Succession and Regulation Factors of Small Eukaryote Community Composition in a Lacustrine Ecosystem (Lake Pavin)

Abstract

The structure and dynamics of small eukaryotes (cells with a diameter less than 5 m) were studied over two consecutive years in an oligomesotrophic lake (Lake Pavin in France). Water samples were collected at 5 and 30 m below the surface; when the lake was stratified, these depths corresponded to the epilimnion and hypolimnion. Changes in small-eukaryote structure were analyzed using terminal restriction fragment length polymorphism (T-RFLP) and cloning and sequencing of the 18S rRNA genes. Terminal restriction fragments from clones were used to reveal the dominant taxa in T-RFLP profiles of the environmental samples. Spumellalike cells (Chrysophyceae) did not dominate the small eukaryote community identified by molecular techniques in lacustrine ecosystems. Small eukaryotes appeared to be dominated by heterotrophic cells, particularly Cercozoa, which represented nearly half of the identified phylotypes, followed by the Fungi-LKM11 group (25%), choanoflagellates (10.3%) and Chrysophyceae (8.9%). Bicosoecida, Cryptophyta, and ciliates represented less than 9% of the community studied. No seasonal reproducibility in temporal evolution of the small-eukaryote community was observed from 1 year to the next. The T-RFLP patterns were related to bottom-up (resources) and top-down (grazing) variables using canonical correspondence analysis. The results showed a strong top-down regulation of small eukaryotes by zooplankton, more exactly, by cladocerans at 5 m and copepods at 30 m. Among bottom-up factors, temperature had a significant effect at both depths. The concentrations of nitrogenous nutrients and total phosphorus also had an effect on small-eukaryote dynamics at 5 m, whereas bacterial abundance and dissolved oxygen played a more important structuring role in the deeper zone.Small phototrophic and heterotrophic eukaryotes (Ͻ5 m) are found throughout the world's oceans and lakes at concentrations between 10 2 and 10 4 cells ml Ϫ1 in the photic zone (11). Small eukaryotes are known to be essential components in marine trophic food webs (20). The small-eukaryote assemblage is formed by picoalgae, which participate in primary production (55), by colorless heterotrophic cells, mostly flagellates, which are considered to be important grazers of prokaryotic and eukaryotic cells (11) and also play a significant role in the mineralization of organic matter, and finally by some small eukaryotes which can be mixotrophs. Despite the ecological importance of small eukaryotes and the general lack of distinct morphological features of these small cells, they have only recently been studied from a molecular perspective (20, 37). Thanks to these techniques, recent studies, conducted in various environments, have revealed a surprisingly high diversity of small eukaryotes and the existence of novel lineages (39). For example, the genetic diversity of small eukaryotes from coastal waters showed the dominance of novel alveolates (from 36% to 62% of total sequences obtained in their libraries) and the importance of novel stramenopiles, ...

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