Grazing of heterotrophic nanoflagellates on the eukaryotic picoautotroph Choricystis sp.

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 ) 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 Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 62: [49][50][51][52][53][54][55][56][57][58][59] 2011 plankton (Christaki et al. 2001), thereby forming the major pathway of picoalgal carbon transport to higher trophic levels (Stockner 1988). Knowledge about grazing on eukaryotic APP is limited, and the majority of studies have concentrated on prokaryotic Synechococcus and Prochlorococcus (Worden et al. 2004). To the best of our knowledge, only 2 studies to date (Parslow et al. 1986, Christaki et al. 2005 have reported species-specific grazing of nanoflagellates on eukaryotic picoalgae, both in marine environments, while no parallel data on the grazing of freshwater HNF on Choricystis exist.Grazing studies have mainly been based on such approaches as size fractionation, dilution, use of specific inhibitors or disappearance of prey analogues (Caron et al. 1991, Liu et al. 1995, Worden & Binder 2003. However, these methods rely on communities and do not consider the different feeding modes of HNF. Investigation of prey items in the food vacuole of a single flagellate at a high resolution enables calculations of ingestion and clearance rates . Additionally, chlorophyll-containing autofluorescing cells of APP can easily be seen in food vacuoles of HNF without using fluorescing dyes (Dolan & 2imek 1998).Here we quantified the grazing impact of HNF on eukaryotic APP and also examined the potential of HNF to control APP at low temperatures. Our simple 2-step experimental set-up consisted of 1 defined prey item, i.e. a rod-shaped unicellular Choricystis sp., and 3 different types of HNF as grazers, i.e. Bodo saltans Ehrenberg, Rhyncho...

Heterotrophic nanoflagellate grazing facilitates subarctic Atlantic spring bloom development

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