18 April 1999

New Bacterial Life Form Discovered

Researchers have found the biggest bacteria ever known off the African Coast. The largest of these single-celled microbes is visible to the naked eye, and is nearly 100 times larger than the previous bacterial record-holder. In addition, the new microbe is an exotic organism that provides firmer evidence of coupling between two key environmental cycles thought until recently to be mutually exclusive in the ocean: the sulfur and nitrogen cycles.

The newly discovered bacteria, named Thiomargarita namibiensis, are the largest prokaryotic organisms yet known, and have been found to be up to 0.75 mm wide. Biologist Heide Schulz from the Max Planck Institute for Marine Microbiology found the microorganisms in sediments off the Namibian coast. In bacterial terms, this is huge.

The bacteria live from sulfide produced in the sea floor and store nitrate from the seawater in an "anaerobic lung".

During an expedition off the coast of Namibia in the search of other known sulfur bacteria, 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 mm wide but some reach up to a size of 0.75 of a millimetre. 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 to name these bacteria Thiomargarita namibiensis, or "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.

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 that 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. 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. And 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 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 bacteria. They live in vertical sheaths in the sediment and move 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 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 also play an important role in the oxidation of the toxic sulfide in Namibian sediments, occurring in high biomass of up to 47 g per square metre. 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 a unique adaptation.