Laboratory and field studies of colloidal iron oxide dissolution as mediated by phagotrophy and photolysis

Abstract

In a previous work, we have employed colloidal ferrihydrite impregnated with an inert radiotracer to probe the mechanistics of iron redox cycling in seawater via phagotrophic and photochemical processes. This paper reports further studies using the inert tracer technique, directed towards obtaining a more quantitative sense of the importance of phagotrophy relative to photolysis as a pathway for the production of bioavailable iron in oxygenated seawater. Our results indicate a maximal (i.e., near‐surface at noon) rate of 12% per day for the photochemically‐mediated dissolution of colloidal ferrihydrite. Protozoan‐mediated dissolution of the same iron oxide phase proceeds at a rate ranging from 1–6% per day, depending on grazing turnover rates. Thus, while photolysis should dominate the redox cycling of refractory iron solids in near‐surface waters under bright daytime conditions, phagotrophy is likely to be a more important process overall when the entire euphotic zone is considered on a time‐averaged basis.