<p>This paper reports the development of innovative magnetic-sensitive biopolymer composites and the subsequent investigation of their rheological properties in relation to in situ optical studies of microstructures. Positively charged iron oxide magnetic nanoparticles (IONP) that had been chemically functionalised by grafting 3-Aminopropylphosphonic acid molecules onto their surfaces were mixed in an entangled aqueous solution of sodium alginate chains. Steady shear flow and viscoelastic measurements were then performed on the resulting nanocomposites using a home-made magneto-opto-rheological device. The increase of low shear viscosity and the linear viscoelastic moduli as the magnitude of the magnetic field increased was clearly demonstrated. This is explained by electrostatic interactions between -NH₃⁺ and -COO⁻ groups at the surface of IONP and polymer chains, respectively. The resulting microstructure, which depends on both the shear rate and the magnetic field amplitude, was observed for the first time using in situ optical microscopy and deeply analysed.</p> Graphical Abstract <p></p>