Metabolic activity of bacterioplankton communities assessed by flow cytometry and single carbon substrate utilization

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 cytometric analysis of bimodal nucleic acid distributions in bacterioplankton stained with a novel fluorescent dye, and (2) utilization of single carbon substrates in Biolog GN Microplates(TM). Both sets of results were compared to bacterial production estimates obtained using a standard technique for H-3-thymidine and H-3-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.