| Sumario: | Some traits of the bacterial community dynamics associated to the rhizosphere of the Mediterranean seagrass Posidonia oceanica, growing in carbonate sediments, were analyzed during a 2-year period in an enclosed bay of the Balearic Islands. The diversity of the bacterial community was studied by the construction of 16S rDNA clone library. For testing temporal and vertical differences in the abundance of total cells and active Bacteria, we used 4',6-diamidino-2-phenylindole (DAPI) staining and fluorescence in situ hybridization (FISH). Moreover, some relevant groups of sulfate-reducing bacteria (SRB) were occasionally assessed by FISH. Despite the observed decrease in the total DAPI-stained cells, bacterial counts, and sulfate reduction rates throughout the sampling time, we found an increase in both the pore-water sulfide concentration and the proportion of SRB. Overall, the results revealed a very high bacterial diversity and indicated shifts in bacterial dynamics that could not be related to temperature-dependent factors, suggesting a link between the documented regression of the seagrass meadow and the decline of the microbial community, likely due to large organic matter inputs to the bay. © 2008 Coastal and Estuarine Research Federation.
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