Heterotrophic bacterial growth and substrate utilization in the oligotrophic Eastern Mediterranean (Aegean Sea)
Heterotrophic bacterial growth and substrate utilization were studied in March and September of 1997 in the oligotrophic Aegean Sea. Maximum velocities of ectoproteolytic activity (ectoaminopeptidase ctivity, EAP), as well as amino acid assimilation and respiration rates (AA-A, AA-R) were measured along with bacterial production (protein synthesis). At the northern stations which are influenced by the input of the Black Sea waters, rates at 5 m depth of EAP, AA-A and bacterial production were 2 to 3 times higher than at southern stations. Influenced by the Black Sea water, mean bacterial numbers in the 0-100 m layer showed typical oceanic concentrations averaging 0.7 x 10 6 cells ml -1 . These values, along with low bacterial production rates (30 ng C l -1 h -1 ) implied slow growth for bacteria and/or that a large number among them were inactive. Neither bacterial abundance nor production were correlated with primary production. The percentage of amino acids respired was higher in September compared to March, particularly in the northern Aegean (mean 69 %). The enzyme kinetic analysis showed a biphasic model, the transition between the high and low affinity enzymes being obtained at 50 ΜM. Ectoaminopeptidase activity was weakly correlated with bacterial production (p < 0.05), but strongly correlated with respiration rates of amino acids (p < 0.001), suggesting that the substrate used was devoted to maintain energy requirements.