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Aquatic Microbial Ecology

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AME 62:49-59 (2011)  -  DOI:

Grazing of heterotrophic nanoflagellates on the eukaryotic picoautotroph Choricystis sp.

Grażyna Bręk-Laitinen*, Anne Ojala

Department of Environmental Sciences, University of Helsinki, Niemenkatu 73, 15140 Lahti, Finland

ABSTRACT: Autotrophic picoplankton (APP, <2 µm), composed of both prokaryotes and eukaryotes, play an important role in the production and transformation of organic carbon in freshwaters. Eukaryotic APP are commonly found in winter and spring, and heterotrophic nanoflagellates (HNF) are regarded as significant consumers of APP. Here, we analysed the grazing impact and the growth ability of the HNF culture derived from a boreal clearwater lake on the picoalga Choricystis sp. For comparison, we used HNF monocultures of Rhynchomonas sp. and Bodo saltans flagellates with distinct feeding behaviours. In the grazing experiments, all HNF cultures ingested picoalgae and reached feeding rates between 0.08 and 0.81 cells HNF–1 h–1. We observed large species-specific differences in grazing rate (0.97 to 1.96 d–1), growth rate (0.35 to 0.85 d–1) and final abundance of HNF (5.6 × 104 to 74.2 × 104 cells ml–1) in the growth experiments. Surprisingly, we found an increase in picoalgal growth in the presence of a grazing HNF population. Although HNF appear to be active herbivores capable of significantly reducing APP stock, flagellates were unable to control the picoalgal population. Overall, this study demonstrates an unexpected role of HNF in microbial food webs by imposing a positive feedback on picoalgal growth through grazing.

KEY WORDS: Heterotrophic nanoflagellates · Picoeukaryotes · Grazing · Choricystis sp. ·Rhynchomonas sp. · Bodo saltans

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Cite this article as: Bręk-Laitinen G, Ojala A (2011) Grazing of heterotrophic nanoflagellates on the eukaryotic picoautotroph Choricystis sp.. Aquat Microb Ecol 62:49-59.

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