Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

© Springer International Publishing Switzerland 2016 307
T. Sime-Ngando et al. (eds.), Lake Pavin, DOI 10.1007/978-3-319-39961-4_18

The Importance of Phagotrophic

Protists in Lake Pavin

Jean-François Carrias , Christian Amblard , Gilles Bourdier ,

and Télesphore Sime-Ngando

Abstract

Phagotrophic protists include unicellular taxa from different lineages of eukaryotes. Most

of these freshwater microorganisms are free-living, heterotrophic forms, consuming bacte-

ria in the water column of lakes. In Lake Pavin, we focused on the seasonal and spatial

dynamic and the diversity of phagotrophic protists and estimated in situ specifi c-grazing

rates of both ciliates and fl agellates. We shown that a large part of bacterivory was the result

of the phagotrophy of attached (i.e. epiphytic) ciliates and fl agellates. Pigmented fl agellates

have also an important grazing impact on bacteria, with clearance rates equivalent to those

estimated for heterotrophic taxa of similar size. Total impact of phagotrophic protists on

bacteria indicates that bacterial productivity might be totally consumed by unicellular

eukaryotes. In a short-term study, we estimated that fl agellate bacterivory dominated the

bacterial mortality in Lake Pavin while viral lysis accounted for less than 10 % of the bacte-

rial mortality. Finally, the use of microcosms suggested that large-size ciliates ingest hetero-

trophic nanofl agellates and may in turn be a key resource for metazoa during spring. We

conclude that phagotrophic protists are involved in the conversion of bacterial biomass and

then constitute a relevant source of carbon for metazoans in the planktonic food web of

Lake Pavin.

Keywords

Protists • Flagellates • Ciliates • Bacterivory • Plankton • Lake • Freshwater

18.1 Introduction

Freshwater phagotrophic protists comprise predominantly
heterotrophic nanofl agellates, phagotrophic phytofl agellates,
and ciliates. These unicellular eukaryotes are recognized as
the primary consumers of bacterioplankton and picophyto-
plankton in freshwater pelagic ecosystems (Sanders et al.
1989 ; Pace et al. 1990 ; Carrias et al. 1996 ). Because of the
lack of accurate methods to count these communities, most
of these cells were usually ignored until 1980–1990s.

Improved interest and new methods have shown the large

diversity of theses planktonic taxa and their key role in the

food web. Firstly, they form a link between picoplankton and

higher trophic levels. Secondly, due to their high metabolic

activity, they play an important role in the recycling of nutri-

ents that limit algal growth (Caron et al. 1988 ). During the

last two decades freshwater protistan ecology has received

increasing attention (Amblard et al. 1993 ; Bennett et al.

1990 ; Carrias et al. 1996 , 2001 ; Riemann and Christoffersen

1993 ). In Lake Pavin, we assessed the distribution of hetero-

trophic nanofl agellates (Carrias et al. 1998a ), the main bacte-

rivores in lakes, and ciliates (Carrias et al. 1998b ). We

estimated the impact of phagotrophic protists on bacteria

using latex beads as tracers with special attention to attached

protists (Carrias et al. 1996 ). The relative importance of viral

lysis and protist bacterivory in bacterial losses was also

J.-F. Carrias (*) • C. Amblard • G. Bourdier • T. Sime-Ngando
LMGE, Laboratoire Microorganismes: Génome et Environnement ,
UMR CNRS 6023 , Université Clermont-Auvergne, Université
Blaise Pascal, BP 80026 , 63171 Aubière Cedex , France
e-mail: [email protected]

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