| Summary: | We have simultaneously studied the abundance, activity and prokaryotic community structure in relation to the quality of oceanic dissolved organic matter (DOM) in the meso- and bathypelagic waters off the Galician Coast (NW Spain, from 43ºN, 9ºW to 43ºN, 15ºW).Distint water masses were identified based on their physical and chemical characteristics. While prokaryotic heterotrophic production decreased from the euphotic layer to the bathypelagic waters by two orders of magnitude, prokaryotic abundance decreased only by one order of magnitude. The relative abundance of SAR11 and Alteromonas, determined by catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH) decreased with depth, while the contribution of SAR 202 and SAR324 to total bacterial abundance as well as the percentage of total prokaryotes identified as Crenarchaeota was significantly higher in the deeper layers (NEADW, North East Atlantic Deep Water and LDW, Lower Deep Water) than in the euphotic zone. Bacterial abundance showed a positive correlation with the absorption coefficient at 254 nm and the dissolved organic carbon, while was negatively correlated to marine humic-like substances. Crenarchaeotal abundance was correlated positively to protein-like fluorescence and the molecular weight of the DOM On the other hand, Bacterial and archaeal community structure were analyzed by ARISA (automatic rRNA intergenic spacer analysis) and T-RFLP (terminal restriction fragment length polymorphism) fingerprints respectively. Both bacterial and archaeal community structures were depth-stratified, with a total of 305 and 48 operational taxonomic units (OTUs) respectively, as revealed. A linear model approach (DISTLM,) revealed that the variability found in both bacterial and archaeal community was mainly described by nitrate, phosphate, temperature, coloured and fluorescent DOM, prokaryotic abundance and prokaryotic leucine incorporation. Our results displayed a prokaryotic community structure based not only on depth-related physicochemical ...
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