In addition, the long-chain hydrocarbons of the archaea are branched. One class of these lipids, with hydrocarbon chains 40 carbon atoms in length, contains glycerol at both ends of the hydrocarbons (Figure 27.18, left). This lipid monolayer structure, unique to the domain Archaea, still fits in a biological membrane because the lipids are twice as long as the typical lipids in the bilayers of other membranes (see Figure 27.18). Lipid monolayers and bilayers are both found among the archaea.

In spite of this striking difference in their membrane lipids, all three domains have membranes with similar overall structures, dimensions, and functions.

Most Crenarchaeota live in hot, acidic places

Most known Crenarchaeota are both ther-mophilic (heat-loving) and acidophilic (acid-loving). Members of the genus Sulfolobus live in hot sulfur springs at temperatures of 70-75°C. They die of "cold" at 55°C (131°F). Hot sulfur springs are also extremely acidic. Sul-folobus grows best in the range from pH 2 to pH 3, but it readily tolerates pH values as low as 0.9. One species of the genus Ferroplasma lives at a pH near 0. Some acidophilic hyperthermophiles maintain an internal pH near 7 (neutral) in spite of their acidic environment. These

Acid Loving ArchaeaHyperthermophiles
27.19 Some Would Call It Hell;Archaea Call It Home Masses of heat- and acid-loving archaea form an orange mat inside a volcanic vent on the island of Kyushu, Japan. Sulfurous residue is visible at the edges of the archaeal mat.

and other hyperthermophiles thrive where very few other organisms can even survive (Figure 27.19).

is increasing in Earth's atmosphere by about 1 percent per year and is a major contributor to the greenhouse effect. Most of the increase is probably due to increases in cattle and rice farming and the methanogens associated with both.

One methanogen, Methanopyrus, lives on the ocean bottom near blazing hydrothermal vents. Methanopyrus can survive and grow at 110°C. It grows best at 98°C and not at all at temperatures below 84°C.

Another group of Euryarchaeota, the extreme halophiles (salt lovers), lives exclusively in very salty environments. Because they contain pink carotenoids, they can be seen easily under some circumstances (Figure 27.20). Halophiles grow in the Dead Sea and in brines of all types: Pickled fish may sometimes show reddish pink spots that are colonies of halophilic archaea. Few other organisms can live in the saltiest of the homes that the extreme halophiles occupy; most would "dry" to death, losing too much water to the hypertonic environment. Extreme halophiles have been found in lakes with pH values as high as 11.5—the most alkaline environment inhabited by living organisms, and almost as alkaline as household ammonia.

Some of the extreme halophiles have a unique system for trapping light energy and using it to form ATP—without using any form of chlorophyll—when oxygen is in short supply. They use the pigment retinal (also found in the vertebrate eye) combined with a protein to form a light-absorbing molecule called bacteriorhodopsin, and they form ATP by a chemiosmotic mechanism of the sort described in Figure 7.12.

The Euryarchaeota live in many surprising places

Some species of Euryarchaeota share the property of producing methane (CH4) by reducing carbon dioxide. All of these methanogens are obligate anaerobes, and methane production is the key step in their energy metabolism. Comparison of rRNA nucleotide sequences revealed a close evolutionary relationship among all these methanogens, which were previously assigned to several unrelated bacterial groups.

Methanogens release approximately 2 billion tons of methane gas into Earth's atmosphere each year, accounting for 80 to 90 percent of the methane in the atmosphere, including that associated with mammalian belching. Approximately a third of this methane comes from methanogens living in the guts of grazing herbivores such as cows. Methane

Methanogens ClassHalophiles
27.20 Extreme Halophiles Commercial seawater evaporating ponds, such as these in San Francisco Bay, are attractive homes for salt-loving archaea, which are easily visible because of their carotenoids.

Another member of the Euryarchaeota, Thermoplasma, has no cell wall. It is thermophilic and acidophilic, its metabolism is aerobic, and it lives in coal deposits. It has the smallest genome among the archaea, and perhaps the smallest (along with the mycoplasmas) of any free-living organism— 1,100,000 base pairs.

In addition to these archaea that are found in amazing habitats, many Crenarchaeota and Euryarchaeota live in environments that are not extreme.

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