Changes in protistan abundance and bacterial activity in response to the addition of eukaryotic inhibitors to natural lake water
Two short-term (24 h and 48 h) microcosm experiments using natural waters from three eutrophic lakes (Masurian Lake District, Poland) were performed to assess the impact of eukaryotic inhibitors (a combination of cycloheximide and colchicine) on the abundance of nanoflagellates and small ciliates in the <15 μm fraction. The results showed that eukaryotic inhibitors were not completely effective against either group of protists; however, they reduced their numbers considerably. At 24 h of the experiment, 41, 15, and 7% of nanoflagellate and 48, 23, and 3% of ciliate abundances were not lysed, depending on the lake from which water was taken. However, after 48 h of incubation, only below 7% of nanoflagellates and 33, 40, and 17% of ciliates were present in the treatments with inhibitors. Our results suggest that inhibitors may indirectly change bacterial growth and activity, but they do not definitively inhibit these processes. It was concluded that eukaryotic inhibitors are more effective against small nanoflagellates than larger nanoflagellates and ciliates. Concentrations of inhibitors higher than 200 and 100 mg l-1 for cycloheximide and colchicine, respectively, and an incubation time longer than 24 h also seemed to be more appropriate to achieve the complete inhibition of protists.