Active versus inactive bacteria: Size-dependence in a coastal marine plankton community

By directly measuring the size distribution of active (cells that took up and reduced the redox dye CTC, 5-cyano-2,3-ditolyl tetrazolium chloride) and inactive cells in a natural coastal bacterial community, we tested the hypothesis that the likelihood of a bacterium being active in marine plankton is a function of its size. The average size of an inactive bacterium was 0.055 mu m(3) while the average size of an active bacterium was 0.12 mu m(3). This average size was constant even after 3 d of incubation in dialysis bags placed in situ, which increased the percentage of active bacteria in the community from 6 to ca 43%. The probability of a bacterium being active was a linear function of its size, from ca 5% for cells of 0.01 mu m(3) to 100% for cells of the largest sizes. These results (1) support the hypothesis of Stevenson (1978, Microb Ecol 4:127-133) that very small bacteria are mainly dormant (inactive) while bigger bacteria are more likely to be active; (2) reconcile 2 apparently opposing observations, (a) commonly found higher specific activities in the larger size classes of bacterioplankton and (b) allometry regularities by which smaller unicellular organisms tend to have higher specific growth rates than larger organisms of similar metabolic mode; and (3) suggest that phagotrophic protists will preferentially crop the active portion of the bacterial community if they select their prey according to size.