GRAZING OF A TETRAHYMENA SP. ON ADHERED BACTERIA IN PERCOLATED COLUMNS MONITORED BY IN-SITU HYBRIDIZATION WITH FLUORESCENT OLIGONUCLEOTIDE PROBES

Predation of attached Pseudomonas putida mt2 by the small ciliate Tetr ahymena sp. was investigated with a percolated column system, Grazing rates were examined under static and dynamic conditions and were compa red to grazing rates in batch systems containing suspended prey. The p rey densities were 2 x 10(8) bacteria per ml of pore space and 2 x 10( 8) bacteria per ml of suspension, respectively. Postingestion in situ hybridization of bacteria with fluorescent oligonucleotide probes was used to quantify ingestion. During 30 min, a grazing rate of 1,382 +/- 1,029 bacteria individual(-1) h(-1) was obtained with suspended prey; this was twice the grazing rate observed with attached bacteria under static conditions. Continuous percolation at a flow rate of 73 cm h(- 1) further decreased the grazing rate to about 25% of the grazing rate observed viith suspended prey. A considerable proportion of the proto zoans fed on neither suspended bacteria nor attached bacteria. The tra nsport of ciliates through the columns was monitored at the same time that predation was monitored. Less than 20% of the protozoans passed t hrough the columns without being retained. Most of these organisms ing ested no bacteria, whereas the retained protozoans grazed more efficie ntly, Retardation of ciliate transport was greater in columns containi ng attached bacteria than in bacterium-free columns. We propose that t he correlation between grazing activity and retardation of transport i s a consequence of the interaction between active predators and attach ed bacteria.