Microbial Extracellular Enzymes and their Role in Dissolved Organic Matter Cycling

Heterotrophic bacteria are faced with the fundamental challenge of obtaining sufficient carbon and energy from the organic substrates in their environment. The outer membrane of Gram-negative bacteria is spanned by porins— trimeric proteins that form inlet channels between the outer membrane and the periplasmic space. The geometry of the porins defines the substrate uptake limit that is on the order of 600 daltons, approximately equivalent to a trisaccharide. A microbial cell in general tightly controls production and activities of enzymes, because enzyme production represents an investment of: carbon, nitrogen, and energy. Several general approaches have been used to measure the activities of extracellular enzymes in aquatic systems. The extent to which dissolved enzymes contribute to the total hydrolytic activity in flesh and marine waters may also be affected by differences in freshwater and marine dissolved organic carbon sources and composition. The precise relationship between potential hydrolysis rates measured with externally added substrates; and the rates at which complex microbial communities in marine systems hydrolyze and ultimately remineralize a spectrum of organic macromolecules actually available as substrates is unknown. More realistic and diverse methods are needed to measure the activities of extracellular enzymes on a wide range of substrates— such as glycoproteins, lipid complexes, and heterogeneous particulate and dissolved organic matter.