During the summer of 1994, bacterioplankton abundance and metabolism were examined in seawater from Bedford Basin, Nova Scotia. Canada. Two new methods were applied to independently assess metabolic activity: (1) flow cytometnc analysis of bimodal nucleic acid distributions in bacterioplankton stained with a novel fluorescent dye, and (2) utilization of single carbon substrates in Biolog GN Microplatcsl'' Both sets of results were compared to bacterial production estimates obtained using a standard technlque for 'H-thymidine and 3H-leucine incorporation. Flow cytometry quantified the relative abundance of the bacterioplankton cells with a high apparent nucleic acid content, which was expressed as an active cell index (ACI) of dividing and/or actively metabolizing cells. The ACI was positively correlated with sole carbon source utilization at 1, 5 and 10 m (p < 0 05). Variations in ACI and sole carbon source utilization followed trends similar to, but could not be directly correlated with, bacterial production throughout the summer period. The new methods provided information that could not be obtained using the standard techniques for bacterial production. Instead, they yielded new and complementary information on the metabolic state of dividing and/or metabolizing cells and insight into the regulation of bacterial production.

Dilution experlments were conducted in the Caribbean and Sargasso Seas in a n attempt to measure bacterial growth under reduced grazing pressure. In this type of experiment, a small volume of the intact plankton sample is diluted into the same seawater from which all organisms have presumably been removed by filtration We noted that d~l u e n t prepared by vacuum filtration (< 100 mm Hg pressure) of natural seawater through 0.2 pm Nuclepore membranes contained bacterla which could be retained by the same membranes upon refiltration Upon ~ncubation, these bacteria rapidly accumulated 3H-amino acids and grew at high rates (0.18 h-') The literature supports the evidence of ultramicrobacteria (those which might be expected to pass through 0.2 pm membranes): these may have passed into the diluent but were subsequently retained on 0 2 pm membranes poss~bly because they increased in cell size, formed ultramicrocolonies, or were aspherical In shape so that the orientation in relation to the membrane pore determined whether the cell was filtered or retained. Although the apparent specific growth rate of bacteria was in inverse relation to the degree of sample dilution, interpretable as a consequence of grazing on bacteria, the growth of bacteria in the diluent made it problematic to consider the dilution experlments in the conventional manner. It IS likely that the 0.22 pm cellulose ester membranes used by many others who have performed such experiments would b e much more effective in retaining ultramicrobacteria.