Daily variations of highly active bacteria in the Northern Adriatic Sea

Nowadays, it is recognized that only a fraction of aquatic bacteria are actively growing, but there is little information about the factors constraining their metabolism. Marine bacterioplankton can rapidly modify their metabolic activity level in response to environmental changes. In this study, we focused on the daily changes in abundance and activity of active bacterial fraction over a 20-day period preceded by intense rainfalls which slightly modified water column conditions. Cells capable of reducing the membrane-penetrable dye 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), estimated by epifluorescence microscopy, are considered very active (CTC+ bacteria). Total bacterial abundance (TBA) ranged from 0.8 to 2.4 ·X 109 cells L-1, whereas CTC+ bacteria were more variable(1.6–9.2 ·X 107 cells L-1), accounting for 1.2–4.4% of TBA. Bacterial activity (BA) quantified as the incorporation of [3H]-leucine varied by more than one order of magnitude over the period (25.0–662.5 pmol L-1 h-1). BA was strongly related to CTC+ bacteria, suggesting that they were mainly responsible for the bacterial community metabolism. Nevertheless, cell-specific activity, scaled to only CTC+ cells, was very high, suggesting that a fraction of cells not detectably CTC+ may be able to assimilate [3H]-leucine. The correlation between salinity and TBA, CTC+ bacteria and BA supported the hypothesis of the active role of freshwater input in enhancing cell activity. Our results suggest that freshwater inputs rather than phytoplanktonic blooms are able to induce shifts in bacterial metabolism over a time scale of days in the area studied.