Iron is recognized as a key element regulating primary production in large regions of the ocean, but nothing is known of its direct effect on higher trophic levels. Growth and metabolism of two species of heterotrophic protozoans fed iron-rich and iron-poor prey were thus examined. Maximum growth rates of Paraphysomonas imperforata and P. butcheri were observed only when Fe quotas of bacterial prey were greater than 70 $ mu$mol Fe:mol C. At lower Fe:C ratios, but at constant prey biomass (C/ml), both species grew significantly slower. Minimum Fe quotas of the flagellates at these slow growth rates ($ sim$10 $ mu$mol Fe:mol C) were similar to those of iron-limited phytoplankton and bacteria. Growth rate reduction was the result of direct elemental limitation by Fe, judging from the protozoans' positive response to Fe additions and from their biochemical characteristics. Filtration and carbon ingestion rates increased under Fe-limitation, but carbon gross growth efficiency (CGGE) decreased when Paraphysomonas imperforata consumed iron-poor bacteria. Ammonium regeneration efficiency was also reduced. The decrease in CGGE was a consequence of reduced activity of the iron-dependent electron transport system, greater DOC excretion, and greater CO$ sb2$ evolution by Fe-limited flagellates. Paraphysomonas imperforata excreted Fe, even when limited by this element, and retained less of the ingested ration and thus had a higher Fe regeneration efficiency than when consuming Fe-rich bacteria. According to recent measurements of biogenic Fe:C in the subarctic Pacific, our results suggest that heterotrophic bacterivorous flagellates may experience iron-limitation in remote oceanic regions. Such limitation could profoundly affect C, N and Fe cycling in the sea.

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