AME 16:27-35 (1998)  -  doi:10.3354/ame016027

ABSTRACT: The fluorogenic tetrazolium dye 5-cyano-2,3 ditolyl tetrazolium chloride (CTC) has been increasingly used as a measure of cell-specific metabolic activity in bacteria, in that it acts as an electron acceptor in the electron transport system (ETS) of bacterial cells. As such, it is taken to be a direct measure of abundance of those bacterial cells that are actively engaged in respiration. Although it would, thus, be anticipated that microbial respiration should be strongly correlated to the number of CTC-active cells, such a relationship has yet to be demonstrated for natural bacterioplankton communities. CTC was used in situ to assay cell-specific respiratory activity within the pelagic community of Chesapeake Bay. Over the course of sampling, the observed variation in CTC-active cell abundance was 25-fold, substantially greater than the 6-fold variation in total cell abundance (estimated by DAPI staining). As a result, the proportion of CTC-active cells ranged from 3.5 to 47.4% of the total bacterial population, with this proportion varying seasonally as well as spatially. Both abundance and proportion of CTC-active cells were highly correlated with respiration rates within the microplankton community (<3 μm size fraction), explaining 80%, or more, of the variations in these rates. Although respiration rates and total bacterial abundance also tended to covary seasonally, the relationship was not as strong, and large spatial differences in respiration along the Bay could not be explained by total abundance alone. This suggests that the large spatial/temporal variations previously observed in respiration rates are the result of changes in the number of active bacteria, rather than total bacterial abundance. Results of the present study thus encourage the view that CTC provides an ecologically meaningful measure of active bacterial abundance in aquatic systems, and that CTC-active bacteria are likely responsible for the bulk of bacterial community metabolic activity.

KEY WORDS: Bacteria · CTC · Respiration