13.11–13.12: Archaea exploit some of the most extreme habitats.

Grand Prismatic Spring hydrothermal vent in Yellowstone National Park, Wyoming. Hot springs are home to many microbes, including thermophilic archaea.

When you look at the photograph of archaea in FIGURE 13-16, you may find it hard to believe that they are even a little bit different from bacteria, let alone profoundly different. Both live as single cells or colonies of cells, both are surrounded by a plasma membrane, and both have species with flagella that twirl like propellers. In fact, until the 1970s, biologists considered the archaea to be bacteria. Only then did comparisons of DNA reveal that archaea are as different from bacteria as humans are.

Figure 13.16: Appearances are deceiving.

Those studies of archaeal nucleotide sequences stimulated other comparisons, which identified additional differences among bacteria, archaea, and eukarya. For example, the chemical compositions of the plasma membranes, cell walls, and flagella of archaea are qualitatively different from those of bacteria. And beyond the large DNA sequence differences and the differences in plasma membranes, cell walls, and flagella, a third difference reflects the phylogenetic positioning of archaea between bacteria and eukarya on the tree of life (see Figure 13-1): eukaryotes have a distinct cell nucleus that is separated from the cytoplasm by a nuclear membrane, whereas bacteria and archaea have neither a nucleus nor a nuclear membrane. The presence of a nucleus protects the chromosomes of a eukaryotic cell and allows the cell to control what molecules interact with its DNA.