A direct viable counting method for measuring tolerance of aquatic microbial communities to Hg2+

Direct counts of Hg2+ resistant cells in natural waters were obtained by fluorescence microscopy after incubation for 20 h in the presence of a growth substrate, nalidixic acid to halt cell division, and inhibiting concentrations of Hg2+. This method discriminated Hg2+ resistant from Hg2+ sensitive Escherichia coli strains. Estuarine samples were used to compare this procedure with two other toxicity measurements that determine the effect of Hg2+ on colony growth and on rates of radioactive thymidine incorporation into cellular material. Toxicity measurements based on direct viable counts and thymidine incorporation rates had comparable sensitivities, and both were 3–4 orders of magnitude more sensitive than the method that utilized colony counts. Thus, the direct enumeration of Hg2+ resistant cells is useful for predicting the potential of aquatic communities to sustain heterotrophic activity, an essential microbial process, in the presence of Hg2+.