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

• Main Authors: F. Van Wambeke, E. Pulido, P. Catala, J. Dinasquet, K. Djaoudi, A. Engel, M. Garel, S. Guasco, B. Marie, S. Nunige, V. Taillandier, B. Zäncker, C. Tamburini
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-04-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/2301/2021/bg-18-2301-2021.pdf

<p>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 <span class="inline-formula">µ</span>M). As a consequence, <span class="inline-formula">Km</span> (Michaelis–Menten half-saturation constant) and <span class="inline-formula">Vm</span> (maximum hydrolysis velocity) parameters were determined for both low- and high-affinity enzymes for alkaline phosphatase, aminopeptidase (LAP) and <span class="inline-formula"><i>β</i></span>-glucosidase (<span class="inline-formula"><i>β</i></span>GLU). Based on the constant derived from the high-LAP-affinity enzyme (0.025–1 <span class="inline-formula">µ</span>M substrate concentration range), in situ hydrolysis of N proteins contributed 48 % <span class="inline-formula">±</span> 30 % to the heterotrophic bacterial nitrogen demand within the epipelagic layers and 180 % <span class="inline-formula">±</span> 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 % <span class="inline-formula">±</span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3af55808dad7e355d8e0b0b2a0272ce7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-2301-2021-ie00001.svg" width="8pt" height="14pt" src="bg-18-2301-2021-ie00001.png"/></svg:svg></span></span> <span class="inline-formula"><i>β</i></span>GLU enzymatic ratios and some of the depth-related trends differed between the use of high and low concentrations of fluorogenic substrates.</p>