Dense populations of a giant sulfur bacterium in Namibian shelf sediments

A previously unknown giant sulfur bacterium is abundant in sediments underl ying the oxygen minimum zone of the Benguela Current upwelling system. The bacterium has a spherical cell that exceeds by up to 100-fold the biovolume of the largest known prokaryotes. On the basis of 16S ribosomal DNA sequen ce data, these bacteria are closely related to the marine filamentous sulfu r bacteria Thioploca, abundant in the upwelling area off Chile and Peru. Si milar to Thioploca, the giant bacteria oxidize sulfide with nitrate that is accumulated to less than or equal to 800 millimolar in a central vacuole.