Characterization of dissolved organic phosphorus in coastal seawater using ultrafiltration and phosphohydrolytic enzymes

Dissolved reactive and organic phosphorus (DRP and DOP, respectively) in samples from river and coastal areas of Tokyo Bay, Japan, was size fractionated into high-molecular-weight (HMW; <0.1 mu m but >10 kDa) and low-molecular-weight (LMW; <10 kDa) size classes using a stirred-cell ultrafiltration system. The LMW fraction accounted for 54-76% of the bulk DOP: LMW-DOP exhibited conservative behavior during mixing with saline waters, with high concentrations at the river mouth decreasing seaward. HMW-DOP was a rather minor component, accounting for 14-36% of the bulk DOP. Concentrations of HMW-DRP, a trace component of all samples, decreased with increasing salinity. Characterization using two phosphohydrolytic enzymes, alkaline phosphatase and phosphodiesterase, demonstrated the presence of three forms of HMW-DOP: easily hydrolyzable mono- and diesters and unhydrolyzable nonreactive DOP. The nonreactive DOP was a significant fraction (up to 67%) of HMW-DOP. Further size fractionation and characterization revealed the importance of hydrophobic compounds (presumably phospholipids) and phosphate esters as the nonreactive DOP. The esters in the nonreactive fraction are potentially labile but may be protected against decomposition by forming submicron particles and/or macromolecular complexes.