Different methods for extracting bacteria from freshwater sediment and a simple method to measure bacterial production in sediment samples

The efficiency of different treatments was tested to extract bacterial cell s from freshwater sediment samples. The influence of sonication, density gr adient centrifugation, fixation by formalin and centrifugation speed on bac terial recovery was investigated. The method developed by Smith and Azam [M ar. Microb. Food Webs 6 (1992) 107] to measure microbial activity on bacter ioplankton (H-3-leucine incorporation), was also evaluated in sediment samp les. After 1 min of sonication bacterial abundance was reduced by about 47% in diluted sediments with tetrasodium pyrophosphate. With the addition of Percoll after sonication, bacterial counts were not significantly different (P < 0.05). Fixation by formalin increased bacterial counts using sonicati on. However, higher bacterial abundance was estimated in non-sonicated samp les. Bacterial abundance in samples centrifuged at 7000 X g with and withou t Percoll was not significantly different (P < 0.05). Highest bacterial abu ndance was obtained after centrifugation at low speed (750 X g). Bacterial abundance decreased with higher centrifugation speed (750, 1500 and 3000 X g), the difference, however, was not significant. Bacterial production rang ed from 0.10 mu g C cm(-3) d(-1) in autoclaved sediment to 0.27 mu g C cm(- 3) d(-1) in untreated sediment. The radioactivity measured in controls of b oth untreated and autoclaved sediment was high (70 and 91%, respectively), indicating a high level of leucine adsorption in sediment particles. In con trast, radioactivity in control samples previously centrifuged was markedly lower (6%). Despite the high values of radioactivity in the controls, bact erial production in untreated sediment was significantly higher than in cen trifuged sediment (P < 0.05). (C) 2000 Elsevier Science B.V. All rights res erved.