The role of heterotrophic bacteria in iron-limited ocean ecosystems

Abstract

IRON availability limits phytoplankton growth in large areas of the world's oceans^1–3 and may influence the strength of the biological carbon pump^4,5. Very little is known of the iron requirements of oceanic heterotrophic bacteria, which constitute up to 50% of the total particulate 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 contain more iron per biomass than phytoplankton. In 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 iron-deficient laboratory cultures, which exhibit reduced elec-tron transport, slow growth, and low carbon growth efficiency. Heterotrophic bacteria therefore play a major role in the biogeo-chemical cycling of iron. In situ iron limitation of heterotrophic metabolism may have profound effects on carbon flux in the ocean.