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 1 0 -~ 1 ind-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.
Growth rate ( p ) , cell volume (CV), chlorophyll a quota (Qchi), and in vivo fluorescence (F) and DCMU-enhanced F (FD) were measured in Fe-limited, semi-continuous cultures of Gymnod~nium sanguineurn, which varied free fenic ion activity (pFe; i.e. -log free ferric ion activity) from 16.0 to 22.2, and in Fe-deplete batch cultures. Selected variables were also determined for cultures grown into nitrogen depletion. The majority of Fe-limited cellular characteristics changed most rapidly over the same pFe range (20.2 to 21.2), with ,U, CV and Qchl declining, while F/chl a and FD/chl a increased. The half-saturation constant for iron-limited growth (K,,) was near the maximum of values calculated for other neritic species examined previously. More importantly, however, the competitive ability of G.
Iron and nitrogen (NO 3 and NH4) uptake by the red tide dinoflagellate Gymnodinium sanguineum Hirasaka were studied in 1988 in Fe-replete and Fe-deplete batch cultures. Saturated rates of Fe transport (~, mol 1-1 cell vol h-1) for cultures grown on NO3 or NH 4 were measured following resuspension in either N source (i.e., NO 3 or NH4). Enhanced Fe uptake capacity developed under Fe stress, and was manifested in all experiments except in that involving a transition from NH4 to NO3 nutrition. Suppression appears to have resulted from a reduced ability of NH4-grown cultures to utilize nitrogen in the form of NO3, thereby causing cells to remain nutritionally stressed (with respect to N rather than Fe, however). Supporting evidence was provided by the complete initial inhibition of NO 3 uptake when Fe-deplete, NH ggrown cells were given saturating iron additions. When NH4 was supplied continuously (i.e., no N source transition), NH4-grown cells showed the greatest iron-stressmediated enhancement (i.e., Fe-replete vs Fe-deplete) of Fe transport (2.5-fold) accompanied immediately by NH4 uptake. Our findings are considered in relation to the potential consequences for this dinoflagellate of reduced iron bioavailability and available nitrogen source.
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