Cover photograph (Copyright © 2008, American Society for Microbiology. All Rights Reserved.): Sediment from oligomesotrophic Lake Hampen (Jutland, Denmark), densely colonized by the submersed macrophyte Littorella uniflora. The plants release oxygen from their roots, as indicated by the lighter color of the oxidized rhizosphere sediment, which is distinct from the darker, reduced sediment layer below the root zone. The oxygen release by L. uniflora enhances nitrification and supports, presumably together with root exudates, a large population of ammonia-oxidizing Archaea in the rhizosphere of this freshwater macrophyte. (See related article on page 3279.)