Algal phagotrophy: Regulating factors and importance relative to phytosynthesis in Dinobryon (Chrysophyseae

Simultaneous measurements of inorganic carbon fixation and phagotrophic particle uptake by Dinobryon in a metalimnetic algal abundance peak showed that this alga depended more strongly on ingested bacteria for nutrition than on photosynthesis. Measurements of the grazing rate at different depths in Lac Gilbert, Quebec, showed that the particle ingestion rate depends on water temperature rather than light availability. Phagocytosis of bacteria proceeded at a similar rate both day and night in most lakes where Dinobryon was found. Since other chrysomonad genera (Chl-sophaerella, Uroglena, Catenochrysis, Ochromonas, Chromulina, and Chrysococcus) were also found to ingest particles, it will not be possible to estimate grazing on bacteria by counting nonpigmented cells and ignoring those containing chlorophyll. In oligotrophic Lac Bowker, 30 % of the phyto-plankton cells were actively ingesting small particles, and it was these phytoplankton and not the less numerous zooplankton that were responsible for most bacterial grazing in the lake. Phagotrophy by algae appears to be an important but unexpected pathway for energy flow in lakes. The flagellated Chrysophyceae, or chry-somonads, bear affinities to both plants an