ABSTRACT-Four different approaches to determine the ultrastructure of amorphous aggregates and their principal components are evaluated for t h e~r ability to resolve structural components as small as 1 nm. Based on extensive comparative analysis, the advantages and major drawbacks of the different methods are discussed. Based on experience with marine snow from the Adriatic Sea, it is concluded that only a combination of fixation and preservation treatments ensures optimal ~nterpretation of the results. Specific emphasis is placed on the use of a hydrophilic resin (Nanoplasta) which allows fixation of high]\ hydrated polysaccharide fibrlls In their original configuration and subsequent visualization on a nm scale.

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 fibrilmediated 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 matnx led us to suggest that diatoms (ma~nly 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 in marine snow, influenc~ng the structure and probably the physical properties of the predominantly algae-derived matrix. While 42% of the marine-snow-assoc~ated bacteria exhibited a capsular envelope larger than their respective cell diameters and only 5% were laclung a capsule, only 12 % of free-living bacteria exhibited a fully developed capsule and 37 % were laclung any visible capsule. Thus we conclude that given the high bacterial abundance in marine snow found in earlier studies, the capsular envelope of marine-snow-attached bacteria might significantly influence the physical and chemical structure of the overall polymeric matrix of marine snow.

Both tannic acid and mercuric chloride increased the fragility of these cells slightly. To determine whether or not any of the differences are statistically significant an analysis was made of 44 control values and 65 observations in each of the most concentrated solutions of tannic acid and mercuric chloride. The values in 0•45 per cent sodium chloride were used since the greatest differences were observed in this solution. Using the Student t-test, the