The distribution and relative abundance of ammonia-oxidizing bacteria in lakes of the McMurdo Dry Valley, Antarctica

Marked differences in the concentrations of major ions and cations, macronu trient chemistry and general trophic status exist among the lakes of the Mc Murdo dry valleys in Antarctica. These differences have been attributed to both variations in stream inputs and in situ lake processes (Priscu, 1995; Lizotte et al., 1996, Spigel and Priscu, 1996). This study examines the rol e of nitrifying bacteria in nitrogen transformations in these lakes. Applyi ng two polymerase chain reaction (PCR) assays targeting the 16S rRNA genes of ammonia-oxidizing bacteria and the active site of the ammonia monooxygen ase gene (amoA), the distribution of ammonia-oxidizers was examined in six Antarctic lakes: Lake Bonney, Lake Hoare, Lake Fryxell and Lake Joyce in th e Taylor Valley, Lake Miers in the the Miers Valley and Lake Vanda in the W right Valley. Using a two stage amplification procedure, ammonia-oxidizers from both the beta and gamma- subclasses of the Proteobacteria were detecte d and their relative abundances were determined in samples collected from a ll sites. Ammonia-oxidizers were detected in all lakes sampled. Members of the gamma subclass were only present in the saline lakes. In general, nitri fiers were most abundant at depths above the pycnocline and were usually as sociated with lower concentrations of NH4 and elevated concentrations of NO 3 or NO2. The distribution of nitrifiers suggests that the primary N2O peak observed in most of the lakes was produced via nitrification. Preliminary data on the rate of nitrification (Priscu et al., 1996) support the occurre nce of nitrification and the presence of nitrifiers at the depth intervals where nitrifiers were detected. In all lakes, except Lake Miers, the data i ndicate that nitrifying bacteria have an important role in the vertical dis tribution of nitrogen compounds in these systems.