Patterns and Mechanisms of Variability in the Physiological State of Bacteria in Marine Pelagic Environments

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Frischer 9906734Bacteria are thought to be second only to phytoplankton in their role of transforming organic carbon in aquatic ecosystems. In open ocean waters, the biomass of bacteria has been reported to exceed that of all other living plankton, and even in continental shelf and coastal waters it is significant compared to other plankton. However, these estimates are all derived from the assumption that all DAPI-stained cells are metabolically active cells. This hypothesis no longer appears tenable: a large fraction of apparent bacteria are either inactive, moribund, or dead. Such a conclusion threatens to precipitate another paradigm shift in conceptual models of ecosystem function. At the very least, it would imply that, at a given time, comparatively few bacteria in situ are growing (and respiring) much faster than previously thought, and that a substantial fraction of DAPI-stained bacteria either exhibit low activity or are operationally a component of detritus.A novel combination of a fluorescent stain and molecular probes has been developed which quantitatively identify those cells with compromised membranes, and those cells containing sufficient rRNA to be considered metabolically active. While neither approach might be considered sufficient to specify active from inactive, moribund, or dead cells, together they comprise a powerful tool to investigate the relative importance of these cell types in situ, and to postulate which sources of bacterial mortality or stimulation may be important at a given time and place. Recent and continuing studies have thoroughly evaluated and confirmed the applicability of the vital stain and probe (VSP) method to quantitatively and unambiguously discriminate among living, nonliving, and low activity cells. In a related but separate study, a quantitative technique also has been developed to determine concentrations of detritus, operationally defined as suspended nonliving POC. These efforts include newly developed image analysis protocols and algorithms to recognize and enumerate the different groups of VSP cells, and also free vs. attached bacteria.Here, mechanistic studies will investigate the effects of environmental parameters on patterns of VSP staining of bacteria; conduct seasonal transects to determine the physiological status of natural bacterial communities across a productivity and detritus gradient; and measure the time-dependent response of natural communities and associated detritus to upwelling-induced phytoplankton blooms on the outer U.S. southeastern continental shelf. Of special interest are evaluating the biological significance (if any) and the triggers of shifts in the fraction of metabolically active populations. Ultimately, the significance may not lie in whether it is 40% vs. 60% of a community which is active, but whether specific functional groups are active or slow growing, e.g. nitrate reducers. To address such issues, the questions posed here must first be answered. Moreover, the results will provide a unique field dataset to evaluate current conceptual models of the role of bacteria in transformations between DOC, detritus, and bacteria.