A Comparison of INT-Formazan Methods for Determining Bacterial Activity in Stream Ecosystems

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Abstract
We investigated different procedures for using a fluorescent stain and dehydrogenase indicator technique for detection of respiring bacteria in two stream ecosystems in the southeastern United States. A modified filter-transfer-freeze (FTF) procedure was useful in detecting iodonitrotetrazolium formazan (INT) crystals in unattached bacteria. This technique did not enhance detection of INT crystals in particle-bound epilithic and epipelic bacteria. No significant differences were found between pre-stained polycarbonate filters mounted with immersion oil and those mounted with glycerol, although we preferred glycerol because of its optical clarity. In contrast to previous studies, we did not find that immersion oil dissolved INT crystals within bacterial cells even after a period of 24 h. Percentages of active bacteria in comparable habitats were not significantly different between a sandstone and a carbonate stream. However, benthic habitats (epilithon and epipelon) had significantly higher percentages of active bacteria (∼15-26%) than overlying stream water (∼2-7%) at all sites on all sampling dates.

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