Is iron(II) an important form of bioavailable iron in seawater?

Fe(III) is the thermodynamically favored form of Fe in seawater under oxygenated conditions. But it forms insoluble oxyhydroxides and strong organic complexes. High affinity Fe(III) transport mechanisms, including siderophore production, appear to be widespread in the upper ocean. Genomic data indicate the presence of an Fe(II) transport gene, feO in many aerobic marine microbes, even though Fe(II) is not stable in oxygenated waters. This suggests Fe(II) is important either as an intermediate or as a species within the water column actively taken up in its own right. Indeed, there are microbial and photochemical pathways for Fe(II) production in surface waters, and even if only a small fraction of Fe is present as Fe(II), it could have dramatic consequences for Fe bioavailability. Here, we examine Fe(II) data in several aerobic regimes. We argue that while some measurements appear to be overestimated due to reactions during analysis with interfering species, there is a small, highly light dependent fraction of Fe present as Fe(II) in oxygenated surface waters. Organisms with high affinity transport pathways for Fe(II) may be well positioned to utilize this source under high light conditions during the day. One constaint however may be the low affinity of Fe(II) for coordination sites associated with Fe transport. Our observation of a pronounced diel cycle is consistent with many previous studies, but also in contrast to some measurements that suggest a low turnover with very modest diel trends. Since large fluctuations in Fe availability on diel scales have important implications for Fe entry into the biological cycle, these issues are important to resolve.