AME 62:49-59 (2011)  -  DOI: https://doi.org/10.3354/ame01457

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