Spatial patterns of ectoenzymatic kinetics in relation to biogeochemical properties in the Mediterranean Sea and the concentration of the fluorogenic substrate used

[摘要] Ectoenzymatic activity, prokaryotic heterotrophic abundances and production were determined in the Mediterranean Sea. Sampling was carried out in the sub-surface, the deep chlorophyll maximum layer (DCM), the core of the Levantine intermediate waters and in the deeper part of the mesopelagic layers. Michaelis–Menten kinetics were assessed using a large range of concentrations of fluorogenic substrates (0.025 to 50  µ M). As a consequence, Km (Michaelis–Menten half-saturation constant) and Vm (maximum hydrolysis velocity) parameters were determined for both low- and high-affinity enzymes for alkaline phosphatase, aminopeptidase (LAP) and β -glucosidase ( β GLU). Based on the constant derived from the high-LAP-affinity enzyme (0.025–1  µ M substrate concentration range), in situ hydrolysis of N proteins contributed 48 %  ±  30 % to the heterotrophic bacterial nitrogen demand within the epipelagic layers and 180 %  ±  154 % in the Levantine intermediate waters and the upper part of the mesopelagic layers. The LAP hydrolysis rate was higher than bacterial N demand only within the deeper layer and only when considering the high-affinity enzyme. Based on a 10 % bacterial growth efficiency, the cumulative hydrolysis rates of C proteins and C polysaccharides contributed on average 2.5 %  ±  1.3  % to the heterotrophic bacterial carbon demand in the epipelagic layers sampled (sub-surface and DCM). This study clearly reveals potential biases in current and past interpretations of the kinetic parameters for the three enzymes tested based on the fluorogenic-substrate concentration used. In particular, the LAP  /   β GLU enzymatic ratios and some of the depth-related trends differed between the use of high and low concentrations of fluorogenic substrates.

[学科分类] 大气科学