BACTERIAL COMMUNITIES IN PLEISTOCENE AND HOLOCENE LAKE SEDIMENTS, LAKE CLARK, ALASKA

We have conducted an exploratory study to determine if bacterial communities inhabiting grain surface biofilms and pore waters in lake sediments are unique to specific lacustrine sedimentary depositional facies. If so, bacterial community analyses may eventually provide a sensitive gauge of environmental change in modern and possibly Quaternary sediments that will directly augment other sedimentary, geochemical, and biological proxies for environmental change in arctic lakes. We collected a 3.9 meter sediment core from a kettle pond adjacent to Dice Bay on the north shore of Lake Clark, Alaska. The core contains two major units: ~70 cm of blue-grey clay at the bottom overlain by ~3m of brown organic lake sediments. This transition to organic deposition occurred when a formerly larger, topographically higher, and glacially fed Lake Clark dropped below the threshold of the basin containing the kettle pond. The upper organic unit contains two tephra layers that are distinct from each other in grain size and composition and serve as recognized markers in other lake cores in the area The older lake clays likely date from deglaciation (~12,000 BP) and the isolation of the pond probably occurred in the early to middle Holocene. Polymerase chain reaction (PCR) amplification of bacterial 16S rRNA genes with universally conserved rDNA primers and terminal-restriction fragment length polymorphism (T-RFLP) analyses were used to determine the phylogenetic diversity of the bacterial communities within each of the depositional facies, including the tephras. Results indicate that each sedimentary horizon contains a unique and diverse microbial community that is more than 90% different from bacterial communities detected in the other sedimentary facies comprising the core. This stratigraphic distribution indicates that bacterial communities observed in each down-core lacustrine sedimentary facies are not associated with modern lake sedimentary environments, and therefore may represent either modern or ancient bacterial communities that inhabit(ed) these sedimentary units.