Remineralization of bioavailable iron by a heterotrophic dinoflagellate

Abstract

The growth of phytoplankton in oligotrophic ocean waters is largely supported by nutri- ents remineralized through microzooplankton grazing. The bioavailability of the micronutrient Fe varies as a function of chemical speciation, but the speciation and bioavailability of Fe recycled by protozoan grazers is poorly characterized. We performed laboratory incubation experiments with cul- tured phytoplankton to examine the bioavailability of Fe produced by the heterotrophic dinoflagel- late Oxyrrhis marina grazing on the centric diatom Thalassiosira pseudonana. Three different phyto- plankton species (Thalassiosira weissflogii, Emiliania huxleyi and Nannochloris sp.) were grown on the remineralized Fe, and Fe uptake rates (measured with the radiotracer 55 Fe) and cell growth were used to assess bioavailability. Following 14 h of grazing, 1.09 to 1.11 nmol l -1 Fe was remineralized in the presence of O. marina compared to 0.30-0.49 nmol l -1 released in grazer-free controls. The size fractionation of dissolved Fe was similar in the grazed (treatment) and non-grazed (control) cultures. The 0.02 to 0.2 µm fraction was the largest (51 to 70%), followed by the <0.02 µm fraction (32 to 44%). Remineralized Fe was rapidly accumulated internally by all phytoplankton species, with most accu- mulation occurring during the first 14 h. More Fe was accumulated in treatments compared to con- trols for all species, and cellular uptake rates were higher for remineralized Fe. These experiments confirm that microzooplankton grazing is a significant source of bioavailable Fe to marine phyto- plankton in the open ocean.