During the yellowing of leaves the porphyrin moiety of chlorophyll is cleaved into colorless linear tetrapyrrolic catabolites, which eventually are deposited in the central vacuoles of mesophyll cells. In senescent cotyledons of rape, Brassica napus, three nonfluorescent chlorophyll catabolites (NCCs), accounting for practically all the chlorophyll broken down, were found to be located in the vacuoles (vacuoplasts) prepared from protoplasts. Transport of catabolites across the tonoplast was studied with vacuoles isolated from barley mesophyll protoplasts in conjunction with a radiolabeled NCC, Bn-NCC-1, prepared from senescent rape cotyledons. The uptake of Bn-NCC-1 into vacuoles was against a concentration gradient and strictly dependent on MgATP and it followed saturation kinetics with a K m of approximately 100 M. Although the hydrolysis of ATP was required, transport was apparently independent of the vacuolar proton pumps: accumulation of the NCC occurred both in the presence of the H ؉ -ATPase inhibitor bafilomycin and after destroying the ⌬pH between the vacuolar sap and the medium. ATP could be replaced by GTP or UTP, and the transport was inhibited in the presence of vanadate. Chlorophyll catabolites isolated from senescent barley leaves competed with the rape-specific substrate for uptake into the vacuoles. Compounds such as the glutathione conjugate of N-ethylmaleimide and taurocholate, which are known to be transported across the tonoplast in a primary active mode, did not significantly inhibit uptake of Bn-NCC-1. Although the heme catabolites biliverdin and bilirubin inhibited the uptake of the NCC, this effect is caused by unspecific binding to the vacuolar membrane rather than to the specific inhibition of carrier-mediated transport. Taken together, the results demonstrate that barley mesophyll vacuoles are constitutively equipped with a directly energized carrier that transports tetrapyrrolic catabolites of chlorophyll into the vacuole.

The

× 10 3 m 3 ) is situated in the Vereina region in the eastern part of the Swiss Alps. We studied microbial grazing on bacteria and bacterial productivity during the ice-free period. The lake normally gets thermally stratified for two months between July and September. In 1996, chlorophyll-a concentrations varied from 0.5 to 2.0 µg l -1 with maximum values just below the thermocline (6 m depth), in 1997, they were between 0.6 and 5.0 µg l -1 with maximum values at 10 m depth -several meters below the thermocline. Bacterial densities varied between 0.7 and 1.7 × 10 6 ml -1 with maxima in the thermocline, one to two meters above the chlorophyll maximum. The areal bacterial biomass (volume beneath 1 m 2 to a depth of 8 m) was 10 µg C l -1 which remained more or less constant for the periods investigated. In 1997, bacterial growth rate and production rates were determined using [ 3 H]-thymidine incorporation. The rates were as low as 0.002 to 0.006 h -1 and 0.01 to 0.03 µg C l -1 h -1 , respectively. We found a carbon ratio of bacteria, phytoplankton, and autotrophic picoplancton (APP) of 1.5:1.1:1 which shows a rather high abundance of bacteria and autotrophic picoplankton (APP) compared to larger phytoplankton. Bacterial growth followed a temperature dependence similar to the one observed for bacteria from Lake Zürich, a prealpine and mesotrophic lake which was studied for comparison. Microbial food web in Jöri Lake III was not top down controlled during the periods of our study and mixotrophic algae like Dinobryon cylindricum var. alpinum and autotrophic nanoflagellates (ANF) were the dominant bacterial grazers observed.