Gene transfer and genetic exchange have been studied in the halophilic archaeon "Halobacterium volcanii" and the hyperthermophilic archaeons "Sulfolobus solfataricus" and "Sulfolobus acidocaldarius". "H. volcani" forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another in either direction. When "S. solfataricus" and "S. acidocaldarius" are exposed to DNA damaging agents, species-specific cellular aggregation is induced. Cellular aggregation mediates chromosomal marker exchange and genetic recombination with high frequency. Cellular aggregation is thought to enhance species specific DNA transfer between "Sulfolobus" cells in order to provide increased repair of damaged DNA by means of homologous recombination. Archaea are divided into 3 subgroups which are halophiles, methanogens, and thermoacidophiles. The first group, methanogens, are archaeabacteria that live in swamps and marshes as well as in the gut of humans. They also play a major role in decay and decomposition with dead organisms. Methanogens are anaerobic organisms, which are killed when they are exposed to oxygen. The second subgroup of archaeabacteria, halophiles are organisms that are present in areas with high salt concentration like the Great Salt Lake and the Dead Sea. The third subgroup thermoacidophiles also called thermophiles, are organisms that live in acidic areas. They are present in area with low pH levels like hot springs and geyers. Most thermophiles are found in the Yellowstone National Park.