Some observations on the copper tolerance of bacterial communities determined by the (3H)-thymidine incorporation method in heavy metal polluted humus

Changes in pH after filtration of bacterial suspensions are important when applying the radioactive thymidine incorporation method to heavy-metal polluted soils with low microbial activity. In the original method (Bååth 1992; Soil Biology and Biochemistry 24, 1157–1165) the blended and centrifuged suspension was filtered through glass wool to remove humus particles from the suspension. When we filtered the bacterial suspension through glass wool the pH increased by 2 units and the thymidine incorporation rate decreased. This made the community copper tolerance measurement ambiguous. When using soil samples with very low activity, we recommend the use of acid-washed glass wool or polyester net filtration which eliminates changes in pH.