Dead or alive? A large fraction of ETS-inactive marine bacterioplankton cells, as assessed by reduction of CTC, can become ETS-active with incubationand substrate addition

The majority of bacteria suspended in seawater do not appear to be metaboli cally active or in good physiological condition as assessed by various meth ods. We tested the idea that a large fraction of 'inactive' bacterial cells can become 'active' with respect to detectable cell-specific electron tran sport system (ETS) activity, determined by the ability of cells to reduce t he fluorogenic tetrazolium salt, CTC, when incubated for periods of time wi th or without additional substrate. Aliquots of 1.0 mu m filtered seawater were amended with mixed antibiotics to inhibit DNA synthesis and thus cell division, and incubated at in situ (12.8 and 16.4 degrees C) temperature or at 20 degrees C. Additions included: phosphate (0.83 mM.P, 5.3 mgP l(-1)), ammonium (1.67 mM N, 23.4 mgN l(-1)), and organic carbon as glucose, mixed amino acids or yeast extract (8.33 mM C, 100 mgC l(-1)). At 20 degrees C, the addition of mixed amino acids and yeast extract resulted in a large inc rease of % ETS-active cells (CTC-positive [CTC+] cells), from 1.9-2.4% at 0 h to 55-87% CTC+ cells by 21 to 28 h. At in situ temperature, the increase in % CTC+ cells was less, and the glucose addition caused the greatest inc rease in % CTC+ cells. Under conditions of increased temperature and high c oncentration of organic substrate, a large proportion of the apparently 'in active' bacteria can become highly ETS-active within a day, suggesting that these cells are in fact alive, and are capable of attaining significant me tabolic activity. The different response patterns of the bacterial assembla ges at 20 degrees C compared to those at 12.8 and 16.4 degrees C suggests t hat temperature can be an important factor in bacterioplankton response to increase in substrate concentration.