Algal phagotrophy: Regulating factors and importance relative to photosynthesis in Dinobryon (Chrysophyceae)1

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 (Chlsophaerella, 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 phytoplankton 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 chrysomonads, bear affinities to both plants and animals and have been claimed for study by zoologists as well as by botanists (Allen 1969). Coexistence of autoand heterotrophy in the chrysomonads and the widespread tendency toward complete loss of chlorophyll have fostered the suggestion that these algae may model progenitors of the Metazoa (Provasoli and Pintner 1953). The diversity and versatility of the group are strongly expressed in the genus Uchromonas, which is capable of phagotrophy, heterotrophy, photoheterotrophy, and photoautotrophy (Pringsheim 19 5 5; Aaronson and Baker 1959). However, Ochromonas is ’ This work was supported by a Natural Sciences and Engineering Research Council of Canada operating grant to J. Kalff and an NSERC postgraduate scholarship to D. Bird. A contribution to the McGill Limnology Research Centre, Lake Memphremagog Project. one of only a few chrysomonads to have been studied extensively in the laboratory, and the nutritional capabilities of other chrysomonads are virtually unknown, especially in their natural setting. Pascher (1943) observed under the microscope that the colonial chrysophycean alga Dinobryon could capture and digest small particles, and thus concluded that at least some part of this alga’s nutrition was heterotrophic. This unusual phagotrophic ability was not studied further until the recent discovery that several Dinobryon spp. are not only phagotrophic in nature, but can be major grazers of bacteria in lakes (Bird and Kalff 1986). Here we present further work on four different aspects of the capabilities of this alga: first, the relative importance of bacterivory and photosynthetic carbon fixation to Dinobryon’s nutrition; second, a comparison of day and night grazing rates; third, the relationship of grazing