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

In a previous work, we have employed colloidal ferrihydrite impregnated wit h an inert radiotracer to probe the mechanistics of iron redox cycling in s eawater via phagotrophic and photochemical processes. This paper reports fu rther studies using the inert tracer technique, directed towards obtaining a more quantitative sense of the importance of phagotrophy relative to phot olysis as a pathway for the production of bioavailable iron in oxygenated s eawater. Our results indicate a maximal (i.e., near-surface at noon) rate o f 12% per day for the photochemically-mediated dissolution of colloidal fer rihydrite. Protozoan-mediated dissolution of the same iron oxide phase proc eeds at a rate ranging from 1-6% per day, depending on grazing turnover rat es. Thus, while photolysis should dominate the redox cycling of refractory iron solids in near-surface waters under bright daytime conditions, phagotr ophy is likely to be a more important process overall when the entire eupho tic zone is considered on a time-averaged basis.