Relation Between Presence-Absence of a Visible Nucleoid and Metabolic Activity in Bacterioplankton Cells

We investigated the report of Zweifel and Hagstrom that only a portion of marine bacteria contain nucleoids-the DNA-containing regions of procaryotic cells-and that such bacteria correspond to the active or viable fraction of bacterioplankton. In Oregon coastal waters, 21-64% of bacteria had visible nucleoids; numbers of nucleoid-visible (NV) bacteria were greater than numbers of metabolically active bacteria, based on cells with active electron transport systems (ETS) and intact cell membranes. During log growth of a marine isolate, proportions of NV and ETS-active cells approached 100%. In stationary growth phase, the fraction of ETS-active cells decreased rapidly, while that of NV cells remained high for 7 d. When starved cells of the isolate were resupplied with nutrient (50 mg liter-1 peptone), total cell number did not increase during the initial 6 h, but the proportion of NV cells increased from 27 to 100%, and that of ETS-active cells from 6 to 75%. In an analogous experiment with a bacterioplankton assemblage, a similar trend was observed: the number of NV cells doubled during the inital 6 h prior to an increase in total cell counts. These results show that some bacteria without visible nucleoids are capable of becoming NV cells, and thus have DNA in a nucleoid region not detectable with the method used here.