EURYARCHAEOTA

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PHYLUM THERMOBACTERIA

INTRODUCTION TO THE THE THERMOBACTERIA

The thermobacteria is a diverse group of variable taxa that have growth maxima often at 80C or higher and differ from the thermoacidophills in that they have cell walls and have growth maxima near neutral pH. In general, the thermobacteria are unicellular (usually coccoid) and motile (Figure A). They respire sulfur and are common members of the microbial flora of sulfotara.

A. TEM image of Thermococcus.

Image taken from: http://www.biologie.uni-regensburg.de/Mikrobio/Thomm/Buttons/bilder/thermococcus-ch-Pt.jpg

SYNOPTIC DESCRIPTION OF THE THERMOBACTERIA

The following description of the Thermobacteria came from Brock et al. (1994), Margulis and Schwartz (1998), Garrity et al. (2001 and 2003), and Black (2002).

I. SYNONYMS: Thermoacidophilic bacteria, archaebacterial mycoplasmas.

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Coccoid, and variable

Cell Wall: Variable Gram + and -.

Motility: Motile or non-motile.

B. Physiology

O₂ Tolerance: Strict anaerobes.

Substrates: Many sulfur compounds; carbon dioxide and hydrogen in Methanopyrus.

Products: Hydrogen sulfide and elemental sulfur; methane in Methanopyrus.

C. Other:

D. Ecology: Grow at neutral pH above 80C. Methanopyrus freezes below 80C.

SYSTEMATICS OF THE THERMOBACTERIA

I have grouped together three classes that might be quite distinct. Thus, I present the following system with much qualification. These taxa had been classified in the Crenarchaeota by Margulis and Schwartz (1999); however, I have followed Garrity et al. (2001 and 2003), who place the these organisms in within the Euryarchaeota. I have followed the relative placement of Garrity et al. (2001) and moved them into the Kingdom Euryarchaeota together with the methanogens and extreme halophiles.

HIERARCHICAL CLASSIFICATION OF THE THERMOBACTERIA

TAXONOMY OF THE PHYLUM THERMOBACTERIA. This system follows that of Garrity et al. (2001 and 2003) who place the thermoplasmas in the Euryarchaeota as a taxon equal to the methanogens and halophiles.

CLASS THERMOCOCCI

Cells spherical (though variable in shape), often in pairs or clustered. Motile with one or more polar flagella (Figure A). Strict anaerobes; sulfur respiration. 80-103C. One order: THERMOCOCCALES

Thermococcus, Pyrococcus.

CLASS ARCHAEGLOBI

Cells cocci to very irregular; Strict anaerobes. Sulfate, sulfite, thiosulfate and nitrate promote growth. Elemental sulfur inhibits growth. One order: ARCHAEGLOBALES.Although extreme thermophiles (thermal optima between 64 and 92C), they differ from the other members of this phylum in the ability to reduce sulfate and generate methane. Indeed, 16S rRNA studies suggest that this group may be more closely allied with the Methanobacteria. The class has 1 order (Archaeoglobales).

Archaeglobus, Ferroglobus.

CLASS METHANOPYRI

Rod-shaped cells; Gram +; strict anaerobes. They use hydrogen to reduce carbon dioxide. Growth above 100C; no growth below 80C. One order: METHANOPYRALES.

These organisms have only been recognized recently as members of the extreme thermophiles. Garrity et al. (2001) recognize this as a separate group with a single order (Methanopyrales).

Methanopyrus.

This page is maintained by Jack R. Holt. Last revised: 02/07/2008.