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 with in the acidic food vacuoles of marine protozoan grazers as evidence of a novel mechanism for the conversion of refractory iron solids to mor e labile forms in oxic surface waters. This paper expands upon those i nitial studies and presents a new technique to study the reaction of i ron oxides in seawater, based on the synthesis of colloidal ferrihydri te containing an inert tracer. Measuring the accumulation of the trace r in the dissolved phase enables the determination of the rate and ext ent of iron oxide reaction, even for kinetically slow processes and re gardless 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 sh own here in a series of co-dissolution studies and in several photoche mical kinetics experiments. In laboratory studies of the dissolution o f colloidal ferrihydrite by protozoan grazers, the inert tracer method enables an improved estimate of the rate of protozoan-mediated iron o xide dissolution, confirming our previous results and providing a usef ul tool for further studies of phagotrophy as a reaction pathway for r efractory iron.