Characterization of dissolved organic phosphorus in coastal seawater usingultrafiltration and phosphohydrolytic enzymes

Dissolved reactive and organic phosphorus (DRP and DOP, respectively) in sa mples from river and coastal areas of Tokyo Bay, Japan, was size fractionat ed into high-molecular-weight (HMW; <0.1 mu m but >10 kDa) and low-molecula r-weight (LMW; <10 kDa) size classes using a stirred-cell ultrafiltration s ystem. The LMW fraction accounted for 54-76% of the bulk DOP: LMW-DOP exhib ited conservative behavior during mixing with saline waters, with high conc entrations at the river mouth decreasing seaward. HMW-DOP was a rather mino r component, accounting for 14-36% of the bulk DOP. Concentrations of HMW-D RP, a trace component of all samples, decreased with increasing salinity. C haracterization using two phosphohydrolytic enzymes, alkaline phosphatase a nd 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. Furt her size fractionation and characterization revealed the importance of hydr ophobic compounds (presumably phospholipids) and phosphate esters as the no nreactive DOP. The esters in the nonreactive fraction are potentially labil e but may be protected against decomposition by forming submicron particles and/or macromolecular complexes.