Within the past decade it has become evident that bacterioplankton contribute significantly to biomass and biogeochemical activity in planktonic systems. Until recently progress in identifying the microbial species which constitute these communities was slow because the majority of the organisms present (direct counts) could not be recovered in culture. Recently introduced molecular techniques are providing the means to study the population dynamics of uncultivated microbial species. This project is based on molecular data which demonstrates the existence of a novel microbial group, the SAR11 cluster, and indicates that this group accounts for about 20% of the non- cyanobacterial microbial activity in the Sargasso Sea. The SAR11 cluster is a related group of species which were detected by cloning 16S ribosomal DNA genetic markers from a natural Sargasso Sea bacterioplankton population. This research will extend earlier observations by providing time-series and depth profile information on the distribution of these organisms at the BATS station in the Sargasso Sea, where they were first observed. The dynamics of seasonal changes in the Sargasso Sea bacterioplankton community will be measured using specific DNA probes for the SAR11 cluster (SAR11R) and marine Synechococcus (SYNF), as well as general probes for eubacterial, eukaryotic and universal small subunit ribosomal RNAs. The specific goals of the project are: 1) to test the hypothesis that the SAR11 cluster population reaches a numerical maximum during summer stratification; 2) to test the hypothesis that the SAR11 cluster is distributed widely in the upper euphotic zone ofoligotrophic oceanic regions; 3) to test the hypothesis that the distribution of the SAR11 cluster is correlated with aerobic photoheterotrophic activity; 4) to provide parallel time series information on the distribution of other planktonic bacterial groups which are being revealed by further genetic studies nearing completion. The theory that bacterial photoheterotrophy is an active process in the open ocean can potentially have a dramatic influence on calculations of carbon flux through the microbial loop. The standing crop of "heterotrophic" bacterioplankton is too large to be explained form primary productivity estimates using a standard 0.6 respiratory conversion factor, but could be better explained using the higher growth efficiencies associated with photoheterotrophy.
