Fermentation of Macroalgae and Halophytes for Animal Feed: Barriers, Bioconversion, Postbiotics, Safety, and Evidence Gaps—A Critical Review

Marine macroalgae and terrestrial halophytes represent a structurally distinct class of feed resources that can be produced without the arable land, fresh water, and synthetic fertilizer inputs required by conventional protein crops. Their direct incorporation into animal diets is, however, constrained by a convergent set of physical and toxicological barriers—viscous polysaccharides, lignocellulosic recalcitrance, anti-nutritional secondary metabolites, and accumulated heavy metals and halogens—that collectively impede practical use. Microbial fermentation can transform some of these barriers into functional resources through enzymatic deconstruction, catabolite conversion, and biosorption—a duality described here as the Barrier–Resource Duality. This critical narrative review synthesizes findings from publications across fermentation microbiology, animal nutrition, food safety, life cycle assessment, and regulatory science. The review analyzes cell-wall architectures of Chlorophyta, Rhodophyta, and Ochrophyta alongside the lignocellulosic matrix of terrestrial halophytes, maps the requisite carbohydrate-active enzyme families and polysaccharide-utilization loci, and evaluates the postbiotic mechanisms—TLR/NF-κB, histone deacetylase inhibition, and metabolite-mediated immunomodulation—by which fermentation-derived compounds may exert host benefit. The evidence base is assessed through a critical narrative synthesis that identifies knowledge gaps across five areas: (i) the absence of co-fermentation studies combining seaweeds and halophytes despite predicted substrate complementarity; (ii) the lack of pilot-scale fermentation data (no study exceeds 500 L); (iii) the single-study dependency of core in vivo efficacy claims; (iv) the absence of validated analytical methods for postbiotic quantification; and (v) the lack of a dedicated regulatory category for fermented marine biomass. It concludes that the field, while mechanistically rich, remains at Technology Readiness Level 2 (technology concept formulated), and that coordinated pilot-scale consortia, multi-institutional replication initiatives, and regulatory pathway design are prerequisites for commercial translation.