Effects of Seasonal Freezing-thawing on the Protistan Communities in a Mountain Lake

Abstract

Background: Freezing-thawing cycles are common phenomena in temperate regions. Such events may have a significant influence on the composition of the protistan communities in a mountain lake. Protists are single-celled eukaryotic microorganisms that act as links in the aquatic microbial food web, affecting the transfer of substances and energy transformation. Yet little is known about the effects of freezing and thawing on the protistan community in a mountain lake. Results: The protistan communities in the lake were mainly composed of Ochrophyta, Ciliophora, Choanoflagellida, Cryptophyta, Chlorophyta, Stramenopiles_X, Cercozoa, Dinophyta, and Haptophyta. Seasonal freezing and thawing affected the community composition and diversity of protists. The change in the protistan community structure resulted from a significant change in organic carbon from the ice-covered to ice-free period. During the ice-covered and ice-free periods, temperature and nitrate were the main causes, respectively, for the changes in protistan community structure at different depths. Water depth also affected the structure of the protistan community, but it was not the most important factor. Conclusions: This study revealed that duration of lake surface icing might affect the function of subalpine lake ecosystems, including the rate of nutrient cycling and energy flow, owing to changes in the structure and biodiversity of the microbial community.