Short-Term Responses of the Natural Planktonic Bacterial Community to the Changing Water Properties in an Estuarine Environment: Ectoenzymatic Activity, Glucose Incorporation, and BiomassProduction.

The possibility that two principal bacterial communities expressing different levels of heterotrophic activity might coexist in an estuarine ecosystem (Ria de Aveiro, Portugal) and could quickly respond to tidal fluctuations of environmental factors was experimentally tested in diffusion chambers by swapping the dissolved components of the natural water between the two communities and comparing their reactivity against the unaltered controls. The results for ectoenzymatic activity (Leu-aminopeptidase and b-glucosidase), glucose incorporation and biomass production after transference of the marine bacterial community to brackish water showed maxima in the range of 241-384% of the control values. The opposite transference of the brackish-water bacterial community to marine water produced maximal decreases to 0.14-0.58% of the control values. In a reverse experiment, designed as the return to the initial conditions after 2 hours of the first exposure, the marine community rapidly re-acquired the characteristic low profile of activity. Contrastingly, the negative effects of 2 hours of exposure to marine water on the activity of the brackish water bacteria persisted, at least for 4 hours, after return to their own water. The apparent short-term irreversibility of the decline in activity of the brackish water bacteria when exposed to marine water, in parallel with the quick and reversible positive response of the marine water bacteria to the brackish water, suggests the development of two distinct bacterioplankton communities adapted to the environmental conditions prevailing at distinct sections of the estuary. The reactivity to environmental changes demonstrated by the two communities allows the prediction of estuarine profiles of bacterial activity steeper than those expected from the conservative transport of bacterial cells associated with tidal currents.