The Dynamic Nature of Protistan Ingestion Response to Prey Abundance

ABSTRACT. Ingestion rate of Paraphysomonas imperforata was found to be a hyperbolic function of prey density. But the same flagellate clone had multiple ingestion responses to prey density, depending on its physiological state and physical stress it suffers. The flagellates in a physiological state of higher growth tended to have higher maximum ingestion and clearance rates than ones in a physiological state of lower growth. The same trend was observed for volume-specific maximum ingestion and volume-specific clearance rate. In response to changing prey density, the growth rate did not change as quickly as the ingestion rate, suggesting imbalance between the two. The tested physical stresses, including shaking, centrifugation, and filtration, also resulted in reduction of ingestion parameters of the flagellates. But half-saturation constants did not show any trend in response to either physiological state or physical stress. In light of the dynamic nature of protistan ingestion response to prey abundance, short incubation, which minimizes the physiological change, and careful handling, which prevents the possible physical stress, should be employed in order to avoid underestimation of in situ ingestion rates. Previously reported ingestion parameters of lab-cultured protists, which are thought to be unrealistic in natural conditions, may represent only one of multiple ingestion responses, probably prey-rich condition.