Marine phosphate oxygen isotopes and organic matter remineralization in the oceans

We show that the isotopic composition of oxygen (delta O-18) in dissolved inorganic phosphate (P-i) reveals the balance between P-i transport and biological turnover rates in marine ecosystems. Our delta(18)Op of P-i (delta(80)Op) measurements herein indicate the importance of cell lysis in the regeneration of P-i in the euphotic zone. Depth profiles of the delta(18)Op in the Atlantic and Pacific Oceans are near a temperature-dependent isotopic equilibrium with water. Small deviations from equilibrium below the thermocline suggest that P remineralization in the deep ocean is a byproduct 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 P-i oxygen. The observed near equilibration of deep water P-i likely calls for continued slow rates of microbial uptake and release of P-i and/or extracellular pyrophosphatase-mediated oxygen exchange between water and P-i along the deep water flow path.