Activity of marine bacteria under incubated and in situ conditions

We evaluated the extent to which the average metabolic activity of open ocean bacterioplankton changed during 2 to 3 d incubations of 1 mu m filtered seawater at in situ temperature. Indices of bacterial activity during incubation experiments were compared to those of in situ bacterioplankton, which were repeatedly sampled at each oceanic site. Indices included: total and cell-specific incorporation rates of tritiated leucine ([H-3]-Leu) and thymidine ([H-3]-TdR), ratio of Leu to TdR incorporation, and percentage of highly active cells as determined by detectable reduction of the fluorogenic dye, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC). Patterns of change in metabolic activity were similar for experiments with water collected in 3 regions: midshelf, slope, and offshore, off the Oregon coast, USA, and incubated at in situ temperatures of 9.5 to 18 degrees C. At the beginning of the experiments, cell-specific incorporation rates of [H-3]-Leu and [H-3]-TdR were lower compared to in situ rates, but after 19 to 28 h, incorporation rates of [H-3]-Leu and [H-3]-TdR increased dramatically, by more than an order of magnitude, compared to in situ activity. When scaled to the abundance of CTC positive (CTC+) cells, rates of Leu and TdR incorporation were 2.8- to 8.1-fold higher during the incubations compared to activity of in situ CTC+ cells. In this study, marine bacteria in 1 mu m, incubated filtrate exhibited a much larger Variation in metabolic activity than did in situ bacterioplankton. In addition, the proportion of CTC+ cells was closely related to cell-specific rates of Leu and TdR incorporation.