Bacterioplankton community structure: Protists control net production and the proportion of active bacteria in a coastal marine community

A series of dialysis experiments was performed to study the relative importance of substrate limitation and grazing in controlling the proportion of active cells of coastal marine bacterioplankton. The grazer community was manipulated by filling dialysis bags with unfiltered water and water serially passed through 150-, 40-, and 0.8-μm pore-size filters. The total number of bacteria, the number of metabolically active cells, bacterial loss rates, and the abundances of heterotrophic nanoflagellates were measured immediately and at 3 and 6 d. Gross growth rates were similar in all treatments, suggesting that ambient nutrient concentrations set an upper limit to the maximum growth rates, whereas grazing determined the net growth rates and the final number of bacteria. Bacterial loss rates, measured as the disappearance of fluorescently labeled minicells, correlated well with the initial density of heterotrophic nanoflagellates in the different treatments. The number of active cells at the end of the experiments varied widely among treatments and reached 2.0 x 10⁶ ml^-1, or over 55% of the total final density in dialysis bags, with little or no grazing by nanoflagellates. The final proportion of active cells was negatively correlated to both the loss rates and the initial nanoflagellate density, and it was estimated that grazing rates on metabolically active bacteria were four or more times higher than those on inactive bacteria. Heterotrophic nanoflagellates thus seemed to control bacterial density by skimming newly growing cells rather than by cropping the standing stock of bacteria.