Iron acquisition by photosynthetic marine phytoplankton from ingested bacteria

Iron is unique among biologically essential trace metals in having a higher particulate than dissolved concentration in ocean surface waters(1). Uptake of dissolved iron is generally considered to be the norm for phytoplankton, as even the smallest iron-bearing particles are unavailable for transport into cells(2,3). But the oceanic dissolved fraction is so small, and the particulate fraction so inert(2), that phytoplankton production is limited by a dearth of available iron in some regions(4). Here we use incubation experiments to show that Ochromonas sp., a common photosynthetic flagellate from the Pacific Ocean, can obtain iron directly in particulate form, by ingesting bacteria. Iron acquisition is highly efficient; Ochromonas assimilates 30% of the ingested ration, acquiring a high intracellular iron concentration and maintaining a significantly faster growth rate than when iron is provided in the dissolved phase. Phytoplankton capable of such phagotrophy (so-called mixotrophic species) may thus be able to assimilate iron in both particulate and dissolved forms in the ocean. Moreover, when iron availability is limited, the iron 'cost' of growth is diminished because Ochromonas derives a greater fraction of its energy from the bacteria. Analysis of standing stocks and clearance rates of plankton in the equatorial Pacific shows that the iron flux through mixotrophic flagellates can amount to 35-58% of the total Fe uptake by the entire autotrophic community. Our results suggest that the phagotrophic ingestion of bacteria may be an effective adaptive strategy for photosynthetic organisms to obtain iron for growth in iron-limited regions of the sea.