Dissolution of iron oxides by phagotrophic protists: Using a novel method to quantify reaction rates

In previous work, we have reported the dissolution of iron oxides within the acidic food vacuoles of marine protozoan grazers as evidence of a novel mechanism for the conversion of refractory iron solids to more labile forms in oxic surface waters. This paper expands upon those initial studies and presents a new technique to study the reaction of iron oxides in seawater, based on the synthesis of colloidal ferrihydrite containing an inert tracer. Measuring the accumulation of the tracer in the dissolved phase enables the determination of the rate and extent of iron oxide reaction, even for kinetically slow processes and regardless of the fate of iron in the system. The validity of the method as a means of following the reaction of iron oxides in seawater is shown here in a series of co-dissolution studies and in several photochemical kinetics experiments. In laboratory studies of the dissolution of colloidal ferrihydrite by protozoan grazers, the inert tracer method enables an improved estimate of the rate of protozoan-mediated iron oxide dissolution, confirming our previous results and providing a useful tool for further studies of phagotrophy as a reaction pathway for refractory iron.