The macroscopic gel phase (mucilage) appears episodically in the northern Adriatic Sea covering tens of square kilometers of sea surface. Current views leave no doubt on phytoplankton production as a proximal source of polymers constituting the gel network, but the mechanism leading to its rapid production remains unknown. We introduced electrochemical sensing of marine microparticles and atomic force microscopy to image supramolecular organization of marine gel network. Our biophysical scenario of mucilage event features self-organization of biopolymers into microparticles ("marine vesicles") that under specific conditions transform to giant gel by a fast vesicle-to-gel transition.