Rapid biologically mediated oxygen isotope exchange between water and phosphate

[1] In order to better constrain the rate of oxygen isotope exchange between water and phosphate via biochemical reactions a set of controlled experiments were conducted in 1988 at the Aquaculture Plant in Elat, Israel. Different species of algae and other organisms were grown in seawater tanks under controlled conditions, and the water temperature and oxygen isotopic composition (delta(18)O(w)) were monitored. The oxygen isotopic composition of phosphate (delta(18)O(p))in the organisms' food source, tissues, and the delta(18)O(p) of dissolved inorganic phosphate (DIP) were measured at different stages of the experiments. Results indicate that intracellular oxygen isotope exchange between phosphorus compounds and water is very rapid and occurs at all levels of the food chain. Through these reactions the soft tissue delta(18)O(p) values become 23-26parts per thousand higher than delta(18)O(w), and delta(18)O(p) values of DIP become similar to20parts per thousand higher than delta(18)O(w). No correlation between delta(18)O(p) values and either temperature or P concentrations in these experiments was observed. Our data imply that biogenic recycling and intracellular phosphorus turnover, which involves kinetic fractionation effects, are the major parameters controlling the delta(18)O(p) values of P compounds dissolved in aquatic systems. This information is fundamental to any application of delta(18)O(p) of dissolved organic or inorganic phosphate to quantify the dynamics of phosphorus cycling in aquatic systems.