Current Trends, Future Prospects and Constraints of Whole Microalgae and Their Fractions as a Functional Feed Ingredient for Animals.

Microalgae are a highly diverse group of unicellular organisms that grow in a wide range of aquatic environments and are widely used as dietary supplements for both human and animal applications. Microalgae are rich in lipids, proteins, carbohydrates and other valuable bioactive components such as pigments, antioxidants and vitamins. Those components have shown bioactivity not only by affecting cell, organ and tissue functionality but also have notable antimicrobial and immunomodulatory properties, positioning microalgae as a potential natural antibiotic substitute. Although production costs of microalgae are high, it has been shown that relatively low (< 1%) inclusion levels of microalgae in the diet of animals affect physiological functions and performance. Microalgae can be fed as whole biomass but also as a fraction (the lipid, protein, carbohydrate or rest fraction). Feedings a fraction of microalgae may be beneficial when only a specific bioactivity of a fraction is required in animals, thereby reducing the cost of feed. For instance, when microalgae are fractionated for human applications, the resulting byproducts or 'side-stream fractions', present a cost-effective feed alternative for livestock. In addition, feeding microalgae or their fractions during periods when young animals are more susceptible to health issues can not only enhance cost-effectiveness but also potentially support their recovery. The aims of this review are (i) to present an overview of the mode of action of the lipid, protein and carbohydrate (rest) fractions of microalgae on whole body physiology, (ii) to summarize previous research on the bioactivity of dietary fractions of microalgae in livestock production and (iii) to propose novel strategies to use whole microalgae biomass or their fractions as functional feed to support resilience in young growing animals during vulnerable health episodes.