Genetic Diversity Of Eukaryotic Picophytoplankton In Freshwater Lakes

Eukaryotic picophytoplankton is the phytoeukaryote with size being less than2μm. It is widely distributed in marine and freshwater ecosystems, and plays a very important role in the material cycle and energy flow in aquatic ecosystem, due to its high surface-volume ratio and high specific carbon uptake. In this study, we studied the genetic diversity, distribution, and effects of environmental factors of eukaryotic picophytoplankton in freshwater lakes.A novel approach based on flow cytometric sorting followed by construction of clone libraries was used to survey the diversity of eukaryotic picophytoplankton of Lake Taihu, Xuanwu Lake and Zixia Lake. The results showed that most of eukaryotic picophytoplankton sequences belong to Cryptophyta, Chysophyta, Bacillariophyta, Chlorophyta and Haptophyta. The composition of eukaryotic picophytoplankton community appeared highly variable in three lakes. Cryptophyta exist in all lakes and Chrysophyta more adapt to the clean water. Eukaryotic picophytoplankton can widely exist in different aquatic ecosystems.The method for the collection of eukaryotic picophytoplankton was improved, with flow cytometry sorting to replace the conventional membrane filtering. This method could greatly reduce the amount of water and the processing time for each sample, and could effectively remove most of bacteria, protists, fungi from eukaryotic picophytoplankton, and thus improve the efficiency of amplification for phytoeukaryotes. In this study, we use clone library and terminal restriction fragment length polymorphism (T-RFLP) methods to study the diversity of eukaryotic picophytoplankton. The T-RFLP method has the advantage of easy operation, rapid and safe extraction, and high resolution, which could be used for a preliminary investigation of microbial community for a number of environmental samples at the same time. We can determine the specific species if combine with construction of clone libraries.Along with eukaryotic picophytoplankton, some heterotrophic protists, such as fungi and alveolate, were also found in the study, and they may coexist with eukaryotic picophytoplankton as parasite or symbionts. Eukaryotic picophytoplankton can exist in all freshwater lakes that were investigated, with different size, different location and different nutritional status, which indicated that eukaryotic picophytoplankton is ubiquitous in freshwater ecosystems. We categorized the lakes of the middle-lower Yangtze River using comprehensive nutrition state index method. There are eight mesotrophic lakes, and the rest of lakes are eutrophic lakes.We found196T-RFs fragments in research, showing that the diversity of eukaryotic picophytoplankton is very high.9%fragments were distributed in more than half of the lakes, and45%fragments were detected only in one to two lakes, which indicated that a small part of eukaryotic picophytoplankton has the widely distribution, but most of them can only survive in the specific habitat.The counting results of eukaryotic picophytoplankton by flow cytometry showed that mesotrophic lakes have a low abundance and eutrophic lakes were more than4-5times of them than mesotrophic lakes. The statistics of T-RFs fragments showed that mesotrophic lakes have an average of43T-RFs, light eutrophic lakes and moderate eutrophic lakes have31T-RFs,32T-RFs, respectively. The results revealed that mesotrophic lakes have low abundance and rich diversity.The canonical correspondence analysis was used to reveal the relationship between the diversity of eukaryotic picophytoplankton and environmental factors. We determine whether they have a linear relationship by detrended correspondence analysis (DCA). The results showed that the concentration of total nitrogen, total carbon, and chlorophyll a had the most important impact on the eukaryotic picophytoplankton communities. We can hypothesize that the genetic diversity of eukaryotic picophytoplankton in the lakes was of great environmental heterogeneity and closely related to the trophic status.The study shows that the genetic diversity of eukaryotic picophytoplankton in freshwater lakes is very abundant, varies greatly among different lakes, and closely related to the trophic status. The results may contribute to understand the community structure of plankton in different lakes comprehensively in the future.