UC Riverside

Abstract of the dissertation

Microbial diversity studies in sediments of perennially ice-covered lakes,

McMurdo Dry Valleys, Antarctica

by

Chao Tang

Doctor of Philosophy, Graduate Program in Microbiology

University of California, Riverside, October 2009

Dr. Brian Lanoil, Chairperson

The primary refuge for life in the McMurdo Dry Valleys (MCM), Antarctica, is a suite of perennially ice-covered lakes, which are entirely microbial ecosystems. Compared with the microbiology studies in the ice-cover, water column and microbial mats, microbial communities in sediments, which are important components for nutrient cycling and prokaryotic diversity in lacustrine systems, have received considerably less study in these lakes.

This is the first study characterizing detailed overall prokaryotic diversity in sediments of MCM lakes. The two selected lakes are Lake Fryxell and west lobe of Lake Bonney (WLB) from Taylor Valley. These two lakes have very distinct benthic geochemistry. Non-culture molecular techniques including clone library and denaturing gradient gel electrophoresis utilizing 16S rRNA gene marker was used. High bacterial diversity and low archaeal diversity was detected in sediments of both lakes; however, the sediment microbial communities are significantly different. Sequences of major bacterial OTUs from WLB sediments exhibited exclusive relatedness to cold environment sequences, suggesting the adaptation to the coldness. Sediment microbial community in Fryxell seemed to be influenced by neighboring marine system and involved in methane cycling.

Interspersed layers of microbial mats and terrigenous particles that penetrated through the ice cover largely represent the sediments stratigraphy in MCM lakes. Sedimentation is very slow. Samples from distant sections of the long sediment core of WLB were used to investigate microbial community change over long geologic period. Different origins of sediments (biogenic or terrigenous) and microbial community's evolution were proposed as explanation for the observed differences in microbial community structure from different sections. A fine scale depth profile of surface sediments from Fryxell was studied. There was clearly microbial population change with depths, likely due to geochemistry transition.

Sulfate reduction was found to co-exist with methanogenesis in Fryxell sediments with very low sulfate. Sulfate reducing bacteria (SRB) diversity was studied by characterizing dsrAB gene, which encodes for dissimilatory sulfite reductase, the conserved enzyme in all SRB. Two novel dsrAB lineages were discovered. Adaptation of the two dominant clusters to low sulfate condition was suggested; other clusters seemed to be related to marine SRB.