BACTERIAL LIPOLYTIC-ACTIVITY IN A HYPERTROPHIC DEAD ARM OF THE RIVER DANUBE IN VIENNA

The kinetic parameters of lipase, bacterial secondary production (BSP) and bacterial numbers (BN) were determined fortnightly during the dev elopment of the summer phytoplankton bloom at two stations of Alte Don au, a hypertrophic stagnant dead arm of the river Danube in Vienna. Un til the middle of August we observed a gradual increase in lipase acti vity as well as BN and BSP rates to the maximum of 19.9 nmol 1(-1) h(- 1), 4.5 x 10(9) cells 1(-1) and 8.1 mu g C 1(-1) h(-1), respectively. At the end of August and during September we found a marked decrease i n all bacterial parameters, coinciding with a progressing increase of chlorophyll a concentrations at both sampling sites. The maximal value s of lipase V-max were determined in the bottom water layer (avg. 13.7 +/- 6.5 nmol 1(-1) h(-1)) probably owing to the predominating importa nce of polymeric matter in the substrate pool for microheterotrophs in this water zone. Differential filtration experiments showed that 20.1 % to 56.3% of the total lipase activity and 4.2% to 9.0% of the total bacterial numbers in Alte Donau water samples occurred in 0.2-mu m fil trate. Further experiments indicated that the high contribution to lip ase activity in the 0.2-mu m filtrate was rather due to the presence o f 0.2 mu m filterable bacteria than to soluble enzyme molecules. Moreo ver, we observed higher bacterial lipase activity in the 0.2 mu m filt rate than in unfiltered samples. The possible influence of limiting fa ctors on the metabolism of bl situ bacteria is discussed.