Exoproteolytic activity in an Atlantic pond (France): estimates of in situ activity

In order to estimate in situ bacterial exoproteolysis and its contribution to bacterial secondary production (BSP), we studied microbial activities (BSP and kinetics of exoproteolytic activity) in relation to dissolved combined amino acids (DCAA) during the survey of an Atlantic marine pond. In this pond, high concentrations of DCAA (mean = 4.27 mu M), probably generated by zooplankton grazing, allowed large bacterial biomass development and high production (max = 10(11) cells l(-1) and 18.8 x 10(9) cells l(-1) d(-1) respectively). Exoproteolytic enzyme activity (mean V-m = 55.9 mu M d(-1)) was tightly cou pled to bacterial production, suggesting that protein utilization was essential for bacterial growth. Calculation, according to the Michaelis-Menten equation, of the dissolved protein in situ hydrolysis rates using the kinetic parameters (V-m and K-m) and the ambient concentration of DCAA proved to be a better estimation of the actual rate of hydrolytic activity than the potential activity based on V-m only. The rates found with this calculation agreed with those obtained from different environments with other methods (DCAA and labelled proteins uptake). Furthermore, our estimates of in situ proteolysis were consistent with BSP (nitrogen fluxes originating from dissolved protein hydrolysis averaging 56% of the N bacterial production), and as a result, these estimates seem to provide a realistic estimation of the actual dissolved protein hydrolysis rates.