How Big is Big, How Small is Small?

Generally, eukaryotic cells are large, and prokaryotic cells are small. These size differences occur because eukaryotic cells have complex substructures and internal architecture that prokaryotic cells lack. Since, we believe that prokaryotic cells depend on diffusion to exchange materials with the environment, they would have to be small and viruses, intracellular parasites, would of necessity be very small. There are, of course, exceptions.

In 1999, a team of scientists from Germany, Spain and The United States isolated large sulfur bacteria named as Thiomargartia namibiensis, the sulfur pearl of Namibia. These bacteria can be almost half millimeter in diameter. Based on the sequence of 16s ribosomal DNA, these bacteria were shown to be close relatives of other marine sulfur bacteria. They are almost one hundred times the volume of the bacteria, previously believed to be largest, Epuliscium fishelsoni, known only from the intestine of the brown surgeon-fish.

 

Mycoplasma pneumoniae

 
     The smallest prokaryotes are the Mycoplasma, at about 0.2µm in diameter, rivaling the viruses’ size. They are animal pathogens and decomposing organisms. The smallest eukaryote, Ostreococcus tauri, a green algae found in the Plankton, was discovered in 1994 from a water sample in French lagoon. These organisms are less than 1µm in diameter. Scientist believe that the lower limit on the size of a cell is about 200nm (0.2µm), based on the size of DNA and ribosome that a cell must contain.

 

Thiomargarita namibiensis

     With Thiomargarita namibiensis as the largest prokaryote, we note that the largest eukaryotic cell with a single nucleus is most likely the Ostrich egg. The largest organisms are the Blue Whale (Balaenoptera musculus), weighing 118,000 kilograms; Giant Redwood trees, Sequiadendron giganteum, 100 meters tall and weighing 5.5 million kilograms; a quaking aspen clone, Bopulus tremuloides, weighing 6 million kilograms; and Armillaria bulbosa (Synonymous with Armillaria gallica), a fungus. In 1992, three scientists from Michigan Technological University, using Restriction Fragment Length Polymorphisms (RFLPs) and Polymerase Chain Reaction (PCR) techniques, showed that the huge hyphal mass of this tree-root colonizing fungus growing in a forest in Northern Michigan was a single organism. It covered about eight hectares, probably weighed more than 10,000 kilograms, and probably has existed for more than 1,500 years.

Although we don’t want to get distracted be the oddities and extremes of nature, size differences are remarkable.

(Content compiled by Raj Kapur Kushwaha for IATSOM blog. He is currently pursuing Master's Degree in Medical Microbiology at St. Xavier's College, affiliated to Tribhuvan University, Nepal.)