Low fractions of active bacteria in natural aquatic communities?

Abstract

The notion that a significant fraction of individ- ual cells within natural bacterial assemblages is not actively engaged in cellular metabolism, although not a new idea, remains fairly contentious. Different approaches for probing the metabolic activity of individual cells often yield widely divergent estimates of the proportion of active cells, with some methods suggesting very low levels of individual cell activity. We comment on 2 aspects of the current discussions regarding cell-specific activity in natural bacterioplankton. First, the apparent perception that most aquatic bacteria must be active is not uniformly supported by the data. In a system- atic survey of the microautoradiography literature, for exam- ple, only 4 out of 23 studies reported a mean proportion of active cells in natural communities that was greater than 50%, and the mean across all such studies was 30%. Second, we propose that the problem of describing bacterioplankton single-cell activity is best approached from the viewpoint that there is a nested hierarchy of physiological states within bacterial communities. The lack of agreement among various methods points to the large range of criteria possible for describing metabolic activity in bacteria. In this regard, the discrete, and over-simplistic, notion of 'active' versus 'inactive' is not particularly useful and should be replaced by a conceptual model in which there exists a continuum of possible single-cell activities.