A purple sulfur bacterium, strain SW26, was isolated in pure culture from intertidal sediments from the Sylt-Rømø Basin, German Wadden Sea, sharing many properties with validated Marichromatium species, but differing significantly by possessing a plasmid, by tolerating up to 16 mM sulfide, and up to 44 °C for growth. Strain SW26 has a DNA base composition of 68.3 mol% G+C, a 16S rRNA gene sequence similarity of >99% to those of Marichromatium species, and shows the highest level of genomic relationship with Marichromatium gracile, despite its remarkably different phenotypic characters. Based upon high genomic similarity but different physiological properties of strain SW26 with respect to the type strain of M. gracile, a novel biotype, designated as M. gracile biotype thermosulfidiphilum is described. © 2008 Elsevier GmbH. All rights reserved.