Microbial degradation at a shallow coastal site: Long-term spectra and rates of exoenzymatic activities in the NE Adriatic Sea

The degradation of organic matter along the water column is mediated by enzymes released into the environment by planktonic organisms. Variations in enzymes profiles (types and levels of activity) reflect the trophic status of the environment and could be caused by shifts in the dominant species or in the level of enzyme expression by the same species in response to changes in the spectrum of organic substrates. To explore this issue, we examined the maximum rates of hydrolysis of 6 different enzymes (protease, α-glucosidase, β-glucosidase, β-galactosidase, alkaline phosphatase and lipase) along the water column (4 depths) at a coastal station in the Gulf of Trieste (northern Adriatic Sea), from 2000 to 2005. Most of the studied enzymes exhibited a pronounced seasonal variability with winter minima and maxima from April to October. During summer, alkaline phosphatase, lipase and protease reached the highest activities, while polysaccharide degradation prevailed in spring and autumn, associated to phytoplankton blooms. Phosphatase/protease activities ratio was generally low, indicating that microbial communities were rarely P-limited, possibly because of the use of organic P sources. A pronounced interannual variability of degradation patterns was found, with maximum rates of protease being the highest in most of the samples, followed by the alkaline phosphatase’s ones. Water column features greatly affected hydrolysis rates, being degradation of linear polysaccharides, lipids, phosphorilated compounds and polypeptides significantly different at different depths during stratified condition. Mixing processes affected especially α-glucosidase activity, possibly as a consequence of resuspension of organic matter from the seabed. Large-impact phenomena such as the 2003 heat wave and mucilage influenced the degradation of specific substrates. Mucilage enhanced lipase, phosphatase and protease, whereas a pronounced inhibition characterised phosphatase and protease during summer 2003.