Phagotrophic sustenance of a metalimnetic phytoplankton peak

Evidence is provided for the importance of phagocytosis to mixotrophic chrysophycean algae and to the phytoplankton community as a whole. First, we confirmed the major assumption of past work that the constituents of ingested particles are incorporated efficiently into algal tissue. Cells of the phagotrophic chrysomonad Dinobryon sertularia collected from Lac Gilbert, Quebec, incorporated carbon from radiolabeled bacterial prey with 54% efficiency (95% C.L. = 40-79%) over a 5-h period. When removed from labeled bacteria, the loss of previously incorporated label from Dinobryon was undetectable over a 4-h period. We conclude that this alga’s quantitative utilization of prey tissue is indistinguishable from that of purely heterotrophic organisms. Second, simultaneous measurements were made of phagotrophic feeding and photosynthesis within a highly concentrated layer of chrysomonads (Dinobryon and Ochromonas) in the metalimnion of Lac Gilbert on 3-4 July 1986. Chrysomonad abundance ranged from 300 cells ml-’ at 6 m, to 9,000 at 7 m, and back to < 100 at 9 m. At the 7-m peak, phagotrophy accounted for 79% of algal community carbon assimilation on this overcast day.