Iron (III) emissions from seawater desalination plants: a new, hitherto overlooked contribution to the iron cycle in coastal seas?

Due to the poor solubility of iron (III) and strong regional differences in terrestrial, riverine or airborne input, iron is a limiting micronutrient in much of the world ocean. Because of increasing living standards and climate change, rainfall is becoming more intermittent, which exacerbates drought conditions. As a result, seawater desalination has a vital and growing role in providing drinking water in many hot and arid regions of the world. Most seawater desalination plants use ferric or ferrate salts as flocculants for removing phyto- and bacterioplankton and suspended organic matter prior to the actual desalination process. Consequently, such seawater desalination plants release large amounts of suspended iron in conjunction with organic matter into coastal waters, with potentially significant consequences for the availability of this micronutrient in coastal marine ecosystems. This review paper provides important background information about the role of iron as a micronutrient in marine ecosystems, its role in the inorganic biochemistry of marine life, as well as key features of its use in seawater desalination, before providing a perspective by exploring potential implications for the physiology and biochemistry of marine life and marine ecosystem functioning.