The largest Bacterium: Scientist discovers new bacterial life form off the African coast

Even visible to the naked eye, newly discovered bacteria named Thiomargarita namibiensis are the largest prokaryotic organisms yet known. Biologist Heide Schulz from the Max Planck Institute for Marine Microbiology in Bremen found these up to 3/4 mm wide microorganisms during a cruise with the Russian research vessel "Petr Kottsov" in sediments off the Namibian coast. The bacteria live from sulfide produced in the sea floor and store nitrate from the seawater in an "anaerobic lung".

Figure: Light-photomicrograph of three cells of Thiomargarita with polarized light. The internal sulfur globules shine white.

It seems rather odd that these bacteria were overlooked so far by marine scientists. During an expedition off the coast of Namibia in the search of other sulfur bacteria known from the pacific coast of South America, biologist Heide Schulz was quite surprised when she looked at her sediment samples more closely. The spherical cells of Thiomargarita namibiensis are generally 0.1 - 0.3 millimeter wide but some reach up to a size of 3/4 of a millimeter. They live in great numbers in the coastal sediments off Walvis Bay (Namibia), and shine white because microscopic sulfur granules, which they store inside, reflect the incident light. Held in line by a common mucus sheath, they look like a thin string of pearls, which inspired the researchers of the Max Planck Institute for Marine Microbiology to name these bacteria Thiomargarita namibiensis, i.e. "Sulfur Pearl of Namibia". Thiomargarita namibiensis has its ecological niche in the oxygen-poor but nutrient-rich sediment and can survive in this environment which is toxic for most animal life due to high levels of hydrogen sulfide.

Figure: A single cell of Thiomargarita photographed next to a fruit fly. The head of the fly and the white cell have about the same diameter (0.5 mm).

In cooperation with the director of the institute, professor Bo Barker Jørgensen, biologist Schulz and her colleagues were able to demonstrate how the bacteria are adapted to this special environment. Under the microscope they discovered that almost the entire volume of the cell is a liquid container, a vacuole. This container makes up 98% of the cells internal volume. It is used to store nitrate which the bacteria utilize to oxidize sulfide. The nitrate concentrations within the cell can be up to 10.000 times higher than in the ambient seawater.

The coast of Namibia and the South American west coast have similar hydrographic conditions which brings vast amounts of nutrients to the sea surface. They belong to the richest areas in the world ocean in terms of phytoplankton (microalgae) productivity and fish stocks. A large proportion of the microalgae that grow in these waters sinks down to the sea bottom where the dead algae are degraded by bacteria. In the sediment, anaerobic bacteria use sulfate to oxidize this organic material, and they produce large amounts of sulfide. This sulfide is a potentially rich source of energy for bacteria when oxidized with nitrate from the oxygen-depleted seawater, but a special trick is used to bring the two chemical compounds together, as sulfide is produced in the sediment, while nitrate is found in the overlying water.

Earlier studies of the Max Planck Institute for Marine Microbiology have revealed this trick for an extensive and dense population of sulfur storing bacteria living along the Pacific coast of South America. The cells of these filamentous bacteria, called Thioploca, are ten times smaller in diameter than the newly discovered Thiomargarita. They live in vertical sheaths in the sediment and shuttle up and down between the surface of the sediment, where they stretch up into the water to take up nitrate for respiration, and deeper parts of the sediment, where they find their energy source, sulfide, that can be stored effectively as sulfur.

Thiomargarita is a close relative of these bacteria, but unlike their smaller cousins they are unable to and do not need to move constantly up and down to get nitrate or sulfide. With the much larger pools of nitrate, stored in the vacuole, and sulfur, stored in the periphery, Thiomargarita can survive for at least three months without external supply of either substrate. In the loose and unstable sediments off Namibia this enables them to lie in the sulfide-rich sediment and wait for times when a passing storm transports nitrate-rich seawater into their surroundings. As an adaptation to this mode of life, they are surprisingly resistant to extreme conditions such as high concentrations of oxygen or sulfide, which would kill Thioploca.

Thiomargarita are not just rare and exotic microorganisms. They occur in high biomass of up to 47 g per square meter, thus playing an important role in the oxidation of the toxic sulfide in Namibian sediments. By coupling the oxidation of sulfide with the reduction of nitrate these bacteria use an energy source which is not accessible for most bacteria in the absence of oxygen. Their special trick, to "hold their breath" and wait until the nutrients become available, is an adaptation unique in the living world.


Published: 8-4-99
Contact: Bo Barker Jørgensen
Max Planck Institute for Marine Microbiology,
Bremen/Germany
Phone: (+49 421) 2028- 602
Phone: (+49 421) 2028 - 580

Contact: Heide Schulz
Max Planck Institute for Marine Microbiology,
Bremen/Germany
Phone: (+49 421) 2028- 646
Phone: (+49 421) 2028 - 580

Contact: Manfred Schloesser, public relation official
Max Planck Institute for Marine Microbiology,
Bremen/Germany
Phone: (+49 421) 2028- 704
Phone: (+49 421) 2028 - 790