IRON ACQUISITION BY PHOTOSYNTHETIC MARINE-PHYTOPLANKTON FROM INGESTEDBACTERIA

Iron is unique among biologically essential trace metals in having a h igher particulate than dissolved concentration in ocean surface waters (1). Uptake of dissolved iron is generally considered to be the norm f or phytoplankton, as even the smallest iron-bearing particles are unav ailable for transport into cells(2,3). But the oceanic dissolved fract ion is so small, and the particulate fraction so inert(2), that phytop lankton production is limited by a dearth of available iron in some re gions(4). Here we use incubation experiments to show that Ochromonas s p., a common photosynthetic flagellate from the Pacific Ocean, can obt ain iron directly in particulate form, by ingesting bacteria. Iron acq uisition is highly efficient; Ochromonas assimilates 30% of the ingest ed ration, acquiring a high intracellular iron concentration and maint aining a significantly faster growth rate than when iron is provided i n the dissolved phase. Phytoplankton capable of such phagotrophy (so-c alled mixotrophic species) may thus be able to assimilate iron in both particulate and dissolved forms in the ocean. Moreover, when iron ava ilability 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 eq uatorial Pacific shows that the iron flux through mixotrophic flagella tes can amount to 35-58% of the total Fe uptake by the entire autotrop hic community. Our results suggest that the phagotrophic ingestion of bacteria may be an effective adaptive strategy for photosynthetic orga nisms to obtain iron for growth in iron-limited regions of the sea.