Cytometric characteristics of bacterioplankton cells

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Heterotrophic bacterioplankton abundance and production were determined along vertical (down to bathypelagic layers) and latitudinal (from 4.9 • E to 32.7 • E) gradients across the Mediterranean Sea in early summer 2008. Abundance and flow cytomet-ric characteristics (green fluorescence and side scatter signals) of high nucleic acid 5 (HNA) and low nucleic acid (LNA) bacterial cells were investigated using flow cytom-etry. Contrarily to what is generally observed, the percentage of total bacteria represented by HNA cells (%HNA, range 30–69%) decreased with increased bacterial production (range 0.15–44 ng C l −1 h −1) although this negative relation was poorly explained (log-log regression r 2 = 0.19). The %HNA as well as the mean side scatter 10 of this group increased significantly with depth in the meso and bathypelagic layers. Our results demonstrated that vertical stratification with regard to chlorophyll distribution above, within or below the deep chlorophyll maximum plays an important role in influencing the distribution of cells, and in the relationships between the flow cytometric parameters and environmental variables such as chlorophyll a or bacterial production. 15 Relationships between green fluorescence and side scatter of both HNA and LNA cells depended largely on chlorophyll distribution over the water column, suggesting that the dynamic link between HNA and LNA cells differs vertically.