Increase in the proportion of metabolically active bacteria along gradients of enrichment in freshwater and marine plankton: implications for estimates of bacterial growth and production rates
It is generally recognized that a fraction of all bacterioplankton cells enumerated using conventional epifluorescence techniques is neither growing, dividing nor metabolically active, but the variation in the proportion of active cells among aquatic systems is not well understood. Here, we hypothesize that the proportion of metabolically active cells increases systematically along gradients of enrichment, and to test this hypothesis the number and proportion of metabolically active planktonic bacteria were investigated during the summer in a set of 24 temperate lakes, which span a considerable range in productivity. The tetrazolium salt 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was used as an indicator of cells with an active electron transport system. The total number of bacteria ranged from 1.88×106 to 7.70×106 ml−1, whereas the number of active cells was more variable and ranged from 0.37×106 to 2.18×106 ml−1 in the study lakes. The proportion of metabolically active cells ranged from 15 to 33%, and tended to increase with nutrient and chlorophyll concentrations, but not with dissolved organic carbon (DOC). Data on the number of total and active bacteria culled from the literature for marine, estuarine and freshwater systems show that the trends we measured in lakes are valid for pelagic systems in general. Over a broad range of aquatic systems, the total number of bacteria varied by three orders of magnitude, whereas the number of active bacteria varied by four orders of magnitude as system productivity increased. The proportion of active cells increased from ultraoligotrophic openocean areas (50%). Our results suggest that in most aquatic systems there is a pool of rapidly growmg cells, embedded in a usually larger matrix of inactive bacteria, and that the relative size of the active and inactive pools varies systematically among bacterial populations along gradients of enrichment.