April 16, 1999
The largest bacterium ever found, a harmless organism that grows as a string of white beads large enough to be visible to the naked eye, has been found in coastal sediments off the coast of Namibia by an international research team. In an article in today’s issue of the journal Science, researchers from the Max Planck Institute for Marine Microbiology in Germany, the University of Barcelona in Spain, and the Woods Hole Oceanographic Institution in Massachusetts suggest the new genus and species be named Thiomargarita namibiensis(“sulphur pearl of Namibia”).
More than three times bigger by volume than the previous record holder and 100-200 times longer than the average bacterium, the new organism breaks records for size among bacteria. In simple terms, each microbe is about the size of the period at the end of this sentence. Genetic sequencing indicates Thiomargarita, collected during a 1997 cruise aboard the Russian research vessel Petr Kottsov, is closely related to the marine species of the filamentous sulphur bacteria, Thioplocaand Beggiatoa. The largest known sulphur bacteria, Beggiatoa spp., are found at hydrothermal vents in the Guaymas Basin, Gulf of California. That genus can reach diameters of 160 microns, while the extremely large forms of Thiomargarita have an average diameter of 180 microns but can reach diameters of up to 750 microns.
“They are very large and interesting organisms with an amazing survival strategy,” team member Andreas Teske of Woods Hole Oceanographic Institution notes. “It would be like people holding their breath for several weeks at a time.” Teske, an assistant scientist in the Institution’s Biology Department, conducted the genetic studies on the new microbe. Thiomargarita is physiologically adapted to its highly dynamic environment with frequent fluctuations of the chemicals needed for food and respiration. “They have evolved to store their food, sulfide, and nitrate for respiration, which is like oxygen to people, when it is plentiful to get them through periods when the surrounding sediments don’t contain enough of what they need to survive,” Teske says. Unlike their close relatives, Thioplocaand Beggiatoa, Thiomargaritaare not mobile and cannot follow fluctuating sulfide and nitrate gradients. Instead, they stay put and wait for the return of more favorable conditions. Their sturdiness allows them to survive and grow in the laboratory in samples of their natural sediment for months to years.
The Namibia shelf off the southwest African coast is an area of strong upwelling, with high plankton productivity and oxygen depleted bottom water. Thiomargaritaoccurred in high numbers in the sediments around Walvis Bay, at a water depth of about 300 feet, with the highest concentrations in the top three centimeters of the sediment. These sediments receive a high load of organic material from the water column; the bacterial degradation of this organic material leads to high sulfide levels in the sediments. Sulfide, a foul-smelling and generally poisonous compound, is used by Thiomargarita,which acts as a sulfide scrubber and detoxifier.
A similar situation occurs off the Pacific coast of Chile and Peru, where large mats of Thioplocacover the sediments. These and other sulfide oxidizers remove sulfide so efficiently that, even in the sediment, its smell is often barely noticeable, allowing animals to exist in what otherwise would be an “off limits” area to most life forms.
“Thiomargaritaand its relatives play an important role in the oceans as nitrate-respiring sulfide-oxidizers; the widespread occurrence and high activity of these bacteria is being recognized only now,” Teske adds. “The discovery of Thiomargaritashows the ocean’s barely-tapped potential for new and exciting findings in the microbial world.”