No abstract

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.

No abstract

Bacterial abundance and activity were followed during a phytoplankton bloom in the high Arctic, the Franz-Joseph Land archipelago (80" to 82" N, 45' to 65" E), in July 1995. At the beginning of July the sea was entirely covered by ice; at the end of July the ice coverage was about 50% Water temperature varied between -1 and -0.6"C and salinity between 32 and 35%. Phytoplankton cells exhibited photoinhibition even during incubation periods when maximum radiation was <400 pE m" S-', indicating adaption to a low radiation level. Phytoplankton biomass, averaged over all 5 sampling sites, was 2.2 pg chl a 1-'; primary production, at the 50% radiation level, was 27.3 pg C 1 -I d-l and mean bacterial abundance was 3.6 X 10' ml-'. Mean bacterial production as measured by thymidine incorporation was 2.46 pg C I-' d-' while for leucine incorporation it was 5.46 pg C 1-' d-' Total organic carbon varied over a narrow range (0.81 to 1.12 mg I-'). Pronounced spatial variations in microbial parameters between waters surrounding different islands were detectable. At Hayes Island phytoplankton biomass and production decreased within 3 wk while bacterial abundance and activity measured as thyrnidine and leucine incorporation increased. During this period the percentage of bacterial (as measured by leucine incorporation) to primary production increased from about 3% to more than 95% and the calculated total organic carbon turnover decreased from 258 to 28 d.

The microspatial distribution of marine bacterioplankton was investigated by comparing the bacterial abundance in 100‐nl subsamples of natural seawater. For sampling, staining with DAPI, and filtering, we designed capillary microtips that allowed pipetting of 100 nl with a sampling error <10%. Highly significant correlations were obtained between the 100‐nl method and the conventional direct‐counting method. Frequency distributions of bacterial cells were determined in experiments with natural seawater during bloom conditions as well as with seawater enriched with diatom cells from cultures of Chaetoceros muelleri. Bacteria in 100‐nl subsamples were normally distributed in each single experiment and in the Formalin‐fixed control. Mean bacterial abundance in subsamples containing at least one phytoplankton cell was never significantly different from samples lacking algal cells. Based on our results we conclude that bacteria are homogeneously distributed in subsample volumes of 100 nl, and therefore we could not verify the current concept that bacteria may cluster around phytoplankton cells.