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Seasonal changes of microbial abundance and associated extracellular enzymatic activity in marine snow and in seawater were studied in the northern Adriatic during a three-year period. Marine snow was present during the entire period of investigation, although in higher concentrations during summer than during winter. Microorganisms densely colonized marine snow and aggregate-associated enzymatic activity was substantially higher (up to 105 times) than in seawater. Alkaline phosphatase activity (APA) and aminopeptidase activity in marine snow showed seasonal variations with higher values in late spring-summer than in autumn-winter, probably in response to changes in the quantity and quality of organic matter. The highest cell-specific bacterial activity was found for phosphatase, followed by peptidase, and the lowest was for glucosidases. Differential hydrolysis of marine snow-derived organic matter points to the well-known phosphorus limitation of the northern Adriatic and indicates preferential utilization of phosphorus- and nitrogen-rich organic compounds by microbes, while hydrolysis of polysaccharides seemed to be less important. In oligotrophic conditions during summer, organic matter released from marine snow might represent a significant source of substrate for free-living bacteria in seawater. For the first time microorganisms producing APA in marine snow were identified, revealing that dense populations of bacteria expressed APA, while cyanobacteria did not. Cyanobacteria proliferating in marine snow could benefit from phosphorus release by bacteria and nanoflagellates.

Seasonal variations in extracellular enzymatic activity in marine snow-associated microbial communities and their impact on the surrounding water