Spatial patterns of biphasic ectoenzymatic kinetics related to biogeochemical properties in the Mediterranean Sea

Abstract. Prokaryotic ectoenzymatic activity, abundance and heterotrophic production were determined in the Mediterranean Sea, within the epipelagic and the upper part of the mesopelagic layers. The Michaelis-Menten kinetics were assessed, using a range of low (0.025 to 1 µM) and high (0.025 to 50 µM) concentrations of fluorogenic substrates. Thus, Km and Vm parameters were determined for both low and high affinity systems for alkaline phosphatase (AP), aminopeptidase (LAP) and β-glucosidase (βGLU). Based on the constant derived from the high AP affinity system, in-situ hydrolysis rates of N-protein contributed of 48 % ± 30 % for the heterotrophic prokaryotic nitrogen demand within epipelagic waters and of 180 % ± 154 % within deeper layers. LAP hydrolysis rate was higher than bacterial N demand only within the deeper layer, and only based on the high affinity system. Although ectoenzymatic hydrolysis contribution to heterotrophic prokaryotic need was high in terms of N, but low in terms of C. Based on a 10 % bacterial growth efficiency, the cumulative hydrolysis rates of C-proteins and C-polysaccharides contributed to a small part of the heterotrophic prokaryotic carbon demand, on average 2.5 % ± 1.3 % in the epipelagic layers. This study notably points out the biases in current and past interpretation of the relative activities differences among the 3 tested enzymes, in regard to the choice of added concentrations of fluorogenic substrates. In particular, enzymatic ratios LAP/βGLU, as well as some trends with depth, were different considering activities resulting from the high or the low affinity system.