ROLE OF PROTOZOAN GRAZING IN RELIEVING IRON LIMITATION OF PHYTOPLANKTON

RECENT evidence indicates that iron is a limiting factor in primary pr oduction in some areas of the oceans(1,2). In sea water, iron is large ly present in the form of particulate and colloidal phases which are a pparently unavailable for uptake by phytoplankton(3-5). Several mechan isms have been proposed whereby non-reactive iron may be converted int o more labile forms (for example, thermal dissolution(6), photochemica l reactions(7,8) and ligand complexation(9)). Here we report that dige stion of colloidal iron in the acidic food vacuoles of protozoan graze rs may be a mechanism for the generation of 'bioavailable' iron from r efractory iron phases. We have demonstrated several grazer-mediated ef fects on colloidal ferrihydrite, including a decrease in colloid size, an increase in colloid lability as determined by competitive ligand-e xchange techniques, and an increase in the bioavailability of colloids to iron-limited diatoms. These results indicate that protozoan grazer s may significantly enhance the supply of iron to marine phytoplankton from terrestrial sources.