The regeneration of highly bioavailable iron by meso- and microzooplankton

The micronutrient iron (Fe) is rapidly cycled in surface waters, and regenerated Fe supports much of the phytoplankton growth in open ocean waters. Meso- and microzooplankton grazing are both important mechanisms to regenerate Fe, but the chemical conditions in the respective digestive systems are different and might affect the bioavailability of Fe. We conducted radiotracer grazing experiments with the copepod Acartia tonsa or the heterotrophic dinoflagellate Oxyrrhis marina feeding on the diatom Thalassiosira pseudonana or the coccolithophore Emiliania huxleyi. Uptake of regenerated Fe-55 by a separate T. pseudonana culture was compared to the uptake of inorganic Fe. Iron regenerated by A. tonsa was taken up 4- to 7-fold faster than inorganic iron. In contrast, no difference was detected between the uptake rate of inorganic Fe and Fe regenerated by O. marina. Ingestion of different prey by A. tonsa revealed that Fe released during diatom digestion was taken up 1.8-fold faster than Fe released from coccolithophore digestion. Digestive systems and the chemical makeup of the ingested prey are crucial in determining the bioavailability of regenerated Fe. Differences in pH and oxygen saturation between digestive vacuoles and guts may affect the speciation of regenerated Fe, and the release of ferrous Fe might contribute to the bioavailability of regenerated Fe. The ecological structure determines the importance of regenerated Fe for a particular ecosystem.