9633712 SHERR Heterotrophic bacteria process a large share of primary production in the sea, and are responsible for a significant portion of respiration and remineralization in marine food webs. Elucidating the roles of bacteria is thus essential to understanding the structure and functioning of marine ecosystems, particularly with respect to the role of marine biota in sequestering atmospheric carbon dioxide. A new epifluorescence, DNA staining method has revealed that bacterioplankton can be divided into two groups of cells: those with, and those without, visible nucleoids (as visualized by the method). Preliminary work suggests that cells with visible nucleoids (NV cells) represent the component of the bacterioplankton assemblage that includes both currently metabolically active and recently active cells. Cells without visible nucleoids (non-NV) appear to be inactive/dormant; however under appropriate conditions such cells can develop visible nucleoids, thus non-NV cells must contain DNA. In coastal bacterioplankton assemblages,the fraction of NV cells (20 ~ - 60%), and of cells separately identified as metabolically active (6% - 40%, was more variable than total bacterial abundance. The ability to identify the currently active + recently active portion of bacterioplankton on a routine basis may provide exciting new component can integrate the response of a bacterial assemblage to environmental change over longer time scales (days-weeks) than would the currently active fraction of the assemblage (time scale of hours-days). In addition, the ratio of currently active:NV cells is a potentially valuable index of the growth state of NV bacteria. This project represents a combined laboratory and field sampling program to evaluate the implications of NV versus non-NV cells in bacterioplankton assemblages. Ideas to be examined include: 1) Non-NV cells can develop visible nucleoids by increasing the amount of DNA/cell. 2) NV cells are subject to greater grazing mortality compared to non-NV cells, which could explain in part the persistence of inactive cells in bacterioplankton. 3) The NV component of bacterioplankton assemblages is more tightly coupled to indices of bottom-up control (chlorophyll-a concentration, bacterial productivity), and of top-down control (abundance of nanoflagellates, rate of bacterivory) than is total bacterial abundance in a coastal upwelling system. The results of this research project should yield important new insights concerning how marine bacterioplankton respond to factors controlling their growth and biomass in the sea. ***
