Comparative physicochemical evaluation of Ca-, Mg-, and Na-alginate from tropical Sargassum spp. for pre-formulation of oral fish vaccine carriers

Oral vaccination in aquaculture remains constrained by antigen degradation and inefficient delivery within the gastrointestinal tract. Alginate-based biomaterials have emerged as promising candidates for antigen encapsulation due to their biocompatibility and gel-forming properties. In this study, alginate extracted from tropical Sargassum spp. was converted into calcium (Ca), magnesium (Mg), and sodium (Na)-alginate and comparatively evaluated as potential carriers for a formalin-killed Vibrio harveyi vaccine.The alginate extracts were characterised using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), and thermogravimetric analysis (TGA). Vaccine premixes were prepared and analysed using SEM and transmission electron microscopy (TEM) to assess structural organisation and antigen integrity. All alginate variants exhibited characteristic polysaccharide functional groups and high compositional purity. Ca-alginate formed dense and compact matrices indicative of strong ionic crosslinking, whereas Na-alginate displayed weak structural cohesion and high residual salts. Mg-alginate exhibited a homogeneous and moderately crosslinked network with enhanced structural flexibility and an intermediate porosity. Encapsulation preserved the morphology of formalin-killed V. harveyi across all matrices.

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