Diversity, Distribution and Dynamics of Picoplankton in Lakes of Different Trophic State - PICO

Coordinating Institution: CRP Gabriel Lippmann
Other Partner(s): Station Biologique de Roscoff (F) , Université de Savoie, Chambéry (F)
From: 01/03/2010
To: 28/02/2013
Budget: 438,000.00€
Contact(s): Hoffmann Lucien

Summary

Picoplanktonic organisms [Traditionally, picoplankton refers to cells with a size ranging between 0.2 and 2 μm] are found in marine and freshwater ecosystems with densities which can reach 104 cell mL-1. They constitute an essential component of the microbial food web and play significant roles in the biogeochemical cycles. Generally, picoplanktonic communities of lakes are dominated by picocyanobacteria whatever the trophic status, autotrophic picoeukaryotes being often about one order of magnitude less abundant than picocyanobacteria.

Despite the low number of marine and freshwater eukaryotic species formally described in this size range (about 100), the few available studies indicate a high phylogenetic diversity among the small eukaryotic population, as well as a general dominance of heterotrophic cells within these small eukaryotic assemblages. Whereas the diversity and the dynamics of the marine picoplankton are well known, a lack of information is observed for freshwater systems. Most studies made on lacustrine systems focused on the molecular characterization at only one selected depth in a single lake.

Therefore, no generalization can be made about the structure, seasonal dynamics and ecology of freshwater picoplankton. In a recent preliminary study of the picoplankton of the Esch-sur-Sûre reservoir, we showed that contrary to most lakes, autotrophic picoeukaryotes dominated the planktonic communities in the lake and showed two peaks of abundance in April and July. TSA-FISH analyses with specific probes showed that among pigmented organisms, Chlorophyta and Haptophyta showed lower contributions while Cryptophytes generally dominated. Among heterotrophic eukaryotes, the dominant groups were the Fungi and the Perkinsea, indicating a quantitative importance of potential parasites in the small heterotrophic eukaryotic assemblages. Furthermore, clone libraries showed that a large part of Fungi was composed of Chytrids. The objective of this project is to study the diversity, seasonal dynamics, and ecology of the picoplankton in freshwater ecosystems varying by their trophic status. The specific questions to be addressed are: - what are the main taxonomic groups represented in the picoplankton - is the spatiotemporal dynamics of the picoplankton the same as the one of the metaplankton - what are the environmental factors determining the dynamics of the picoplankton and of its constitutive taxonomic groups - is the picoplankton composition dependent on the trophic status of the lake.