AME 42:139-147 (2006)  -  doi:10.3354/ame042139

ABSTRACT: To examine the relative importance of nanoflagellates and ciliates as grazers of bacteria, we examined seasonal changes in their consumption of bacteria in a coastal sea, where the oligotrichous ciliates Strombidium and Strobilidium are dominant throughout the year. The numbers of these 2 dominant taxa accounted for 16 to 71% (average 55%) of the total ciliate numbers. Other ciliate taxa which seasonally dominated were photosynthetic Myrionecta rubra, the aloricate oligotrichs Laboea and Lohmaniella, the loricate oligotrichs Tintinnopsis, Cyttarocylis, and scuticociliates. Fluorescently labelled 0.5 µm diameter beads were used to determine grazing rates. Bacterivory was detected for nanoflagellates, Strombidium, Strobilidium, Lohmaniella, Tintinnopsis and Cyttarocylis. Ingestion rates on bacteria by nanoflagellates ranged between 0.3 and 2.2 bacteria protistan cell^–1 h^–1. Ingestion rates of the dominant ciliate taxa Strombidium and Strobilidium, when feeding on bacteria, varied from 7 to 34 bacteria protistan cell^–1 h^–1, though the rates were low relative to those of other dominant ciliate taxa (14 to 50 bacteria protistan cell^–1 h^–1). Significant relationships (p < 0.01) were found between the cell numbers of bacteria and ingestion rates of nanoflagellates, and between the cell numbers of bacteria and ingestion rates of Strombidium, whereas the relationship between the cell numbers of bacteria and ingestion rates of the total ciliates was not significant. Turnover rates of bacteria due to grazing by the total ciliates (0 to 0.63% d^–1) were lower than those by nanoflagellates (1 to 15% d^–1). Thus, it is likely that the dominant ciliates in the bay are not important grazers of bacteria, and that the main grazers of bacteria are nanoflagellates.

KEY WORDS:Microbial food web · Nanoflagellates · Oligotrichous ciliates · Grazing · Coastal sea · Japan

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