ALBERT

Description: Organic material entering the oceanic mesopelagic zone may either reenter the euphotic zone or settle into deeper waters. Therefore it is important to know about mechanisms and efficiency of substrate conversion in this water layer. Bacterial biomass, bacteria secondary production (BSP). extra­cellular peptidase activity (EPA) and particulate organic nitrogen (PON) were measured in vertical pro­files of the North Atlantic (46° N 18° W; 57° N 23° W) during the Joint Global Ocean Flux Study (JGOFS) cruise in May 1989. The magnitude of these parameters decreased differently with depth. The strong­est decreases were observed for bacterial production (3H-thymidine incorporation) and peptide turn­over (using the substrate analog leucine-methylcoumarinylamide). Bacterial biomass and peptidase potential activity were not reduced as much in the mesopelagic zone. Peptidase potential per unit cell biomass of mesopelagic bacteria was 2 to 3 times higher than that of bacteria in surface water. Nevertheless bacterial growth at depth was slow, due to slow actual hydrolysis. Values of theoretical PON hydrolysis were calculated from PON measurements and protein hydrolysis rates. These corre­sponded well to bacterial production rates, and the degree of correspondence increased from a factor of 0.63 (PON hydrolysis/ESP) in the mixed surface layer to 0.87 in the mesopelagic zone. Thus we hypothesized an effective coupling between particle hydrolysis and uptake of hydrolysate by bacteria, which depletes the deeper water of easily degradable substrates as hydrolysates usually are. The low enzymatic PON turnover rate of 0.04 d- 1 in the subeuphotic zone suggests that residence time of parti­cles within a depth stratum may be important for its contribution to export. storage and recycling of organic matter.

Description: The responses of natural bacterial populations in the waters of the Kiel Fjord, Germany, and in Lake Kinneret, Israel, to additions of organic substrates were followed by determining changes over 24 h either in direct cell counts or in 3H-thymidine incorporation, and in the Kiel Fjord additionally in 3H-leucine incorporation. In parallel, 1 ym filtered water samples were stored for 3 or 4 d in order to starve the indigenous bacterial populations prior to repeating the substrate addition experiments. Generally, upon substrate addition, relatively higher incorporation of radiotracers was noted in the preincubated samples. Growth response to substrate addition even in starved populations was only significant after 24 h. Incorporation rates of 3H-thymidine and 3H-leucine were more sensitive indicators of bacterial response to substrate additions than cell counts. Continued cell replication in unsupplemented controls, and insignificant increase over time of radiotracer incorporation in most of the fresh samples with added supplements, indicated that the indigenous bacterial populations in Kiel Fjord and Kinneret were apparently not substrate limited. Comparison of actual bacterial production after 24 h (direct counts) to that predicted by 3H-thymidine incorporation after 1 h showed that although reasonably good predictions of daily production were obtained in the unsupplemented samples, this was usually not the case when substrates were added.

Description: The near-bottom water layer is influenced by events in the sediment and by sedimentation from the productive surface layer. Microbial activity in this layer shows occasionally strong gradients from the pycnocline down to the sediment and it reacts to seasonal variations in oxygen content in the adjacent to the sediment. Comparison of mean values of bacterial stock and activity parameters in the productive surface layer and in the near-bottom layer shows, that despite of similar patterns of bacterial biomass and bacterial production in both layers, uptake velocity of leucine, peptidase activity, as well as turnover rates of leucine and hydrolysis rates of peptides are considerably lower in the near-bottom water layer. This is explained by effects of temperature, nutrient quality and oxygen depletion.

Description: During the RV "Meteor" expedition in the Central Arabian Sea (MINDIK 87) the vertical distribution of particulate organic carbon and nitrogen, bacterial abundance, heterotrophic activity for glucose and the bacterial production determined by [3H-methyl]-thymidine incorporation were surveyed. At the time of observation (April-May) the study area was characterized by a stable pycnocline at 35 m and a nutricline at 50 to 55 m depth. Maxima for all biomass measurements were observed in the nutricline, whereas highest rates were detected in the nutrient-depleted surface layer above the nutricline. Based on these hydrographic conditions, a double vertical zonation was established also for microbiological and planktological events in the water column. The integrated bacterial production in the mixed surface layer (0-30 m) was half that in the layer underneath (30-100 m). This upper zone can be considered as a more or less closed sub-system and a significant amount of primary production (approximately 30 %) was channelled through the bacteria. Corresponding to the higher biomass in the lower zone the turnover of organic material was somewhat slower. This observation together with the presence of nutrients in the chemocline indicated a subsystem of more open character for exchange processes with underlying waters.