study done by Hurst and Merchant (2001) showed no correlation between higher GC content in prokaryotes and increased optimal growing temperatures. However, their analysis did show that there was higher GC content for the third nucleic acid within the codon. This demonstrates that within the wobble position there is likely a selective pressure for more hydrogen bonds to increase stability within the DNA, but less selective pressure for GC pairings within the DNA as a whole. This supports what is seen in P. fumarii. The majority of the DNA is composed of G and C nucleotides, but the DNA still contains many A and T nucleotides. These results likely indicate that along with increasing GC pairing in the wobble position, thermophilic archaea have other mechanisms for stabilizing their DNA at such high temperatures. One possible mechanism for stabilizing DNA at such high temperatures are proteins such as a type I topoisomerase that supertwists the DNA making spontaneously untwisting of the DNA more difficult. The presence of this protein in multiple evolutionarily distant organisms supports the hypothesis that this protein plays a role in DNA stabilization. Membrane adaptations All microbes ranging from the smallest bacteria to the largest multicellular eukaryote contains a membrane with phospholipids. A phospholipid molecule is composed of a long fatty acid, often called the tail of the molecule, and a phosphate group, which serves as the head of the molecule. Phospholipid membranes can range widely in the structure of the fatty acid tail, which is composed of mostly hydrocarbons. These phospholipid molecules form bilayers with the polar phosphate groups facing the aqueous solution inside or outside of the cell with the hydrocarbons facing inward interacting with each other. The membrane, along with proteins, controls which molecules are allowed in or out of the cell. For this reason, the membrane plays a crucial role in the survival of the cell. A faulty membrane can allow too many solutes into the cell, resulting in cell death. Different organisms have devised different strategies to control what goes in and out of the cell. Bacteria and eukaryotic cells contain phospholipid bilayers containing ester linkages, while archaea contain ether linkages. While these mechanisms work very well for organisms that live in mesophilic environments, they do not work for extremophiles. Mesophiles are organisms that live within relatively moderate temperatures (20–45 °C). They are organisms that live around sea level and can survive around