Temporal variation of the small eukaryotic community in two freshwater lakes: Emphasis on zoosporic fungi

Abstract

Applications of molecular approaches to the study of microbial eukaryotic communities in freshwater lakes are transforming our understanding of these ecosystems. One of the most unexpected discoveries is that zoosporic fungi significantly dominate the planktonic fungal diversity. Although zoosporic fungi are now recognized as an important component of aquatic microbial food webs, our knowledge of their community structure and temporal variability remains poor. The objectives of our study were (1) to compare and describe the contribution of zoosporic fungi to the eukaryotic diversity in 2 lakes differing in their trophic status during the mixing and the stratified seasons and (2) to phylogenetically identify the recovered zoosporic fungal sequences. The small eukaryotes (0.6 to 8 μm) of the euphotic zone of the oligotrophic Lake Tusca - loosa and meso-eutrophic Lake Lurleen (Alabama, USA) were collected over 1 yr. Analyses of the 28S rDNA clone libraries showed that zoosporic fungi dominated the small planktonic fungal community and were more diverse in the meso-eutrophic lake and during the thermal stratification. Although the overall structure of the eukaryotic community was similar between the 2 lakes, at lower taxonomic levels, community composition differed. Analyses of the retrieved fungal sequences revealed that zoosporic fungi mostly affiliated with Rhizophydiales and Chytri diales or formed environmental clades. Although the phytoplanktonic community was also monitored, zoosporic fungal parasites were rarely observed on algae. These results provide new insights into the diversity and seasonality of the zoosporic fungal community in lake ecosystems. © Inter-Research 2012.