COUPLING BETWEEN RATES OF BACTERIAL PRODUCTION AND THE ABUNDANCE OF METABOLICALLY ACTIVE BACTERIA IN LAKES, ENUMERATED USING CTC REDUCTION AND FLOW-CYTOMETRY

In natural bacterioplankton assemblages, only a fraction of the total cell count is active, and, therefore, rates of bacterial production sh ould be more strongly correlated to the number of active cells than to the total number of bacteria. However, this hypothesis has seldom bee n tested. Herein we explore the relationship between rates of bacteria l production (measured as leucine uptake) and the number of active bac teria in 14 lakes in southern Quebec. Active bacteria are defined as t hose cells capable of reducing the tetrazolium salt CTC to its fluores cent formazan; these cells were enumerated using now cytometry. Bacter ial production varied two orders of magnitude in the lakes studied, as did the number of active bacteria, whereas the total number of bacter ia varied by only sixfold. The number and proportion of active bacteri a were similar among lake strata, but rates of bacterial production we re highest in the epilimnion and lo west in the hypolimnion. As expect ed, bacterial production was better correlated to the number of active cells, and bacterial growth rates calculated for active cells ranged from 0.7 to 1.8 day(-1), on average threefold higher than those calcul ated on the basis of total bacterial abundance. Growth rates scaled to active cells were, on average, similar among lake strata and did not show any pattern along a gradient of increasing chlorophyll concentrat ion, so there was no systematic change of bacterial growth rates with lake productivity. In contrast, growth rates scaled to the entire bact erial assemblage were positively correlated to chlorophyll, were tenfo ld more variable among lakes than growth rates of active cells, and sh owed larger differences among lake strata. Scaling bacterial productio n to either the total number or the number of active cells thus result s in very different patterns in bacterial growth rates among aquatic s ystems.