Significance of exoenzymatic activities in the ecology of brackish water: measurements by means of methylumbelliferyl-substrates

Abstract

Exoenzymatic activity is an important factor for macromolecular substrate turnover in marine environments. It serves as a balance of the DOC [dissolved organic C] pool and supplies microheterotrophs with easily degradable organic substrates. Velocities of hydrolysis of glucosides, protein-like compounds, glucosaminides and organic P compounds and hydrolyzation rates of naturally occurring substrate analogues were measured by means of fluorogenic methylumbelliferyl (MUF)-compounds. Field observations of exoenzymatic activity together with the measurements of other bacterial activity parameters were derived from 2 eutrophied Baltic Sea fjords and adjacent offshore waters. Protease activity was dominant in all water samples. .alpha.-D-glucosidase and glucosaminidase activities showed strongest variations between fjords and offshore waters. These enzymes appear suitable as indicators for organic pollution and eutrophication. A preliminary combination experiment with MUF-compounds and radioactively labeled substrates exhibited a discrepancy between the amount of substrate hydrolyzed by exoenzymes and the incorporation of this substrate by bacteria. This may give an indication towards medium-scale changes in the DOC pool and the mode of equilibration of small molecules and macromolecules in the DOC pool.