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9 pages, 6 figures, 2 tablesGrowth and grazing mortality of marine heterotrophic bacteria were measured in the summer of 2000 in coastal waters of the NW Mediterranean Sea. Serial-dilution experiments were performed with water from surface and deep chlorophyll maximum (DCM) layers. Bacterial abundances (mean ± SD) were very similar at the surface (7.2 ± 2.9 × 10^5 cells ml–1 and DCM (7.4 ± 1.1 × 10^5 cells ml–1). Intrinsic bacterial growth rates (mean ± SD) were 0.88 ± 0.43 d–1 in the surface layer and 0.71 ± 0.23 d–1 at the DCM. Grazing rates on bacteria (mean ± SD) were 0.75 ± 0.23 and 0.58 ± 0.29 d–1, in the surface and DCM layers, respectively. Nucleic acid content analysis by flow cytometry revealed different intrinsic growth rates and grazing pressure on bacteria of high (HNA) and low (LNA) content depending on their location in the water column. Generally, growth and grazing rates were balanced in both groups in both layers. At the surface, HNA bacteria revealed significantly higher intrinsic growth rates than LNA bacteria (1.18 ± 0.60 and 0.47 ± 0.28 d–1, respectively). Average growth rates at the DCM were higher for LNA (0.90 ± 0.46 d–1) than for HNA (0.36 ± 0.23 d–1), but not significantly. At the surface, grazing rates on HNA bacteria were also significantly higher than on LNA bacteria (1.02 ± 0.31 and 0.37 ± 0.19 d–1, respectively). At the DCM, the opposing tendency, though not statistically significant, was observed (0.26 ± 0.17 and 0.77 ± 0.50 d–1). Bacteria were responsible for a large portion of the C flux through the system. Bacterial C flux was funneled mostly by HNA bacteria at the surface (70%) and by LNA at the DCM (80%). HNA bacteria were the most active component of the bacterial community in the surface layer. However, we found that LNA bacteria were also active, particularly at the DCM, suggesting differences in structure and functioning of the corresponding microbial networks. The most significant result was the clear relation between depth and activity of each bacterial fraction.Field work was supported by research projects ARO2000 (MAR99-1202) and DILEX (MAR98-0855), and writing by EFLUBIO (REN2002-04151-C02-01), all funded by the Spanish Ministry of Education and Science. We thank the captain and crew of the RV 'García del Cid'.Peer reviewe

aquatic microbial ecology

Growth, grazing and carbon flux of high and low nucleic acid bacteria differ in surface and deep chlorophyll maximum layers in the NW Mediterranean Sea