Differential Dissolved Organic Nitrogen Availability and Bacterial Aminopeptidase Activity in Limnic and Marine Waters

Abstract

Nitrogen often limits primary production in marine ecosystems and its loading from terrestrial sources is the major cause of enhanced coastal eutrophication worldwide. About 70% of nitrogen transported by rivers globally is dissolved organic nitrogen (DON). Therefore, terrestrial DON is potentially an important component of the N dynamics in aquatic ecosystems, but the bioavailability of this organic nitrogen is poorly known. Bacterial extracellular hydrolysis of polymers is a bottleneck in the utilization of natural dissolved organic matter, mostly consisting of high molecular weight compounds. To study the bacterial utilization and extracellular enzymatic hydrolysis of DON, we developed a bioassay employing natural DON as the only N source, and N as the limiting nutrient. Bacterial cell density and activity of an unspecific aminopeptidase (AMPase) were followed in the cultures. Natural DON stimulated the cell-specific AMPase activity. Furthermore, refractory and humus-rich DOM caused a stronger stimulation than labile DOM. We propose that the previously reported inhibitory effect of humic substances on enzyme activity was outweighed by the induction of enzyme synthesis caused by refractory substrates. AMPase activity and the estimated DON bioavailability were more than twofold higher in seawater than in freshwater with identical substrate additions. This indicates that hydrolysis and turnover of land-derived DON is enhanced when it enters coastal marine waters, enabling it to support elevated bacterioplankton and phytoplankton growth.