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Borgonie, Gaetan Nematology Section, Department of Biology, Ghent University, Ghent, Belgium.
- Habitat and ecology
- Additional observations
- Related Primary Literature
- Additional Reading
The deep subsurface biosphere, extending more than 3 km (1.86 mi) under the Earth's surface, comprises a significant fraction of the global planetary biosphere. Limitations imposed by temperature, energy, oxygen, and space led to the conviction that the deep subsurface was populated exclusively by bacteria and viruses with a total biomass equaling that of the surface. However, no multicellular organisms were believed to be able to survive at these depths. Although many lower invertebrate phyla have a reputation for being able to survive in harsh conditions, the phylum Nematoda (also termed Nemata, comprising the roundworms) is second to none. (Roundworms are not to be confused with earthworms, which belong to the phylum Annelida.) Nematodes have been recovered from the seafloor of deep oceans, hot springs, and acidic seeps (pH 0), and some species recover easily from deep freezing or decades of desiccation. However, the most impressive example of stress resistance in nematodes was demonstrated by the survival of the species Caenorhabditis elegans during the 2003 breakup of the space shuttle Columbia upon reentry, and the subsequent free fall and impact of the biological container in which the species was contained. In addition to the ability of nematodes to undergo anabiosis (a state of suspended animation induced by desiccation and reversed by the addition of moisture) for extended periods, these organisms continue to metabolize aerobically in hypoxic (oxygen-deficient) environments where the partial pressure of oxygen (O2) is only 0.4 kPa (2% of the atmospheric value). Therefore, nematodes were regarded as prime candidates to look for in the deep subsurface because they are one of the most successful metazoan phyla on the surface with respect to their abundances, distribution, and physiological tolerances.
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