Loose ligands and available iron in the ocean.

T he ocean covers ≈70% of the Earth’s surface and plays a major role in regulating global climate and sequestering carbon dioxide from the atmosphere. Diverse populations of phytoplankton, namely photosynthetic microalgae and cyanobacteria, abound in sunlit surface waters of the ocean and account for half of global primary production. Phytoplankton require a variety of essential nutrients, including iron, to fix carbon dioxide and fuel ocean food webs, but the oxidized form of iron, Fe(III), that prevails in the ocean is only sparingly soluble in oxygenated seawater. The solubility of iron is enhanced through chelation with organic ligands, and nearly all of the dissolved iron in seawater is bound to natural ligands (1). The molecular identity and reactivity of seawater ligands has largely eluded discovery by the iron workers, leaving a gaping hole in our understanding of the availability of iron to phytoplankton. This has ramifications for the global C cycle, because phytoplankton productivity is limited by iron availability in vast regions of the surface ocean that have an ample supply of other critical nutrients, such as N and P (2). In PNAS, Hassler et al. (3) provide experimental evidence indicating saccharide-enhanced iron utilization by phytoplankton cultures and natural planktonic communities in surface seawater. Apparently, saccharides chelate iron in a bioavailable form, an observation that is likely to transform current thinking about iron cycling in the ocean. Saccharides (also known as carbohydrates) are abundant, bioreactive components of dissolved organic matter (DOM) that are produced by phytoplankton in the surface ocean (4, 5). It seems that saccharides play a critical role in sustaining ocean productivity through a positive feedback that links the C and Fe cycles (Fig. 1A). The mix of natural organic ligands in seawater has been categorized into two classes, strong and weak, according to their binding affinities for iron (1). Much attention has been focused on the stronger binding ligands, particularly siderophores—a group of designer ligands biosynthesized by marine bacteria (6). Siderophores can grab iron from weaker ligands and lock iron in a complexed form that seems to be directly bioavailable only to bacteria. The recent discovery (7) that photolysis of a specific bacterial siderophore can be coupled with the uptake of released iron by marine phytoplankton provides a fascinating mechanism to make siderophore-bound iron available to marine microalgae, but it seems that siderophores supply a minor fraction of the iron requirements of the phytoplankton community (6). Common biochemicals, such as saccharides and amino acids, have functional groups that can form weak complexes with iron and other trace metals in seawater. Relatively little is known Fe FeTr Fe