Dynamics of herbivorous grazing by the heterotrophic dinoflagellate oxyrrhis marina

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Abstract
In a series of batch experiments in the dark the heterotrophic dinoflagellate Oxyrrhis marina grazed three phytoplankton prey (Phaeodactylun tricornutum, Isochrysis galbana and Dunaliella teriolecta) with equal efficiency. Growth rates of the dinoflagellate ranged between 0.8 and 1.3 day−1 Maximum observed ingestion rates on a cell basis varied according to the size of the prey from about 50 cells flagellate−1 day−1 when D.tertiolecta was the prey to 250–350 cells fiagellate−1 day−1 when the other species were eaten. However, when compared on a nitrogen basis, ingestion rates were independent of prey type. Both ingestion and growth ceased when prey cell concentrations fell below a threshold concentration of about 105 cells ml−1. Maximum specific clearance rates were 0.8×1040ndash;5.7×104 it day which is considerably lower than that found for heterotrophic dinoflagellates in oceanic waters and may explain why O.marina generally thrives only in productive waters. The timing of NH regeneration was linked to the C:N ratio of the prey at the start of grazing. Regeneration efficiencies for NH4. never exceeded 7%; during the exponential phase and were ≤45% well into the stationary phase. These results are comparable to those obtained with heterotrophic flagellates and demonstrate that the bioenergetic patterns of grazing and nutrient cycling by different protozoa are very similar. Moreover, they support the notion that to achieve 90+% nutrient regeneration in the open ocean, as is currently believed, the microbial food loop must consist of multiple feeding steps. Alternatively, nutrient regeneration efficiencies may be considerably lower than 90%.