Organic Nitrogen Utilization by Phytoplankton: The Role of Cell-Surface Deaminases.

摘要

Many phytoplankton can catalyze the decomposition of organic phosphates using cell-surface phosphates and subsequently take up the phosphorus for growth. The nitrogen analogs, cell-surface deaminases, have not been reported in phytoplankton, except for a low affinity asparaginase. In fact, high affinity cell-surface L-amino acid and amine deaminases (oxidases) exist in at least three phytoplankton genera: Amphidinium, Pleurochrysis, and Prymnesium. One type of enzyme oxidizes L-amino acids, a second type primary amines; the end products are hydrogen peroxide, an organic product (an alpha-ketoacid or an aldehyde), and NH4+ which is assimilated by the cell. The regulation of the amino acid and amine oxidases under different growth conditions and their inhibition by various reagents are discussed. A method was developed for measuring H2O2 in seawater samples and was subsequently used to obtain field data showing dark production of H2O2. This phenomenon may have been due to the presence of organisms with cell-surface oxidases. Since cell-surface deaminases would not have been detected by standard field methods used to study amino acid and primary amine cycling, the existence of these enzymes suggests that phytoplankton may have a more important role in recycling organic nitrogen than currently believed. These results thus have wide-ranging implications for understanding nitrogen cycling; primary production; the geochemistry of H2O2, organic acids and aldehydes; and algal-invertebrate symbioses in aquatic environments. (kt)