Ingestion and digestion of an autotrophic picoplankter, Synechococcus, by a heterotrophic nanoflagellate, Bodo saltans

We investigated the process of digestion and estimated ingestion from diges tion rate and food vacuole content in a planktonic freshwater bacterivore. Digestion was examined in feeding and nonfeeding Bodo flagellates previousl y exposed to Synechococcus and in "naive" flagellates. Digestion of Synecho coccus by Bodo was estimated for flagellates in different growth phases. Fo od vacuole contents declined exponentially, and digestion rate was relative ly constant, averaging about 1% food vacuole content min(-1) at 22 degrees C, regardless of feeding history or growth phase. There was close agreement among ingestion rates based on digestion rate and rates estimated from the disappearance or direct uptake of Synechococcus. We also examined ingestio n of Synechococcus by Bodo. Ingestion was similar in cells of different gro wth phases and in the presence or absence of fluorescent microspheres. In c ontrast, in the presence of Synechococcus, microspheres were ingested at lo wer rates in late-exponential-phase cells and not at all in mid- and late-s tationary-phase flagellates. Results of a preliminary experiment with marin e Synechococcus and mixed marine flagellates gave results similar to those obtained with Bodo in terms of an exponential decline in average cell conte nts and a digestion rate of about 1% cell contents h(-1). Our results sugge st that food vacuole content can be used to estimate ingestion of autotroph ic picoplankton by heterotrophic nanoflagellates.