General Microbiology (MICR300)

Lecture 8Microbial Diversity: Archaea

(Text Chapters: 13.1-13.12)

Archaea

Comparing Bacteria and Archaea

Characteristic Bacteria ArchaeaProkaryotic cell structure Yes Yes

Cell wall Muramic acid present Muramic acid absentMembrane lipids Esther-linked Ether-linked

Ribosomes 70S 70SInitiator tRNA Formylmethionine Methionine

Operons Yes YesRNA polymerases One (4 subunits) Several (8-12 subunit each)Methanogenesis No Yes

Growth above 1000C No Yes

Phylogenetic Overview

Archaea consists of four phyla, the Euryarchaeota, the Crenarchaeota, the Korarchaeota, and the Nanoarchaeota, with the first two phyla being the major ones.

Figure 13.1 shows a phylogenetic tree of Archaea.

Metabolism

With the exception of methanogenesis, bioenergetics and intermediary metabolism in species of Archaea are much the same as those in various species of Bacteria.

Several Archaea are chemoorganotrophic and thus use organic compounds as energy sources for growth. Chemolithotrophy is also well established in the Archaea, with H2 being a common electron donor.

Metabolism

The capacity for autotrophy is widespread in the Archaea and occurs by several different pathways. In methanogens, and presumably in most chemolithotrophic hyperthermophiles, CO2 is incorporated via the acetyl-CoA pathway or some modification thereof.

Phylum Euryarchaeota

Extremely Halophilic Archaea

Extremely halophilic Archaea require large amounts of NaCl for growth. These organisms accumulate high levels of KCl in their cytoplasm as a compatible solute.

These salts affect cell wall stability and enzyme activity. The light-mediated proton pump bacteriorhodopsin helps extreme halophiles make ATP (Figure 13.4).

Methane-Producing Archaea: Methanogens

A large number of Euryarchaeota produce methane (CH4) as an integral part of their energy metabolism. Such organisms are called methanogens. Methanogenic Archaea are strictly anaerobic prokaryotes.

Habitats of methanogenic Archaea are listed in Table 13.4.

Thermoplasmatales: Thermoplasma, Ferroplasma, and

Picrophilus

Thermoplasma, Ferroplasma, and Picrophilus are extremely acidophilic thermophiles that form their own phylogenetic family of Archaea inhabiting coal refuse piles and highly acidic solfataras.

Cells of Thermoplasma and Ferroplasma lack cell walls and thus resemble the mycoplasmas in this regard.

Thermoplasma

To survive the osmotic stresses of life without a cell wall and to withstand the dual environmental extremes of low pH and high temperature, Thermoplasma has evolved a unique cell membrane structure (Figure 13.11).

Phylum Crenarchaeota

Habitats and Energy Metabolism of Crenarchaeotes

Table 13.7 summarizes the habitats of Crenarchaeota. They include very hot and very cold environments.

Most hyperthermophilic Archaea have been isolated from geothermally heated soils or waters containing elemental sulfur and sulfides. Hyperthermophilic Crenarchaeota inhabit the hottest habitats currently known to support life.

Habitats and Energy Metabolism of Crenarchaeotes

Cold-dwelling crenarchaeotes have been identified from community sampling of ribosomal RNA genes from many nonthermal environments

Crenarchaeotes have been found in marine waters worldwide and thrive even in frigid waters and sea ice.

Hyperthermophiles from Terrestrial Volcanic Habitats

Sulfolobales and Thermoproteales are two representative orders of hyperthermophilic Archaea from Terrestrial volcanic habitats

Two phylogenetically related organisms isolated from these environments include Sulfolobus and Acidianus. These genera form the heart of an order called the Sulfolobales.

Key genera within the Thermoproteales are Thermoproteus, Thermofilum, and Pyrobaculum.

Hyperthermophiles from Submarine Volcanic Habitats

Submarine volcanic habitats are homes to the most thermophilic of all known Archaea. These habitats include both shallow-water thermal springs and deep-sea hydrothermal vents.

Pyrodictium and Pyrolobus are examples of archaea whose growth temperature optimum lies above 100ºC. The optimum for Pyrodictium is 105ºC and for Pyrolobus is 106ºC.

Hyperthermophiles from Submarine Volcanic Habitats

Cells of Pyrodictium are irregularly disc-shaped and grow in culture in a mycelium-like layer attached to crystals of elemental sulfur.

Other notable members of the Desulfurococcales include Desulfurococcus and Ignicoccus.

Like Pyrodictium, Desulfurococcus is a strictly anaerobic S0-reducing bacterium, but it differs from Pyrodictium in that it is much less thermophilic, growing optimally at about 85°C. Ignicoccus grows optimally at 90ºC, and its metabolism is H2/S

0 based.

Heat Stability of Biomolecules

Protein and DNA stability in hyperthermophiles is critical to surviving high temperature. Because most proteins denature at high temperatures, much research has been done to identify the properties of thermostable proteins.

Hyperthermophilic prokaryotes typically produce special classes of chaperonins that function only at the highest growth temperatures. In cells of Pyrodictium, for example, the major chaperonin is a protein complex called the thermosome.

Heat Stability of Biomolecules

All hyperthermophiles produce a DNA topoisomerase called reverse DNA gyrase. Reverse gyrase introduces positive supercoils into DNA (in contrast to the negative supercoils introduced by DNA gyrase, found in all nonhyperthermophilic prokaryotes).