Rapid biologically mediated oxygen isotope exchange between water and phosphate

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 (δ¹⁸O_w) were monitored. The oxygen isotopic composition of phosphate (δ¹⁸O_p) in the organisms' food source, tissues, and the δ¹⁸O_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 δ¹⁸O_p values become 23-26‰ higher than δ¹⁸O_w, and δ¹⁸O_p values of DIP become ∼20‰ higher than δ¹⁸O_w. No correlation between δ¹⁸O_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 δ¹⁸O_p values of P compounds dissolved in aquatic systems. This information is fundamental to any application of δ¹⁸O_p of dissolved organic or inorganic phosphate to quantify the dynamics of phosphorus cycling in aquatic systems.