by Whit Gibbons

July 14, 2013

Q: Blue whales reach a length of 100 feet and are the biggest animals on earth as far as scientists know. But several years ago you wrote about some type of microorganism that got enormous as well. What was the story?

A: In 1999 a type of bacteria a hundred times larger than any other known bacteria was discovered in the Atlantic Ocean off the coast of Namibia in southwestern Africa. Compared to other bacteria, that would be equivalent to a whale more than a mile long.

H.N. Schulz of the Max Planck Institute for Marine Microbiology in Germany reported finding a new species that approached the enormous size (for bacteria) of a period (like the one at the end of this sentence). Schulz and her research team were amazed to find the previously unknown giant in samples taken more than 300 feet deep in ocean sediments. "These giant bacteria grow as a string of pearls," they said in Science magazine, "which shine white ... and are large enough to be visible to the naked eye." Yet each individual of the species has only one cell.

The big bacterium was given a big name - Thiomargarita namibiensis (which means "sulfur pearl of Namibia"). The species belongs to a group known as sulfur bacteria that thrive on sulfur compounds from which they obtain energy. However, whereas some sulfur bacteria require oxygen for the energy conversion process, the giant bacteria live in sediments with little or no oxygen present. Thiomargarita rely instead on nitrogen compounds. Thiomargarita live in deep waters where both sulfur and nitrogen accumulations can be high, but both elements may not always be abundant at the same time. Hence, the bacteria often store one or the other until suitable levels of both are available. The enormous size of Thiomargarita bacteria is explained as a necessity for storage while the organism waits out periods when either nitrogen or sulfur compounds are in short supply.

The unusually large size of the single cells of Thiomargarita was at first a mystery in itself. For species such as bacteria that do not breathe with lungs or gills, the factor limiting an increase in body size is a phenomenon known as volume to surface ratio. That is, the maximum volume of the organism is constrained by the ability of the organism's surface to provide the necessary exchange of gases with the surrounding environment.

The volume to surface ratio results in a physical law in which the bulk of an object increases at a proportionally greater rate than its surface. Thus, the ultimate size the organism can reach is limited. The volume to surface ratio is also one factor limiting the size of lungless animals like insects, assuring that most must remain small because the required oxygen must diffuse from the outside surface of the body to the inside.

The giant size of Thiomargarita is accomplished as a result of a hollow interior, where the sulfur and nitrogen compounds are stored. The actual living matter of the cell forms a thin layer with a typical outside surface area but also an inside surface area that is available for gas exchange. Therefore, the volume to surface ratio differs from that of the smaller bacteria that can only use the outer surface.

The discovery of such a bacterium offers prospects for controlling certain forms of water pollution. A bacterial species that thrives on high levels of nitrates and sulfides that are harmful to most animal species could be very useful. The bacteria convert the harmful compounds into harmless sulfur and nitrogen, thus cleansing the habitat. Such systems of biological control are seldom understood and developed overnight, but research directed at such a problem could result in major strides in environmental cleanup technology, which is encouraging news.

But of even greater interest is the fact that the existence of such a phenomenal organism was unknown until the last decade of the millennium. What else is out there that we still know nothing about?

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