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. Two species of heterotrophic protozoa were thus fed iron-rich
and iron-poor bacterial prey and their growth and metabolism examined. Maximum growth rates of Paraphysomonas
imperforata and Paraphysomonas butcheri were observed only when iron quotas of bacterial prey were >70 μmol
Fe mol C-1. At lower Fe: C ratios, but at constant prey biomass, both species grew significantly slower. Iron quotas
of the flagcllatcs at these slow growth rates (-10 μmol Fe mol C-1) were similar to those of iron-limited phytoplankton
and bacteria. Growth rate reduction was likely the result of direct, elemental limitation by iron, judging
from the positive response of the protozoa to iron addition and their biochemical characteristics. Filtration and
carbon ingestion rates increased under iron limitation, but gross carbon growth efficiency (GCGE) decreased when
P. imperforata consumed iron-poor bacteria. Ammonium regeneration efficiency was also reduced. The decrease in
GCGE was a consequence of reduced activity of the iron-dependent electron transport system, greater dissolved
organic carbon excretion, and greater CO, evolution by iron-limited protozoa. P. imperforata excreted iron, even
when limited by this element, and retained less of the ingested ration than when consuming iron-rich bacteria.
Coupled with 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.