The impact of ultraviolet-B (UV-B) radiation on bactenal density and production and on extracellular enzymatic activity was investigated the northern Adriatic Sea. Samples were incubated in quartz bottles and exposed to natural solar radiation (0.5 W m-2) as well as to artificial UV-B (0.4 W m-2) sources. Exposure to artificial UV-B sources over a penod of 12 h revealed a constant decline in bactenal density to about 60% of the corresponding dark value. Total lipase and leucine-aminopeptidase activity showed a decrease to 38.8 and 21.9%, respectively, of the dark control; dissolved leucinearninopeptidase activity was significantly more affected (15.3 % of the corresponding dark value) than hssolved lipase activity (43.2 % of the corresponding dark value). Samples exposed for 6 h to artificial UV-B or for 4 h to natural solar radiation exhibited rapid recovery during subsequent dark incubation Following UV-B exposure (0.4 W m-2) bactenal density recovered rapldly from 74.6% to 84.1 %, lipase activity recovered from 64 % to 80% and leucine-aminopeptidase activlty from 53% to 7 1 % of the corresponding dark values dunng 6 h of subsequent dark incubation. Recovery of bacteria followng exposure to natural solar UV-B radiation with similar intensity was even higher. In these experiments bacterial density reached smllar values as in the dark control, bacterial production even exceeded the dark control production rates after 6 h of dark incubation following UV-B exposure. This difference might be attributed to photorepair induced by UV-A and to increased availability of dissolved organic matter due to UV-B mediated photolysis. UV-B radiation levels of 0 4 W n1r2 as used in this study are detectable in the surface layers of the northern Adnatic Sea up to 0.5 m depth for at least 3 to 5 h d -' during summer Thus our results suggest that microbial life might be affected by UV-B radiation and consequently also the carbon and energy flow in aquatic systems.

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.

The integrity of the intracellular structures and the presence and dimension of the capsular envelope were investigated in marine snow-associated and marine free-living bacteria by transmission electron microscopy and special fixation techniques. Three categories depending on the presence of internal structures were differentiated. In marine snow, 51% of the marine snow-associated bacterial community was considered intact, 26% had a partly degraded internal structure, and 23% were empty with only the cell wall remaining. For the free-living bacterial community, 34% were intact cells, 42% exhibited damage, and 24% of the cells were lacking any internal structure. We also investigated the morphology and the extent of the bacterial capsular envelope. More than 95% of all intact marine snow-associated bacteria were surrounded by a capsule while Ϸ55% of empty marine snow-associated bacteria had no capsule. For free-living bacteria, Ϸ65% of the intact cells had a capsule while Ϸ80% of the empty free-living bacteria lacked a capsule. Thus there is a clear trend from intact cells which are commonly surrounded by a capsular envelope to empty bacteria for which only the cell wall is remaining. Since bacterioplankton represent the largest living surface in the ocean, it is concluded that the release of intracellular material from bacteria into the environment as well as the release of extracellular capsular material might fuel the dissolved organic matter pool of the ocean.

Bacterial transformation of leucine into high molecular weight dissolved organic matter (HMW-DOM) material was studied in seawater cultures. Three different types of cultures were used: sodium azide poisoned cultures served as control, eukaryotic inhibitors to prevent bacterivory, and untreated cultures were used to determine the effect of grazing on bactena on the release of HMW-DOM. All these cultures were labeled wlth I4C-leucine and the distribution of incorporated label into particulate organic matter (POM) and I4C recovered in dissolved organic matter (DOM) was measured over a period of 10 d as well as the transformation into HMW-DOM. Fractionation of DOM was performed using dialysis membranes (molecular weight cutoff 50 000 daltons) and by gel filtration for substances with a molecular weight below l500 daltons. There was a shift to HMW-DOM discernible in grazed and ungrazed cultures and this DOM fraction accumulated to ca 6 % of the initially added leucine in the grazed culture within 8 d. Flagellates appeared to have only a minor influence on the formation of HMW-DOM but might be important in consuming bacterially derived HMW-DOM, representing a novel, nonpredatory way of carbon and energy transfer from bacteria to flagellates.