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

The reactivity of spatially distinct bacterial communities within an estuar ine gradient to contrasting water properties was evaluated in the field and experimentally tested in diffusion chambers. In field conditions, it was o bserved that total and active bacterial numbers were, on average, 3 times h igher in brackish water than in marine water. The fraction of active bacter ia was, however, similar in both zones (26.6 % on average). Total leucine i ncorporation, as a measure of biomass productivity, was 3,5 times higher in the brackish water zone following the increase in bacterial population siz e. Productivity seemed also to depend on the level of activity of individua l cells since specific leucine incorporation per active cell was 19 % highe r in the brackish water zone. Experimental assays in diffusion chambers sho wed that when the marine bacterial community was exposed for 6 h to brackis h water, the fraction of active bacteria and leucine incorporation increase d (20 to 60 % and 50 to 220 %, respectively), The opposite was observed whe n the brackish water community was exposed to marine water (20 % decrease i n the fraction of active bacteria and 50 to 80 % decrease in leucine incorp oration). The positive response of marine bacteria to the brackish water co nditions was higher when the salinity of the brackish water was artificiall y increased to 34 psu. Brackish water bacteria, however, responded negative ly to this increase in salinity. When the marine community was exposed for only 2 h to brackish water, it promptly showed increased activity. The imme diate transfer of this community to marine water for an extra 4 h period in duced 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 w ater was irreversible when transferred back to brackish water. Bacteria rem ained at a low activity level for the extra 4 h. The high bacterial abundan ce and production in mid-estuary and the similar patterns of variation of t otal and active bacteria throughout the system seemed to indicate conservat ive transport of a euryhaline bacterial community from its main source in t he mid-estuary. However, the experimental assays with diffusion chambers in validated this hypothesis. They suggest the presence of 2 communities: a nu trient-limited marine zone community that grows optimally at high salinity and a nutrient-replete brackish water community requiring salinities below 25 psu.