Ultrastructure of marine snow. II. Microbiological considerations

Marine snow from the northern Adriatic Sea was examined using transmission electron microscopy (TEM) and non-disturbing embedding techniques in combination with ultrathin sectioning to visualize the fibrillar matrix of the snow and of the associated microorganisms at a resolution of ca 1 nm. Despite the high degree of heterogeneity of marine snow, TEM images clearly show fibril-mediated associations between algae, bacteria and embedded organic and inorganic particles. The similarity in morphology between diatom-derived polysaccharides and the dominant fibrils in the marine snow matrix led us to suggest that diatoms (mainly Chaetoceros sp.) were the most important producers of mucilage in the northern Adriatic Sea in summer 1993. Bacteria, however, also produced copious amounts of fibrillar material n marine snow, influencing the structure and probably the physical properties of the predominantly algae-derived matrix. While 42% of the marine-snow-associated bacteria exhibited a capsular envelope larger than their respective cell diameters and only 5% were lacking a capsule, only 12% of free-living bacteria exhibited a fully developed capsule and 37% were lacking any visible capsule. Thus we conclude that given the high bacterial abundance in marine snow found in earlier studies, the capsular envelope at marine-snow-attached bacteria might significantly influence the physical and chemical structure of the overall polymeric matrix of marine snow.