32 No. 2
Giant Sulfur Bacteria
A remarkable discovery, an unknown bacterium about 100 times larger than most common bacteria and big enough to be seen with the naked eye, was made in oxygen-depleted marine sediments off the coast of Namibia in Southeastern Africa in April 1997. A true giant among unicellular microorganisms, Thiomargarita namibiensis was found forming delicate strings of pearly spheres, some growing up to three-quarters of a millimeter in diameter. In addition to its abnormal size, this fascinating microbe is unusual in several other ways, including its ability to accumulate large quantities of nitrate ions within the cell, sometimes in concentrations 10 000 times higher than in the surrounding seawater. Significantly, nitrate is used to oxidize sulfide ions derived from the degradation of organic matter by sulfate-reducing bacteria, a process that constitutes the key source of energy for the cells and nicely links the natural cycles of nitrogen and sulfur.
The stamp illustrated in this note is part of a set issued by Namibia in 2003 to celebrate recent biological discoveries in the country, including a new species of catfish and an insect long thought to be extinct. The stamp features a photomicrograph of three cells of Thiomargarita, each about 0.2 mm in diameter, and is similar to the one depicted on the cover of the 16 April 1999 issue of Science, where the finding was initially disclosed. The small yellow dots are globules of elemental sulfur formed by the oxidation of sulfides and are deposited only in the thin outer layer of the cell—comprising only 2 percent of the cell’s volume—that surrounds the large central reservoir of nitrate ions. Although a proposal to use Thiomargarita in the future to clean up pollution caused by agricultural nitrate-rich effluents may be far fetched, it is clear that the role that such sulfur bacteria play in the ecology of coastal waters should not be underestimated.
Written by Daniel Rabinovich <email@example.com>.
last modified 6 April 2010.
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