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Reduction of iron from the ferric state to the ferrous state is one strategy employed by microorganisms in near-neutral environments to increase its biological availability. In recent years, the existence of mobile reducing agents produced by microorganisms to promote iron reduction,known as electron shuttles, has been demonstrated. Production of electron shuttles has been shown for several organisms, employing a variety of mostly organic molecules as the electron carrier. Here we show that the coastal cyanobacterium Lyngbya majuscula produces iron-reducing superoxide radicals (O-2(center dot)-) and that this facilitates increased iron uptake. We suggest that superoxide is a useful electron shuttle because it reacts rapidly and almost indiscriminately with Fe(III)-organic complexes and its precursor, dissolved oxygen, is ubiquitous in the photic zone. We further suggest that, for these reasons, the generation of superoxide by marine oxygenic photosynthetic microorganisms and its use in facilitating iron uptake may be a reasonably widespread process.