Episodes of massive mucilage formation in the northern Adriatic Sea have been recorded for over a century but their cause is still a matter of conjecture and debate. It is generally thought that mucilage forms due to copious polysaccharide exudation by phosphorus limited algae. In this paper we develop the thesis that bacteria play major roles in mucilage formation. We argue that mucilage is largely produced as a consequence of bacteria-organic matter interactions and bacterial capsular polysaccharide synthesis. Ectohydrolytic enzymes of bacteria are critical in producing long-lived polysaccharides. Further, bacteria cause efficient P regeneration, particularly intensely in microscale features e.g. phycospheres, detritus and aggregates. Bacteria thus help sustain high rates of primary production despite vanishingly low levels of phosphorus in the bulk phase seawater. We integrate these roles of bacteria into a conceptual model which emphasizes microscale interactions of microbes within a seawater gel matrix as the basis for a mechanistic understanding of the accumulation of long-lived polysaccharide to form mucilage.