VARIABILITY OF THE BACTERIOPLANKTON COMMUNITY DRIVEN BY ENVIRONMENTAL FACTORS IN FROZEN DALI LAKE NORTH CHINA

The lakes in cold areas usually undergo a long period of frozen conditions during which the physicochemical environment and biological community will undergo significant changes. To understand the characteristics of the bacterioplankton community in a frozen lake, we collected 28 ice samples and 39 water samples from under the ice sheet (including two ice layers and three water layers) in Dali Lake, an inland closed lake on the Inner Mongolia Plateau, Northern China. Based on 16S-rRNA gene-based high throughput sequencing coupled with comparisons of physicochemical proxies between "ice" and "water" samples, the vertical characteristic of the bacterioplankton community from ice to water were investigated in detail. The results of the analysis show that the contents of total nitrogen, total phosphorus, dissolved inorganic phosphorus, total dissolved phosphorus, electrical conductivity, and dissolved oxygen were visibly higher in water samples than in ice samples. Correspondingly, the ice sample abundance indices (the Chao and Ace proxies) were higher than those in water, but the water sample evenness and diversity indices (the Shannon evenness, Shannon, and Simpson indices) were higher than those in ice. In addition, the Proteobacteria, Actinobacteria, Cyanobacteria, Bacteroidetes, Verrucomicrobia, and Tenericutes were the dominant bacterioplankton phyla both in ice and water samples. However, the Proteobacteria was the most abundant bacterioplankton phylum in ice samples (the average relative contents were 87.8% and 67.8% in surface ice and bottom ice, respectively), and the Actinobacteria was the most abundant bacterioplankton phylum in water samples (the average relative contents were 42.0%, 43.1%, and 45.6% in surface water, middle water, and bottom water, respectively). The ecological network of bacterioplankton was significantly different in five types of samples during the freezing period. The results revealed a higher proportion of positive interactions in the entire lake and a higher connectivity in ice than in water. Actinobacteria was the most connected and most critical phylum for maintaining community function. Finally, the bacterioplankton community composition was evidently controlled by environmental factors. In Dali Lake, the changes in phosphorus had higher correlations with bacterioplankton in ice samples, and the changes in nitrogen had higher correlations with bacterioplankton in water samples. Concomitantly, the genera that played key roles in ice samples were not the same as those in water samples that had experienced the frozen conditions. Taken together, these data will provide evidence as to how a bacterioplankton community responds to environmental changes in a frozen lake.