Cell-free extracellular enzymatic activity is linked to seasonal temperature changes: a case study in the Baltic Sea

Abstract. Extracellular enzymatic activities (EEAs) are a crucial step in the degradation of organic matter. Dissolved (cell-free) extracellular enzymes in seawater can make up a significant contribution of the bulk EEA. However, the factors controlling the proportion of dissolved EEA in the marine environment remain unknown. Here we studied the seasonal changes in the proportion of dissolved relative to total EEA (of alkaline phosphatase (APase), β-glucosidase (BGase), and leucine aminopeptidase (LAPase)), in the Baltic Sea for 18 months. The proportion of dissolved EEA ranged between 37 and 100, 0 and 100, and 34 and 100 % for APase, BGase, and LAPase, respectively. A consistent seasonal pattern in the proportion of dissolved EEA was found among all the studied enzymes, with values up to 100 % during winter and  <  40 % during summer. A significant negative relation was found between the proportion of dissolved EEA and temperature, indicating that temperature might be a critical factor controlling the proportion of dissolved relative to total EEA in marine environments. Our results suggest a strong decoupling of hydrolysis rates from microbial dynamics in cold waters. This implies that under cold conditions, cell-free enzymes can contribute to substrate availability at large distances from the producing cell, increasing the dissociation between the hydrolysis of organic compounds and the actual microbes producing the enzymes. This might also suggest a potential effect of global warming on the hydrolysis of organic matter via a reduction of the contribution of cell-free enzymes to the bulk hydrolytic activity.