Microbial Diversity in Soil Cores From the Yukon River Basin, Alaska

Understanding the microbial environment in permafrost areas is important for understanding processes that release carbon and other nutrients from soils as a result of permafrost melting. Soils were collected in August 2005 from two sites in the Yukon River Basin, Alaska, and examined for microbial diversity as part of a larger project to investigate carbon cycling within the river basin. One site was located at the Bonanza Creek Long- Term Ecological Research Site near Fairbanks in an area of discontinuous permafrost and the other site was collected 400 kilometers to the north near Coldfoot in an area of continuous permafrost within the Arctic Circle. Both sites are characterized as black spruce forest and permafrost is 42-55 cm below land surface. Soil pore waters in the active layer at the Bonanza Creek site had a higher pH (5.06 versus 4.35), lower SO4 and DOC, and higher dissolved CH4 compared to the Coldfoot site. Dissolved oxygen was measured at >1.0 mg/L in water pumped from piezometers at both sites. Soil samples were collected from a range of depths above and below the permafrost and analyzed for total bacteria, for most probable number (MPN) of nine metabolic types of microorganisms, and for five metabolic types of microorganisms by quantitative polyermase chain reaction (QPCR). Soil geochemistry and climatic conditions affected the microbial abundances and distributions found at these two sites. The total number of bacteria by direct count ranged from 105 to 107 cells per gram dry weight (gdw) sediment with living cells comprising 1.4 to 98% of the total enumerated bacteria. In near-surface samples (top 40 cm), the MPN results indicate that aerobes, fermenters, humic acid reducers, and iron reducers account for most of the total bacteria. Nitrifiers and denitrifiers were found in a few samples, whereas sulfate reducers and methanogens were below our detection limit using the MPN method. The QPCR results indicated the presence of methanogens in 9 of 14 samples in concentrations of 102 to 104 cells per gdw sediment, sulfate reducers in all samples in concentrations of 101 to 103 cells per gdw sediment, and Geobacter (as an iron reducer) in 13 of 14 samples in concentrations of 103 to 104 cells per gdw sediment. Methanogens, sulfate reducers, and iron reducers (from the QPCR method), and denitrifiers (from the MNP method) were found in greater abundance and at more soil depths at the Bonanza Creek site than at the colder climate of the Coldfoot site. At both sites, there was a gradual decrease in abundance of microorganisms from the surface to the deepest sample (1 meter). Although the pore water sampled from the cores was oxic, significant numbers of living or recently living microbes that normally function in suboxic or anoxic environments were present in samples above and in the permafrost. This suggests that the soil was anoxic, perhaps under the winter snowpack. These results indicate a microbially active and diverse ecosystem in soils that are <10 degrees C in the Yukon River Basin. These microbially diverse ecosystems may play an important role in metabolizing carbon in soils as permafrost melting increases.