The majority of bacteria suspended in seawater do not appear to be metabolically active or in good physiological condition as assessed by various methods. We tested the idea that a large fraction of 'inactive' bacterial cells can become 'active' with respect to detectable cell-specific electron transport system (ETS) activity, determined by the ability of cells to reduce the fluorogenic tetrazolium salt, CTC, when incubated for periods of time with or without additional substrate. Aliquots of

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... 0 pm 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"C) temperature or at 20°C. Additions included: phosphate (0.83 mM P, 5.3 mgP 1-'), ammonium (1.67 mM N, 23.4 mgN l-'), and organic carbon as glucose, mixed amino acids or yeast extract (8.33 mM C, 100 mgC 1-l). At 2OoC, the addition of mixed amino acids and yeast extract resulted in a large increase 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 increase in % CTC+ cells. Under conditions of increased temperature and high concentration of organic substrate, a large proportion of the apparently 'inactive' bacteria can become highly ETS-active within a day, suggesting that these cells are in fact alive, and are capable of attaining significant metabolic activity. The different response patterns of the bacterial assemblages at 20°C compared to those at 12.8 and 16.4"C suggests that temperature can be an important factor in bactenoplankton response to increase in substrate concentration.