Algal Biomass and Algal-Derived Preparations in Livestock Nutrition: Gut Microbiota Modulation, Host Molecular Pathways, and Bioprocess Innovations

Algal biomass and algal-derived preparations have attracted increasing interest in livestock nutrition because of their potential to modulate the gut microbiota, which plays a crucial role in animal health, immunity, and growth performance. This review focuses primarily on microalgae and cyanobacterial preparations, while also considering selected macroalgae relevant to livestock feeding, without restricting its scope to marine sources. In this narrative review, we summarize recent advances in the physicochemical properties and biological activities of key algal components, such as polysaccharides, polyunsaturated fatty acids, and carotenoids. We assess evidence for changes in potentially beneficial bacteria, including Lactobacillus and Bifidobacterium, and for inhibitory effects against selected pathogenic bacteria, including Salmonella and pathogenic strains of Escherichia coli. We also examine proposed links between these compounds, microbial short-chain fatty acid and bile acid metabolism, and host immune and growth responses. This review discusses practical applications of algal biomass and algal-derived preparations in livestock production, with a focus on inclusion levels and safety assessments. Effects are evaluated in relation to the source organism, preparation, and animal model, rather than assumed to be shared across algal groups. Current challenges and future research directions are also identified to facilitate the efficient utilization of algal resources in sustainable animal husbandry. This review identifies the evidence supporting specific feeding applications and distinguishes observed animal responses from proposed mechanisms.