Scientist around the world are trying to find life on other planets. But is "life" something we expect it to be? Recently found species don't use the basic ingredient - oxygen.
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The discovery of the new species, which live buried in sediment under the Mediterranean seafloor, is significant in that it marks the first observation of multi-cellular organisms (metazoans), that spend their entire life under permanently anoxic conditions. A few metazoans have been known to tolerate anoxic conditions, but only for limited periods of time.
The team of Italian and Danish researchers that discovered the new life forms has identified the creatures as belonging to the animal phylum Loricifera, the most recently described animal phylum. Loriciferans, which have a length of less than one millimetre, typically live in sediment.
Despite belonging to previously known taxonomic groups, the new species possess some radical differences compared with other metazoans. Most significantly, the new species do not have mitochondria, the cellular organelles that use oxygen and sugar to generate the cell’s energy. Instead, the new Loriciferans have organelles that resemble hydrogenosomes, which are used by some single-celled eukaryotes to generate energy without oxygen. However, this is the first time that these organelles have been observed in multi-cellular organisms. Previous research has indicated that hydrogenosomes may have evolved from mitochondria, while other research suggests they evolved independently.
To find the new species, the researchers carried out three oceanographic expeditions from 1998 to 2008 to search for life in the extreme environments located more than 3,000 meters (for our American friends: two miles) under the Mediterranean Sea. The researchers focused on an area called the L’Atalante basin, which is located off the southern coast of Greece.
This type of "deep hypersaline anoxic basin" was created by the flooding of mineral sediments from 5.5 million years ago. For the past 50,000 years, the basin has possessed a dense hypersaline brine layer up to 60 meters thick. The brine serves as a physical barrier that prohibits oxygen exchange between the water and sediment, making the basin completely oxygen-free. In addition, the basin is rich in methane and hydrogen sulphide, and is also home to a diverse assembly of prokaryotes that have adapted to these conditions.
Because previous studies have reported the presence of cadaverous metazoans that had sunk to anoxic deep-sea sediments in the Black Sea, the researchers here stained the newly collected specimens with Rose Bengal, a protein binding stain that colors living organisms with a much greater intensity than deceased organisms, demonstrating that the new species were indeed alive. In addition, the scientists found the data that proved evidence of reproduction.
The results support the hypothesis that inhabitants of the anoxic sediments of the L’Atalante basin have developed an anaerobic metabolism and specific adaptations to live without oxygen. So, while we are searching for life on another planets, it would be wise to think again about the definition of life. ■