ASLO/TOS/AGU Ocean Sciences Meeting, Honolulu, HI, February 2006.

Phosphate oxygen isotopes and microbial phosphorus cycling in the oceans

A. S. Colman¹, R. E. Blake², Y. Liang², D. M. Karl³, K. K. Turekian², M. L. Fogel⁴

¹University of Maryland Biotech. Inst., Center of Marine Biotech. 701 E. Pratt St, Baltimore, MD 21202

²Yale University, Dept. Geology & Geophysics, 210 Whitney Ave., New Haven, CT 06511

³Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

⁴Carnegie Institution of Washington, Geophysical Laboratory 5251 Broad Branch Rd., Washington, DC 20015

We have measured the isotopic composition of oxygen in dissolved inorganic phosphate (Pi) in coastal waters and the deep ocean. Our measurements, coupled with laboratory experiments involving a variety of phosphatase enzymes, implicate cell lysis and extracellular pyrophosphatase activity as significant processes in the isotopic cycling of marine phosphate oxygen. Small deviations from isotopic equilibrium below the thermocline suggest that P remineralization in the deep ocean is a by-product of microbial carbon and energy requirements. However, isotope effects associated with phosphohydrolase enzymes involved in P remineralization are quite large and could potentially lead to significant disequilibration of Pi oxygen. We introduce a non-steady state box model that tracks phosphate oxygen isotope compositions along deep water flow paths, and we use this model to constrain deep water microbial phosphate fluxes. The observed near equilibration of deep water Pi likely calls for continued slow rates of microbial uptake and release of Pi and/or extracellular pyrophosphatase-mediated oxygen exchange between water and Pi in the deep ocean.