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 i mportance of substrate Limitation and grazing in controlling the propo rtion of active cells of coastal marine bacterioplankton. The grazer c ommunity was manipulated by filling dialysis bags with unfiltered wate r and water serially passed through 150-, 40-, and 0.8-mu m pore-size filters. The total number of bacteria, the number of metabolically act ive cells, bacterial loss rates, and the abundances of heterotrophic n anoflagellates were measured immediately and at 3 and 6 d. Gross growt h rates were similar in all treatments, suggesting that ambient nutrie nt concentrations set an upper limit to the maximum growth rates, wher eas grazing determined the net growth rates and the final number of ba cteria. Bacterial loss rates, measured as the disappearance of fluores cently 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 trea tments and reached 2.0 x 10(6) 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 bo th the loss rates and the initial nanoflagellate density, and it was e stimated that grazing rates on metabolically active bacteria were four or more times higher than those on inactive bacteria. Heterotrophic n anoflagellates thus seemed to control bacterial density by skimming ne wly growing cells rather than by cropping the standing stock of bacter ia.