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Achaea. Figure 19.2. Figure 19.5. Crenarchaeota. The name Crenarchaeota means “scalloped archaea.” - Are often irregular in shape All crenarchaeotes synthesize a distinctive tetraether lipid, called crenarchaeol . Figure 19.6. Crenarchaeota. Desulfurococcales

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The name Crenarchaeota means “scalloped archaea.”

- Are often irregular in shape

All crenarchaeotes synthesize a distinctive tetraether lipid, called crenarchaeol.

Figure 19.6



- Lack cell walls, but have elaborate S-layer

- Reduce sulfur at higher temperatures

Desulforococcus mobilis

- Hot springs

Ignicoccus islandicus

- Marine organism

Figure 19.8


Barophilic hyperthermophiles

- Grow near hydrothermal vents on the ocean floor

- A common feature is the black smoker.

- Crenarchaeotes that are vent-adapted:

- Pyrodictium abyssi

- Pyrodictium occultum

Figure 19.9



- Include species that respire by oxidizing sulfur (instead of reducing it)

- Found within hotsprings

- Sulfolobus solfataricus

- A “double extremophile”

- Grows at 80oC and pH 3

- Oxidizes H2S to sulfuric acid

Figure 19.13


The crenarchaeote Cenarchaeum symbiosum inhabits the sponge Axinella mexicana.

- The relationship is unclear, but they can be co-cultured in an aquarium for many years.

Figure 19.17

Euryarchaeota methanogens
Euryarchaeota: Methanogens

Euryarchaeota means “broad-ranging archaea.”

Are dominated by methanogens

- All are poisoned by molecular oxygen and therefore require complete anaerobiosis.

- Major substrates and reactions include:

Carbon dioxide: CO2 + 4H2→ CH4 + 2H2O

Acetic acid: CH3COOH → CH4 + CO2

Methanol: 4CH3OH → 3CH4 + CO2 + 2H2O

Methylamine: 4CH3NH2 + 2H2O →

3CH4 + CO2 + 4NH3


Anaerobic Habitats for Methanogens

Methanogens grow in:

- Anaerobic soil of wetlands

- Especially rice paddies

- Landfills

- Digestive tracts of animals

- Termites

- Cattle

- Humans

- Marine benthic sediments

Figure 19.22A

Figure 19.22B


Biochemistry of Methanogenesis

Biochemical pathways of methanogens involve unique cofactors.

- These transfer the hydrogens and increasingly reduced carbon to each enzyme in the pathway.

Figure 19.25


Biochemistry of Methanogenesis

The process fixes CO2 onto the cofactor methanofuran (MFR).

- The carbon is then passed stepwise from one cofactor to the next, each time losing an oxygen to form water, or gaining a hydrogen carried by another cofactor.

Figure 19.26

Euryarchaeota halophiles
Euryarchaeota: Halophiles

Main inhabitants of high-salt environments are members of the class Haloarchaea.

Figure 19.28

- Their photopigments color salterns, which are used for salt production.

- Most are colored red by bacterioruberin, which protects them from light.

Halophilic archaea require at least 1.5M NaCl.

Figure 19.29B


Animation: Light-Driven Ion Pumps and Sensors

Retinal-Based Photoheterotrophy

Click box to launch animation


H. salinarum glycoprotein cell wall


The smallest known euryarchaeotes.

Nanoarchaeum equitans

- Is an obligate symbiont of the crenarchaeote Ignicoccus hospitalis

- Host and symbiont genomes have been sequenced, revealing extensive coevolution.

Figure 19.36