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This is the first qualitative and quantitative evidence showing that the marine mixotrophic flagellate Prymnesium parvum (Prymnesiida = Prymnesiophyceae p.p.) is able to graze on small (5 pm) planktonic diatoms of the genera Minidiscus and Thalassiosira. Flagellate grazing and diatom species preferences were determined quantitatively in monoxenic batch cultures under nutrient depletion. Video microscopy coupled with digital film was used to visualise the ingestion process. Prey-switching was also investigated using bacteria as an alternative prey and the results were compared with flagellate growth dynamics in the absence of any prey. P. parvum started to graze on diatom cells about 5 min after the diatoms were introduced into the flagellate culture flasks. A reduction of about two-thirds of the initial diatom population occurred in both diatom taxa during the first 2 h of contact in flasks without bacteria. High interim grazing rates occurred during the first 8 h (0.30 diatom flagellate -1 h -1 for M. trioculatus and 0.74 diatom flagellate -1 h -1 for Thalassiosira sp.); when bacteria were added, prey-switching (from diatom cells to bacteria) was observed in the M. trioculatus experiment. Grazing rates on both diatom species for the duration of the experiment (144 h) were not statistically different (0.017 diatom flagellate -1 h -1 ); when bacteria were added these rates decreased (0.003 diatom flagellate -1 h -1 for Minidiscus trioculatus and 0.002 diatom flagellate -1 h -1 for Thalassiosira sp.). Bacterial grazing by P. parvum was also similar in both diatom flasks (0.17 bacteria flagellate -1 h -1 ). Addition of diatoms to the cultures did not enhance flagellate growth. Diatom capture usually involved the production of a pseudopodium-like structure at the posterior end of flagellate cells. Overall, these results suggest that predation by mixotrophic prymnesiophyte flagellates may be an important factor in regulating populations of nanoplanktonic diatoms in marine environments.

Grazing by Prymnesium parvum on small planktonic diatoms