Digestion of bacterial macromolecules by a mixotrophic flagellate, Ochromonas sp., compared with that by two heterotrophic flagellates, Spumella pudica and Bodo saltans

Digestion of bacterial biomass by three species of phagotrophic flagellates was studied using radioactive tracer techniques and short-term feeding exp eriments. Macromolecules of two different bacterial strains and natural lim nic bacterioplankton were pulse-chase-labelled with one of the following pr ecursors H-3-thymidine, S-35-/C-14-methionine or C-14-leucine, before bacte ria were fed to flagellates and radioactive labels were traced into flagell ate macromolecules. The concentrations of prey and predators were monitored by flow cytometry. The aim of the work was to compare efficiencies of bact erial macromolecule accumulation by mixotrophic (Ochromonas) and heterotrop hic (Spumella and Bodo) flagellates. We observed that flagellate accumulati on efficiency of bacterial macromolecules labelled with thymidine (mean 15- 30%, depending on flagellate species) was lower than of bacterial macromole cules labelled with amino acids (mean 26-68%). Heterotrophic flagellate spe cies had similar accumulation efficiencies of bacterial molecules, when eit her leucine (26-42%) or methionine (31-41%) was used as a tracer. In contra st the mixotrophic flagellate accumulated significantly more residues of la belled methionine (68%) than of labelled leucine (54%). Methionine seems to be accumulated as an intact molecule and possibly Ochromonas preferentiall y accumulated methionine as an additional source of reduced sulphur. Protoz oan accumulation efficiencies did not differ significantly whether the puls e-labelled bacterial prey were from growing or long-term starvation culture s. Our results suggest that labelled amino acids are more appropriate than labelled thymidine for studying transfer of bacterial biomass within food w ebs.