MINERALS, MATS, PEARLS AND VEILS: A PANOPLY OF GIANT SULFUR BACTERIA

The biogeochemical cycling of sulfur has been at the heart of microbial ecology since the mid-19th century. This is due, at least in part, to the striking forms of many of the organisms involved in the transformation of reduced sulfur species. Giant sulfur bacteria were among the earliest micro-organisms to capture the interest of microbiologists exploring the links between geochemical cycling of the elements and the microbiota responsible. Consequently, giant sulfur bacteria were among the first bacteria described. Organisms resembling Beggiatoa (‘ Oscillatoria alba’) were described as early as 1803 (Vaucher, 1803), but were included in the genus Beggiatoa some time later (Trevisan, 1842). Thiothrix (Rabenhorst, 1865; Winogradsky, 1888), Achromatium (Schewiakoff, 1893) and Thioploca (Lauterborn, 1907) were all described by the early 20th century and Winogradsky (1887, 1888) had already formulated the principles of lithotrophic growth based on sulfide oxidation, from his work on Beggiatoa species. Surprisingly for such conspicuous organisms, novel giant sulfur bacteria are still being described (Guerrero et al., 1999; Schulz et al., 1999).

Achromatium

Bacteria of the genus Achromatium are remarkable. Cells of up to 125 μm in length have been reported (Babenzien et al., 2005; Head et al., 2000a) and, in addition to characteristic sulfur globules that become visible on treatment of the cells with dilute acid, their large oval cells are typically filled with enormous inclusions of calcium carbonate (Fig. 1).