Light-induced reduction of natural iron(III) oxide and its relevance to phytoplankton

Abstract

Primary productivity in natural waters may sometimes be limited by the availability of iron^1–4. To understand the process it is necessary to know which form(s) of iron can be utilized by phytoplankton. Here we present evidence, from algal culture work and photochemical experiments, which suggests that the prevalent form of iron in most natural waters—participate iron(III) oxide—is an adequate source of iron for algae, even though iron is taken up only in its soluble, ionic form⁵. This is because the oxide undergoes light-induced reduction, providing a continuous supply of soluble (available) iron(II). Humic substances (HS), co-precipitated with the oxide, participate in the reduction process by reducing the iron(III) directly and/or by maintaining it in a reducible state. These findings suggest that, at least in freshwaters, cessation of phytoplankton growth is rarely due to lack of iron.