Distribution of Bacterial Decomposers in Lake Khuvsgul, Mongolia

Abstract

To understand the ecological function of heterotrophic bacterial community in water column of large freshwaterlakes in the permafrost zone, we investigated the structure and function of bacterial community in Lake Khuvsgul,Mongolia. Species composition of overall bacterial community was analyzed by denaturing gradient gelelectrophoresis (DGGE) of 16S rRNA gene fragments, and bacteria that can be cultured at 10oC were isolatedand characterized. Based on the depth profile of environmental parameters, thermocline and chemocline wererecognized at the 5~10 m zone of the water column. The stratified DGGE profile indicated that the discontinuityof water properties might influence the structure of bacterial community: band profiles in the 0~5 m zone werediverse with large change by depth, but the profile was relatively stable at the ？‰？10 m zone, with predominance ofthe band identified as Acidovorax facilis. Bacterial cultures were screened for protease, cellulase, amylase and lipase activity, and 23 isolates were selected for high activity of the hydrolytic enzymes. The isolates were identifiedbased on their 16S rRNA gene sequences. In the surface water (zero meter depth), Acidovorax defluvii andSphingobacterium faecium with high cellulase activity were present. Flavobacterium succinicans, Mycoplanabullata and A. facilis were stably predominant isolates at 2 m, 5 m, and ？‰？10 m depths, respectively. F. succinicansisolates showed high protease activity while M. bullata isolates showed moderate levels of protease andcelluase activity. A. facilis isolates showed either cellulase or lipase activity, exclusively to each other. Accordingto the profile of growth rates of the isolates in the temperature range of 0~42oC, the surface-zone (0~5 m)isolates were facultative psychrophiles while isolates from ？‰？10 m depth were typical mesophiles. This stratificationis believed to be due to stratified availability of organic materials to the bacterial decomposers. In thewater column below the chemoline, the environment is extremely oligotrophic so that the trait of rapid growth inlow temperature might not be demanded by deep-lake decomposers. The stratified distribution of communitycomposition and decomposer activity in Lake Khuvsgul implies that ecological functions of bacterial communityin lakes of cold region are sharply divided by water column stratification.