At 24 stations in the Weser Estuary and the German Bight the Most Probable Numbers (MPN/g dry wt. sediment) of nitrate-dissimilating (= denitrifying) and of nitrate plus nitrite-dissimilating bacteria were recorded. The numbers of nitrite-dissimilating bacteria, i. e. denitrifiers not capable of reducing nitrate to nitrite, were calculated by subtraction of the MPN for nitrate-dissimilating from the MPN of nitrate plus nitrite-dissimilating bacteria. By determining the percentages of these bacteria in relation to the number of the heterotrophs, the ecological importance of denitrification, especially the nitrite dissimilation, was estimated. The results showed the MPN of nitrate-dissimilating bacteria to be in the range of 0-156 (up to 0.8 % of heterotrophic bacteria). An exception was the sediment of one station with a MPN of 1849, or 5.2 % of the heterotrophs. The amounts of nitrite-dissimilating bacteria were between 0 and 2352 (up to 13 % of heterotrophic bacteria). In the estuary the numbers of nitrate-dissimilating and of nitrite-dissimilating bacteria showed a decreasing tendency with distance from Bremerhaven. The highest numbers were found in the Weser off Bremerhaven and also at 3 stations in the German Bight, south of the Isle of Helgoland.

The basal medium consisted of Bacto Nutrient Broth (DIFCO) with the addition of 17.6 g/l NaCl. This medium, abbreviated as NB in the parameter comment, was used to determine the Most Probable Numbers (MPN) of heterotrophic bacteria, whereas the addition of 0.5 % KN03 (NB +KN03 ) or 0.2 % KN02 (NB + KN02 ) made possible the records of nitrate-dissimilating and of nitrate plus nitrite-dissimilating bacteria, respectively. The MPN obtained with a fourth medium containing NB, 0.5 % KN03 and 0.2 % KN02 (NB + KN03 + KN02 ) were compared with the MPN from NB + KN02. The media were filled in 9 ml portions in test tubes and in each test tube a Durham tube was inserted for the detection of gas formation. The media were sterilized at 121 °C for 20 minutes in an autoclave.

Supplement to: Tan, Tjhing Lok; Rüger, Hans-Jürgen (1979): Denitrifiers in sediments of the Weser Estuary and the German Bight: Densities of nitrate-dissimilating and nitrite-dissimilating bacteria. Veröffentlichungen des Instituts für Meeresforschung Bremerhaven, 17, 189-197