THE ROLE OF HETEROTROPHIC BACTERIA IN IRON-LIMITED OCEAN ECOSYSTEMS

IRON availability limits phytoplankton growth in large areas of the wo rld's oceans(1-3) and may influence the strength of the biological car bon pump(4-5). Very little is known of the iron requirements of oceani c heterotrophic bacteria, which constitute up to 50% of the total part iculate organic carbon in open ocean waters(6,7) and are important in carbon cycling as remineralizers of dissolved organic matter and hence producers of CO2 (ref. 8). Here we report that oceanic bacteria conta in more iron per biomass than phytoplankton. Tn the subarctic Pacific, they constitute a large fraction of biogenic iron and account for 20- 45% of biological iron uptake, Bacterial iron quotas in the field are similar to those of iran-deficient laboratory cultures, which exhibit reduced electron transport, slow growth, and low carbon growth efficie ncy. Heterotrophic bacteria therefore play a major role in the biogeoc hemical cycling of iron. In situ iron limitation of heterotrophic meta bolism may have profound effects on carbon Bur in the ocean.