IMPACT OF PRESSURE ON BACTERIAL-ACTIVITY IN WATER COLUMNS SITUATED ATTHE EUROPEAN CONTINENTAL-MARGIN

The vertical profiles of bacterial abundance and activity were measure d in water columns situated at the European continental margin of the Celtic Sea in July 1993 and January 1994. The stations lay in the midd le of a submarine canyon (depth: 3713 m) and at the base of the contin ental slope (depth: 4480 m). The bacterial numbers were counted using epifluorescence microscopy, the cell multiplication was determined as H-3-thymidine uptake, the accumulation of proteinaceous biomass was me asured as H-3-leucine uptake, and the impact of pressure on bacterial activity was studied in experiments under pressure conditions between 1 and 450 atm. In upper water layers or in July, the ratio of leucine uptake compared to thymidine uptake was generally higher than in deep layers or in January. The populations of all water layers generally pr eferred their in situ pressure, especially in July, while in January t he barotolerance of the upper communities in 10 to 150 m water depth i ncreased. A comparison of per-cell activity under in situ pressure sho wed that cells close to the bottom occasionally possessed higher speci fic activities than in the euphotic water layer. These findings indica te that the performance of experiments under elevated pressure is high ly recommended, if the detection of phenomena such as those shown here in the deep sea is wanted.