Feeding by the zooflagellate Pseudobodo sp. on the picoplanktonic prasinomonad Micromonas pusilla

The trophodynamic interaction between a heterotrophic zooflagellate, Pseudobodo sp. (2 to 4 pm) and a small (2 pm) prasinophyte, Micromonas pusilla, was studied in continuous culture. This zooflagellate was capable of phagocytizing Micromonas and causing a rapid decline in cell numbers of the latter. Maximum growth rate of the zooflagellate was about 2 d-I and maximum clearance rate was about 1 X 10-~ 1ind-I d-l. A video system was used to record its feeding behaviour. We attempted to estimate population parameters by deliberately perturbing the experimental system and using systems identification procedures to fit non-linear dynamic models to the resulting time series. Our results suggest that the response of zooflagellates to fluctuating food densities is intrinsically more complicated than observations of steady-state growth would suggest. These complexities include both time lags in the response of ingestion to increasing food density, and a complex response of cell size, physiological state and depth rate to decreasing food density. Present observations suggest that considerable caution should be exercised in the use of steady-state chemostat results to predict or model zooflagellate populat~ons in the field.

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