Iron availability, cellular iron quotas, and nitrogen fixation in Trichodesmium

Iron availability is suggested to be a primary factor limiting nitrogen fix ation in the oceans. This hypothesis is principally based on cost-benefit a nalyses of iron quotas in the dominant nitrogen-fixing cyanobacteria, Trich odesmiun spp., in the contemporary oceans. Although previous studies with T richodesmium have indicated that iron availability enhanced nitrogen fixati on and photosynthesis, no clear relationship has been reported between cell ular iron quotas and nitrogen fixation. We re-examined the proposed link, b etween iron availability and nitrogen fixation in laboratory isolates and n atural populations collected from coastal waters north of Australia. In lab oratory cultures grown under iron-limiting conditions, we measured a declin e in cellular iron quotas, photochemical quantum yields, the relative abund ance of photosystem I to photosystem II reaction centers, and rates of nitr ogen fixation. Nitrogen fixation displayed a critical threshold of the diss olved sum of total inorganic Fe species ([Fe ']) of ca. log[Fe '] = -9.7. F ield populations of Trichodesmium, collected during bloom conditions, showe d high iron quotas consistent with high nitrogen fixation rates. Using seas onal maps of aeolian iron fluxes and model-derived maps of surface water to tal dissolved Fe, we calculated the potential of nitrogen fixation by Trich odesmium in the global ocean. Our results suggest that in 75% of the global ocean, iron availability limits nitrogen fixation by this organism. Given present trends in the hydrological cycle, we suggest that iron fluxes will be even more limiting in the corning century.