Physiological responses of marine and brackish water bacterial assemblages in a tidal estuary (Ria de Aveiro, Portugal)

The reactivity of spatially distinct bacterial communities within an estuarine gradient to contrasting water properties was evaluated in the field and experimentally tested in diffusion chambers. In field conditions, it was observed that total and active bacterial numbers were, on average, 3 times higher in brackish water than in marine water. The fraction of active bacteria was, however, similar in both zones (26.6 % on average). Total leucine incorporation, as a measure of biomass productivity, was 3,5 times higher in the brackish water zone following the increase in bacterial population size. Productivity seemed also to depend on the level of activity of individual cells since specific leucine incorporation per active cell was 19 % higher in the brackish water zone. Experimental assays in diffusion chambers showed that when the marine bacterial community was exposed for 6 h to brackish water, the fraction of active bacteria and leucine incorporation increased (20 to 60 % and 50 to 220 %, respectively), The opposite was observed when the brackish water community was exposed to marine water (20 % decrease in the fraction of active bacteria and 50 to 80 % decrease in leucine incorporation). The positive response of marine bacteria to the brackish water conditions was higher when the salinity of the brackish water was artificially increased to 34 psu. Brackish water bacteria, however, responded negatively to this increase in salinity. When the marine community was exposed for only 2 h to brackish water, it promptly showed increased activity. The immediate transfer of this community to marine water for an extra 4 h period induced a return to the initial low activity level. In contrast, the negative response of the brackish water community after 2 h of exposure to marine water was irreversible when transferred back to brackish water. Bacteria remained at a low activity level for the extra 4 h. The high bacterial abundance and production in mid-estuary and the similar patterns of variation of total and active bacteria throughout the system seemed to indicate conservative transport of a euryhaline bacterial community from its main source in the mid-estuary. However, the experimental assays with diffusion chambers invalidated this hypothesis. They suggest the presence of 2 communities: a nutrient-limited marine zone community that grows optimally at high salinity and a nutrient-replete brackish water community requiring salinities below 25 psu.