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Prokaryotic abundance and activity are commonly assessed by dividing them into two size-fractions: free-living and attached to particles. Nevertheless, organic matter, essential for the growth of heterotrophic prokaryotes, is present in the environment in a continuum of sizes, from purely dissolved to large particles. Therefore, defining the activity of the prokaryotic community would be more accurate by considering all the distinct size fractions. To achieve this, we measured prokaryotic abundance (PA), heterotrophic prokaryotic activity (as leucine incorporation) and extracellular enzyme activities at a coastal site in the NW Mediterranean Sea. We conducted measurements in both bulk seawater and size fractionated samples sequentially passing through 5 different filter types: 0.2-0.8-3-5-10 μm pore size. Our results indicate that the fraction <0.8 μm contained the highest percentage of cells (91.6 ± 1.1 %) and leucine incorporation rates (72.2 ± 3.5 %). Most of the extracellular enzyme activity appeared in the dissolved fraction (<0.2 μm; 19.8-79.4 %), yet the specific activity of the enzymes (per cell activity) was 100-1000 times higher in the particulate (>0.8 μm) than in the free-living (0.2-0.8 μm) fraction. The size fraction with highest specific activities for leucine incorporation and most of the enzyme activities (β-glucosidase, esterase, Leu-aminopeptidase and alkaline phosphatase) was the 5-10 μm fraction. In contrast, the higher specific chitobiase activity in the >10 μm fraction, suggests that the prokaryotic community colonizing large particles might be more specialized in the hydrolysis of organic matter of zooplanktonic origin than the community colonizing smaller particles., Competing Interests: Declaration of competing interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

The SAR11 cluster and the Group I of marine Archaea represent probably the best two examples of uncultured marine prokaryotes of widespread occurrence. To study their microdiversity and distribution, a total of 81 and 48 clones, respectively, were sequenced from Mediterranean and Antarctic waters at different locations and depths. The DNA regions chosen for the analysis were the last third, approximately, of the 16S rRNA gene and the 16S-23S intergenic spacer (also known as internal transcribed spacer [ITS]). There was a high concordance in both, even with the extremely variable ITS, where potential probes have been proposed for the identification and isolation of these micro-organisms. In terms of community structure, our results show that although depth-related factors seem to be predominant in the final associations of the clones, geography also plays a significant role. A major group of surface-associated sequences was found in both SAR11 and marine Archaea. In both cases this group was relatively homogeneous containing little diversity in terms of sequence, while sequences retrieved from deep samples and some surface clones contained much more heterogeneity. As a whole, both groups of prokaryotes seem to fall within the limits of well-defined taxonomic units.