Abstract
The surface of the Mediterranean Sea is a low-phosphate-low-chlorophyll marine area where marine het-erotrophic prokaryotes significantly contribute to the biogeo-chemical cycles of all biogenic elements such as carbon, notably through the mineralization of dissolved organic compounds. Cell-specific leucine incorporation rates were determined in early summer in the open stratified Mediter-ranean Sea. The bulk leucine incorporation rate was on average 5 ± 4 pmol leu l −1 h −1 (n = 30). Cell-specific 3 H-leucine incorporation rates were assayed using flow cy-tometry coupled to cell sorting. Heterotrophic prokaryotes (Hprok) were divided into cytometric groups according to their side scatter and green fluorescence properties: high nucleic acid containing cells (HNA) with high scatter (HNA-hs) and low scatter (HNA-ls) and low nucleic acid containing cells (LNA). Cell-specific leucine incorporation rates of these cytometric groups ranged from 2 to 54, 0.9 to 11, and 1 to 12 × 10 −21 mol cell −1 h −1 , respectively. LNA cells represented 45 to 63% of the Hprok abundance, and significantly contributed to the bulk leucine incorporation rates, from 12 to 43%. HNA/LNA ratios of cell-specific leucine incorporation were on average 2.0 ± 0.7 (n = 30). In surface layers (from 0 m down to the deep chlorophyll depth, DCM), cell-specific rates of HNA-hs were elevated (7 and 13 times greater than LNA and HNA-ls, respectively). Nevertheless , on average HNA-hs (26%) and LNA (27%) equally contributed to the bulk leucine incorporation in these layers. Prochlorococcus cells were easily sorted near the DCM and displayed cell-specific leucine incorporation rates ranging from 3 to 55 × 10 −21 mol leu cell −1 h −1 , i.e. as high as HNA-hs'. These sorted groups could therefore be defined as key-players in the process of leucine incorporation into proteins. The mixotrophic features of certain photosyn-thetic prokaryotes and the high contribution of LNA cells to leucine incorporation within the microbial communities of the Mediterranean could be reinforced.