9709012 Giovannoni The focus of this study is the ecology of bacterioplankton in Crater Lake, Oregon. The motivation for the study originates from the observation of pronounced vertical structure in the distributions of uncultured bacterioplankton "species" in open oceans. This study will test the hypothesis that populations of uncultured bacterioplankton in freshwater ecosystems may be similarly structured along the vertical axis, and that such structure may play an important role in processes such as the remineralization of organic matter and nutrient export from the epilimnion. The proposed work will be integrated with the ongoing Crater Lake Long Term Monitoring program. Crater Lake is the centerpiece of Crater Lake National Park. This oligotrophic caldera lake is one of the clearest freshwater environments ever recorded, with secchi depths ranging from approximately 25 to 36 m over typical annual cycles. The extreme clarity of the lake's surface water is attributable to low nutrient input from its small, highly eroded catchment, and to its extraordinary depth, which allows for long-term export of nutrients to abyssal regions during periods of seasonal stratification. During the progress of seasonal stratification, the chlorophyll maximum migrates to the upper hypolimnion. In many respects, Crater Lake is atypical of freshwater environments but closely resembles the intensively studies, oligotrophic open ocean systems typified by the Sargasso Sea. A major component of the investigative approach will be the application of ribosomal RNA (rRNA) methods to identify abundant bacterioplankton species and to measure spatial and temporal variation in their activities and distributions. PCR amplification and cloning of rRNA genes in lake water, automated gene sequencing, the design and testing of rRNA probes, and hybridizations to high density pin bolts of 16S rRNA gene clones and environmental RNA samples will be used to identify key microbial species, assess their phylogenetic relationships , and determine their distributions. These approaches will be augmented with radiotracer studies of bacterial production, dissolved organic carbon measurements, direct cell counts, and experiments to identify bacteria that exhibit specialized preferences for dissolved organic carbon from different depths, to provide an integrated perspective on Crater Lake bacterioplankton ecology.
