A COMBINED MICROCOSM AND MESOCOSM APPROACH TO EXAMINE FACTORS AFFECTING SURVIVAL AND MORTALITY OF PSEUDOMONAS-FLUORESCENS AG1 IN SEAWATER

The survival of Pseudomonas fluorescens Ag1 in seawater of Roskilde Fj ord (Denmark) was evaluated by a series of laboratory microcosm and fi eld-based mesocosm experiments. In sterile seawater microcosms, cultur ability of Ag1 was negatively influenced by high salinity (34 versus 8 .5 parts per thousand). In microcosms with 0.2 mu m-filtered seawater, addition of a carbon + nitrogen + phosphorus nutrient mixture was nee ded to induce proliferation of Ag1. In nutrient-amended microcosms the Ag1 population maintained viability, as determined by the direct viab le counts method, at a level dose to 100%. In natural water microcosms , Ag1 decreased by two to three orders of magnitude in three days. Fie ld experiments in 5300-litre seawater enclosures demonstrated a less p ronounced decline. The observed average decline rate agreed well with the calculated average predation potential of heterotrophic nanoflagel lates using fluorescence-labelled Ag1 as prey. During the experiment, mesocosms were amended with nutrients as in microcosms to attempt indu ction of Ag1 cell proliferation in situ, but the decrease rate of Ag1 remained unchanged. Viability remained above 25% throughout the experi ment, emphasizing that the decline of Ag1 did not result from extensiv e cell death. The combination of micro and mesocosms proved useful as a test scenario for fate studies of microorganisms introduced into an aquatic environment.