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The Small Red Pear of the Sea
By Kate Dalke

A microbe formerly known as Pirellula strain 1 is moving up in the scientific world.

The marine bacterium was plucked from the Baltic Sea over 20 years ago and then largely forgotten. Little was known about it, and it didn’t even have a name—only a number: strain 1.

Colonies of R. baltica living in the lab.

Now researchers have sequenced the genome of this bacterium and honored it with a proper name, Rhodopirellula baltica, which means the small, red pear of the Baltic. The single-celled microbe emits a reddish sunscreen that protects it from ultraviolet light on the surface of the water.

R. baltica can be found floating at the ocean surface, but also below the surface and in sediment. The bacterium sticks to (and eats) small bits of nutrient-rich waste in the ocean called marine snow particles. These particles slowly descend to the ocean floor with the bacteria, which have genes that allow them to survive in the oxygen-starved environment.

When the food pickings are slim, the bacteria can grow flagella that allow them to swim toward new sources of food.

At this early stage in the research, scientists simply want to learn about what R. baltica does in the world. Nonetheless, the creature may be of interest to biomedical researchers because it produces natural antibiotics.

“Our interest in the organism is completely based on [building our] knowledge,” says Frank Oliver Glöckner, who led the sequencing at the Max Planck Institute of Marine Microbiology in Bremen, Germany.

R. baltica has the largest circular chromosome of any bacterium sequenced so far: 7.1 million base pairs, almost twice as many as the common human gut bacterium Escherichia coli.

It is also the first representative of an abundant and diverse group of bacteria called Planctomycetales to be sequenced. Planctomycetes play an important role in the global carbon and nitrogen cycles and can break down ammonia in wastewater.

The genomes of two other planctomycetes are being sequenced now. The Institute for Genomic Research (TIGR) in Rockville, Maryland, is sequencing a freshwater isolate, Gemmata obscuriglobus, and Integrated Genomics, Inc., in Chicago, Illinois, is working on another strain, Gemmata sp. Wa1-1.

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Glöckner, F. O. et al. Complete genome sequence of the marine planctomycete Pirellula sp. strain 1. Proceedings of the National Academy of Sciences USA 100, 829-8303 (July 8, 2003).

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