a high content of antinutritional factors (ANF), such as phytic acid (myo-inositol hexaphosphate), which limits the nutrient phosphorus’s bioavailability (P). By forming binary and ternary complexes with proteins and vital minerals like calcium (Ca), zinc (Zn), magnesium (Mg) and iron (Fe), phytate, the salt of phytic acid, can hinder fish growth by lowering feed conversion and decreasing nutrient digestibility. However, adding P to the diet to compensate for the mineral deficiency raises feed costs and threatens aquaculture’s sustainability by increasing P loss through eutrophication. Microbial phytase (MP, myo-inositol hexaphosphate phosphohydrolase) cleaves phytate P and increases the proportion of available P to satisfy the dietary P requirement for growth and bone mineralization of farmed fish species; additionally, MP enhances the digestibility of nutrients, reduces their excretion and promotes environmental sustainability in aquaculture. This chapter discusses recent advances in the role of MP in increasing the growth, gut function and antioxidant-immune indices of many aquaculture fish species. The chapter discusses several factors that limit the activity of MP and explores recent trends in improving the functionality of MP in diets, including phytase super-dosing, a multienzyme approach to improve phytase activities, the use of phytase with an organic acid (OA) and the role of phytase-producing probiotic bacteria in low-cost agro-waste fermentation processes. The paper suggested future directions and opportunities to promote genetic engineering of novel phytases to increase their stability and efficacy in fish feed.