An iron curtain in the Atlantic Ocean forms a biogeochemical divide.

The concept of biogeochemical provinces in the ocean was implicit in Longhurst’s (1) classic monograph, Ecological Geography of the Sea. Longhurst identified distinct oceanic zones based on the relationships of oceanic biology (using chlorophyll, a proxy for phytoplankton biomass, and primary production) to physical forcing (e.g., surface currents, mixing depth, and upwelling). As we have advanced over the last several decades in our understanding of the nutrient biogeochemistry of the seas, it has become evident that there are also distinct provinces with respect to nutrient dynamics. Many of these zones map directly on Longhurst’s eco-geography. For example, Sohm et al. (2) recently proposed regions of the sea with characteristic populations of nitrogen fixers and distinct controls on these populations. In PNAS, Schlosser et al. (3) now provide compelling field evidence of two distinct biogeochemical provinces of the tropical Atlantic Ocean with respect to the distribution of nitrogen fixation. These provinces are primarily defined by another physical phenomenon, the Atlantic InterTropical Convergence Zone (ITCZ). In tandem, Schlosser et al. (3) also firmly establish the importance of iron as a major control on nitrogen fixation in these two ocean basins. During the late 1990s, analysis of the comprehensive ocean nutrient surveys that had taken place over the previous decades provided evidence of excesses of regenerated nitrate relative to phosphate in subeuphotic waters of the North Atlantic. This process led to the hypothesis that nitrogen fixation in the overlying surface waters was responsible for this excess and, moreover, might be a quantitatively more important process in the marine nitrogen cycle than thought to be at that point (4). Given the concurrent developing recognition of the heightened cell quota of marine nitrogen fixers for iron (5), the substantial flux of aeolian iron into the North Atlantic from northwest African deserts was thought to be a driver of this excess (4–6). Schlosser et al. (3) undertook two major research expeditions, which sampled the North and South Atlantic across the equator, and have identified a sharp ecological boundary for nitrogen fixation associated with the ITCZ. North of the ITCZ the biomass of ITCZN Summer ITCZN Winter A