Hydrothermal vents release heat and hydrogen sulfide, allowing extremophiles to survive using chemolithotrophic growth. Archaea are generally similar in appearance
to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA.
By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually
more phylogenetically related to eukaryotes. As their name suggests, Archaea comes from a Greek word archaios, meaning original, ancient, or primitive.
Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was
likely exposed to. Examples of these Archaean extremophiles are as follows:
- Thermophiles, optimum growth temperature of 50 °C-110 °C, including the genera Pyrobaculum, Pyrodictium, Pyrococcus, Thermus aquaticus and Melanopyrus.
- Psychrophiles, optimum growth temperature of less than 15 °C, including the genera Methanogenium and Halorubrum.
- Alkaliphiles, optimum growth pH of greater than 8, including the genus Natronomonas.
- Acidophiles, optimum growth pH of less than 3, including the genera Sulfolobus and Picrophilus.
- Piezophiles, (also known as barophiles), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera Methanococcus and Pyrococcus.
- Halophiles, grow optimally in high salt concentrations between 0.2 M and 5.2 M NaCl, including the genera Haloarcula, Haloferax, Halococcus.
Methanogens are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals. This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of adenosine triphosphate. They are the only known organisms capable of producing methane. Under stressful environmental conditions that cause DNA damage, some species of archaea aggregate and transfer DNA between cells. The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell.
Diagram of Archaea
