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Iron (Fe) plaque formation is a well known phenomenon in wetland, freshwater and salt marsh species; however there are no reports about Fe plaque occurrence in seagrasses. Here we review the main factors regulating Fe deposition on the roots and rhizomes of plants from reduced sediments/soils, and discuss these factors in relation to marine environment. Moreover, we present some early observations and quantification of Fe plaque on the tropical seagrass Cymodocea serrulata. Based on these first results and literature data, we compare the seagrass C. serrulata with the freshwater macrophyte Lobelia dortmanna performance and efficiency with regard to plaque formation. The comparison shows that regardless of the lower oxygen release to the sediment and less favourable sediment conditions with high pH, low organic matter content, high carbonate content and actively ongoing sulphate reduction, the seagrass is able to develop significant Fe plaques on roots and rhizomes. Thus we conclude that seagrasses have the necessary characteristics for plaque formation, and that marine sediments may be a favourable environment for Fe plaque development. Moreover, we suggest the likelihood of finding Fe deposits also on other species than C. serrulata and in other geographical locations.