Phosphomonoesterase and phosphodiesterase activities in the Eastern Mediterranean Sea during stratified versus mixed conditions

Abstract

Abstract. Dissolved organic phosphorus hydrolysis by marine planktonic microorganisms is a key process in the P cycle, particularly in P-depleted, oligotrophic environments. The present study assessed spatio-temporal variations of phosphomonoesterase (PME) and phosphodiesterase (PDE) activities using concentration kinetics in the eastern Mediterranean Sea during 2 contrasted situations: the end of winter (including a small bloom period), and autumn. The distribution and regulation of the maximum hydrolysis rates (Vm) and half-saturation constants (Km) of both ectoenzymes were assessed in relation to the vertical structure of the epipelagic layers. PME reached their maximum activities (Vm) after 1 µM MUF-P addition whereas, for PDE, it was necessary to add up to 50 µM bis-MUF-P to reach saturation state. On average, the Km of PDE was 33 ± 25- times higher than that of the PME. Vm of PME and Vm of PDE were linearly correlated. Conversely to the Km values, Vm were on the same order of magnitude for both ectoenzymes, their ratio (Vm PME:Vm PDE) ranging between 0.2 and 6.3). Dissolved organic phosphorus (DOP) and the phosphomonoesterease hydrolysable fraction of DOP explained mostly no variability of Vm PME nor Vm PDE. On the contrary, Vm of both phosphohydrolase enzymes was inversely correlated to DIP concentration. The particular characteristics of concentration kinetics obtained for PDE (saturation at 50 µM, high Km, high turnovertimes) are discussed with respect to possible inequal distribution of PDE and PME among organic material size continuum, and accessibility to phosphodiesters.