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PHYLUM METHANOBACTERIA

INTRODUCTION TO THE METHANOBACTERIA

The Methanobacteria may be among the most abundant organisms on earth.  They occur in sediments or any other substrate low in oxygen and with enough water to support them.  In such environments they reduce carbon dioxide with hydrogen, formate, acetate, or methanol (Margulis and Schwartz 1998) and release methane as a waste product.  Methanogens are the sources of methane bubbles known as marsh gas common to wetlands and anoxic lake sediments.  Also, they occur as commensals in the guts of animals such as termites and cows.  All in all, methanogens likely are the primary contributors to atmospheric methane, a very powerful greenhouse gas.

A. Methanosarcina (cocci) and Methanosaeta (Methanothrix?, rods).
Image taken from
A: http://www5.gtz.de/gate/techinfo/biogas/basics/microbiol.html

SYNOPTIC DESCRIPTION OF THE METHANOBACTERIA

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

I. SYNONYMS: Methanogenic bacteria; methanogens.

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Variable; cocci, rods and spirals.

Cell Wall: Archaebacterial; gram+ or gram-.

Motility: Non-motile.

B. Physiology

O2 Tolerance: Obligate anaerobes.

Substrates: Reduce CO2 with H2; formic acid, acetic acid and methanol can also serve as H-donors.

Products: Methane.

C. Other: Have a unique type of t-RNA and characteristic co-enzymes (M and F420).

D. Ecology: Found in aquatic sediments and in the digestive tracts of animals. Some require very high temperatures. For example, Methanococcus jannaschii has a thermal optimum of 85C.

SYSTEMATICS OF THE METHANOBACTERIA

This phylum almost certainly is paraphyletic since all of these organisms are lumped together based only on the ability to generate methane by the reduction of carbon dioxide.  Garrity et al. (2001 and 2003) separate the methanogenic taxa into three groups that they refer to as classes within their phylum Euryarcheota.  Margulis and Schwartz (1998) also define the Methanobacteria as a group within their phylum Euryarcheota which also includes the halobacteria.  I have kept them as a separate group or phylum as a provisional group until more work can clarify their relationships.  I do believe, however, that the differences in physiology between the extreme halophiles and the methanobacteria warrant placing them into separate phyla. 

HIERARCHICAL CLASSIFICATION OF THE METHANOBACTERIA

This taxonomy is based in part on Margulis and Schwartz (1998) but modified by the system of Garrity et al. (2001 and 2003).  The relationships between members of this group are far more complex than indicated by the taxonomy here which should be regarded as very tentative.   Margulis and Schwartz (1998) lump the methanobacteria with the halophiles into a taxon called Euryarcheota (B-1) which I treat as a kingdom.
CLASS METHANOBACTEREI

Cells variable (short to long); all nonmotile.  Strict anaerobes, they grow by metabolizing hydrogen.  No growth above 70C.

ORDER METHANOBACTERIALES

Methanobacterium, Methanobrevibacter, Methanomonas, Methanosphaera, Methanothermobacter, Methanothermus.

CLASS METHANOCOCCI

Cells variable in shape, coccoid to spirals and sheathed rods.  They are strict anaerobes.

ORDER METHANOCOCCALES

Coccoid and mainly marine. Walls of protein.  They use hydrogen or formate to reduce carbon dioxide. 

Methanococcus, Methanothermococcus, Methanotorris.

ORDER METHANOMICROBIALES

Cells coccoid, rod, or sheathed rods.  They use hydrogen, formate or alcohols to reduce carbon dioxide.  May contain glycerol.

Methanomicrobium, Methanoculleus, Methanofolis, Methanogenium, Methanolacinia, Methanoplanus, Methanocorpusculum, Methanospirillum, 

ORDER METHANOSARCINALES

Coccoid, cell bundles, or sheathed rods.  Use methyl groups as food sources.  May use hydrogen to fix carbon dioxide.

Methanosarcina, Methanococcoides, Methanohalobium, Methanohalophilus, Methanolobus, Methanomicrococcus, Methanosalsum, Methanosaeta (=Methanothrix).

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