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

• Published in: Biogeosciences 2021-04, Vol.18 (7), p.2301-2323
• Main Authors: Van Wambeke, France, Pulido, Elvira, Catala, Philippe, Dinasquet, Julie, Djaoudi, Kahina, Engel, Anja, Garel, Marc, Guasco, Sophie, Marie, Barbara, Nunige, Sandra, Taillandier, Vincent, Zäncker, Birthe, Tamburini, Christian
• Format: Article
• Language: English
• Subjects: Affinity; Alkaline phosphatase; Aminopeptidase; Bacteria; Biodiversity and Ecology; Biogeochemistry; By products; Carbon; Cellobiase; Chlorophyll; Chlorophylls; Demand; Environmental Sciences; Enzyme kinetics; Enzymes; Glucosidase; Hydrolysis; Intermediate water; Kinetics; Low concentrations; Nitrogen; Ocean, Atmosphere; Parameters; Phosphatase; Phosphatases; Polysaccharides; Prokaryotes; Proteins; Saccharides; Saturation; Sciences of the Universe; Substrates; β-Glucosidase
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-18-2301-2021

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 µM). 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 (βGLU). Based on the constant derived from the high-LAP-affinity enzyme (0.025–1 µM 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 / βGLU enzymatic ratios and some of the depth-related trends differed between the use of high and low concentrations of fluorogenic substrates.