Growth rates of natural bacterioplankton assemblages and their relationship to protistan presence

In the natural aquatie environment, baeterioplankton assemblages are subjeet to protistan predatory aetion, whieh is eonsidered a dominant biotie eontrol (WEISSE & STOCKNER 1993, SHERR & SHERR 1994). This aetion is size seleetive in relation to the larger eells (SHERR et al. 1992, HAHN & HOFLE 1999) and has an influenee on the frequency of dividing eells (FDC), whieh has been used to assess baeterial growth (HAGSTROM et al. 1979). The FDC has been reported to inerease more rapidly and reaeh higher values in ungrazed water samples in eomparison to samples with an intaet protistan population (SHERR et al. 1992). Therefore, seleetive protistan grazing and the resulting deerease of baeterial FDC may explain, in part, the low growth rates generally observed for assemblages of suspended baeteria (GoNzALEZ et al. 1990). The results of a laboratory experiment, performed in May 2000, on ehanges of some eharaeteristics of baeterial population dynamics (size, abundanee, biomass, growth rates and FDC) related to rhe presenee or absenee of protists, are reported herein.