Dense Populations of a Giant Sulfur Bacterium in Namibian Shelf Sediments

A previously unknown giant sulfur bacterium is abundant in sediments underlying 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 16 S ribosomal DNA sequence data, these bacteria are closely related to the marine filamentous sulfur bacteria Thioploca , abundant in the upwelling area off Chile and Peru. Similar to Thioploca , the giant bacteria oxidize sulfide with nitrate that is accumulated to ≤800 millimolar in a central vacuole.