Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

• Published in: Nature 2004-05, Vol.429 (6989), p.292-294
• Main Authors: Geider, Richard J, Mills, Matthew M, Ridame, Celine, Davey, Margaret, La Roche, Julie
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Animals; Atlantic Ocean; Bioassays; Biological and medical sciences; Biological Assay; Biological Availability; Carbon Dioxide - metabolism; Chlorophyll - analysis; Cyanobacteria - drug effects; Cyanobacteria - metabolism; Cyanophyta; Dust; Fundamental and applied biological sciences. Psychology; Humanities and Social Sciences; Iron; Iron - analysis; Iron - pharmacology; Katagnymene; letter; Marine; multidisciplinary; Nitrates - pharmacology; Nitrogen; Nitrogen - metabolism; Nitrogen fixation; Nitrogen Fixation - drug effects; Nutrients; Ocean, Atmosphere; Oceans; Oscillatoria; Phosphorus; Phosphorus - analysis; Phosphorus - pharmacology; Phytoplankton; Phytoplankton - drug effects; Phytoplankton - metabolism; Plankton; Quaternary Ammonium Compounds - pharmacology; Science; Science (multidisciplinary); Sciences of the Universe; Sea water ecosystems; Seawater - chemistry; Ships; Synecology; Tropical Climate
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature02550

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments and subsequently confirmed by large-scale iron fertilization experiments. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales, providing an alternative to phosphorus as the ultimate limiting nutrient. Oceanographic observations have been interpreted both to confirm and refute this hypothesis, but direct experimental evidence is lacking. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs and strongly impacted by Saharan dust input. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic.