Unexpected importance of potential parasites in the composition of the freshwater small-eukaryote community

Abstract

The diversity of small eukaryotes (0.2 to 5 mum) in a mesotrophic lake (Lake Bourget) was investigated using 18S rRNA gene library construction and fluorescent in situ hybridization coupled with tyramide signal amplification (TSA-FISH). Samples collected from the epilimnion on two dates were used to extend a data set previously obtained using similar approaches for lakes with a range of trophic types. A high level of diversity was recorded for this system with intermediate trophic status, and the main sequences from Lake Bourget were affiliated with ciliates (maximum, 19% of the operational taxonomic units [OTUs]), cryptophytes (33%), stramenopiles (13.2%), and cercozoa (9%). Although the comparison of TSA-FISH results and clone libraries suggested that the level of Chlorophyceae may have been underestimated using PCR with 18S rRNA primers, heterotrophic organisms dominated the small-eukaryote assemblage. We found that a large fraction of the sequences belonged to potential parasites of freshwater phytoplankton, including sequences affiliated with fungi and Perkinsozoa. On average, these sequences represented 30% of the OTUs (40% of the clones) obtained for each of two dates for Lake Bourget. Our results provide information on lacustrine small-eukaryote diversity and structure, adding to the phylogenetic data available for lakes with various trophic types.

FIG. 1.

Relative levels of nano- and microalga groups and microcyanobacteria on 16 May 2005 and 10 August 2005 in Lake Bourget (depth, 0 to 20 m).

FIG. 2.

Rarefaction curves determined for the two 18S rRNA gene libraries generated for Lake Bourget. The number of different RFLP patterns was determined after digestion with restriction endonuclease HaeIII.

FIG. 3.

Phylogenetic tree for small-subunit rRNA gene sequences covering the diversity of Chlorophyceae, Cryptophyta, and Cercozoa. Sequences derived in this study are indicated by bold type. The names of the lakes in which sequences were identified are indicated in parentheses.

FIG. 4.

Phylogenetic tree for opisthokonts. See the legend to Fig. 3 for details.

FIG. 5.

Phylogenetic tree for alveolates. See the legend to Fig. 3 for details.

FIG. 6.

Phylogenetic tree for stramenopiles. See the legend to Fig. 3 for details.