Hybridization measurements made with phylogenetic group-specific probes for bacterioplankton ribosomal RNAs have shown that several major bacterioplankton groups form highly stratified populations in the lower ocean surface layer in the northwestern Sargasso Sea. Other studies at the same site have uncovered evidence of a seasonal cycle in which dissolved organic carbon (DOC) accumulates in the upper surface layer during the vernal period and is exported to the lower surface layer during winter mixing. This study proposes to examine the relationship between the composition of bacterioplankton communities and the source, amount and lability of DOC in the water column by conducting seawater culture experiments that combine water and inocula from different depths. High-resolution electropherograms of amplified ribosomal RNA genes will be used to survey community composition. Electropherogram results will be confirmed by hybridizations of total community RNA to phylogenetic group-specific probes. Data from this study will be compared to ongoing measurements of bacterioplankton production, biomass and DOC that are a part of the Bermuda Atlantic Time-Series Study.
