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In the paper 'Coherence of microbial respiration rate and cell-specific bacterial activity in a coastal planktonic community' by Smith (1998), Aquat Microb Ecol 16:2?-35, the author, using the fluorogenic tetrazolium dye 5-cyano-2,3 ditolyl tetrazolium chloride (CTC)-as a measure of cell-specific metabolic activity, and comparing the numbers of CTC reducing cells with respiratory activity, reported that CTC-active (CTC+) cell abundance was highly correlated with respiration rates within the microplankton community, explaining 80% or more of the variations in these rates. This finding is a new and interesting observation which had not previously been reported. However, the fraction of CTC+ cells made up on average only 1 4 % of the total bacterial numbers although a eutrophic site, Chesapeake Bay, was investigated. The author concluded that this low fraction of CTC+ bacteria '...is likely responsible for the bulk of bacterial community metabolic activity...'. On the other hand, the portion of active bacteria has been reported to be 25 to 94% of total bacterial numbers using 2(p-iodo-phenyl)-3-p-(nitropheny1)-5 phenyltetrazolium chloride INT in the same environment (Tabor & Neihof 1984). To explain this discrepancy the less direct coupling of INT to respiratory activity compared to CTC was cited (Smith & McFeters 1996, 199?), necessarily leading '...to lower estimates of that fraction of the total population quantitatively engaged in respiratory metabolism ...' But does this mean that INT-positive bacteria can be neglected with respect to the metabolic activity of bacterial communities? There are, indeed, a number of methods which yield distinctly higher levels of metabolically active bacterial cells compared to the CTC method. In a recent study of

Is the CTC dye technique an adequate approach for estimating active bacterial cells?