Magnetic nanoparticles are a versatile platform to create functional materials in many fields of research. In particular, they can be functionalized with biocompatible molecules for applications to life sciences. As an example, this strategy has been recently proposed to improve the potential of iron- and zinc- oxides as physiology fostering agents toward plants. In addition, if appropriate polymers are chosen from natural sources, optimally engineered delivery systems can be designed with the possibility of large-scale production while ensuring sustainability. Here we propose to study the high resolution structure and the hydrophobic/electrostatic interactions in composite aggregates obtained by combining iron oxide nanoparticle or zinc-iron oxide nanoparticles and polysaccharides extracted from autotrophic micro-algae or from the rinds of succulent plants