Role of protozoan grazing in relieving iron limitation of phytoplankton

Abstract

RECENT evidence indicates that iron is a limiting factor in primary production in some areas of the oceans^1,2. In sea water, iron is largely present in the form of particulate and colloidal phases which are apparently unavailable for uptake by phytoplankton^3–5. Several mechanisms have been proposed whereby non-reactive iron may be converted into more labile forms (for example, thermal dissolution⁶, photochemical reactions^7,8 and ligand complexation⁹). Here we report that digestion of colloidal iron in the acidic food vacuoles of protozoan grazers may be a mechanism for the generation of 'bioavailable' iron from refractory iron phases. We have demonstrated several grazer-mediated effects on colloidal ferrihydrite, including a decrease in colloid size, an increase in colloid lability as determined by competitive ligand-exchange techniques, and an increase in the bioavailability of colloids to iron-limited diatoms. These results indicate that protozoan grazers may significantly enhance the supply of iron to marine phytoplankton from terrestrial sources.