Methane-forming bacteria are known by several names (Table 3.1) and are a mor-phologically diverse group of organisms that have many shapes, growth patterns,and sizes. The bacteria can be found as individual rods, curved rods, spirals, and cocci(Figure 3.1) or grouped as irregular clusters of cells, chains of cells or filaments, and sarcina or cuboid arrangements (Figure 3.2). The range in diameter sizes of individual cells is 0.1–15 mm. Filaments can be up to 200mm in length. Motile and
nonmotile bacteria (Figure 3.3) as well as spore-forming and non-spore-forming bacteria can be found.
Methane-forming bacteria are some of the oldest bacteria and are grouped in the domain Archaebacteria (from arachae meaning “ancient”) (Figure 3.4). The domain thrives in heat. Archaebacteria comprise all known methane-forming bacteria, the extremely halophilic bacteria, thermoacidophilic bacteria, and the extremely ther-mophilic bacteria. However, the methane-forming bacteria are different from all other bacteria. Methane-forming bacteria are oxygen-sensitive, fastidious anaerobes and are free-living terrestrial and aquatic organisms. Although methane-forming bacteria are oxygen sensitive, this is not a significant disadvantage. Methane-forming bacteria are found in habitats that are rich in degradable organic compounds. In thesehabitats, oxygen is rapidly removed through microbial activity. Many occur as symbionts in animal digestive tracts. Methane-forming bacteria also have an unusually high sulfur content: Approximately 2.5% of the total dry weight of the cell is sulfur. The of methane-forming bacteria are classified in the domain Archaebacteria because of several unique characteristics that are not found in the true bacteria or Eubacteria. These features include 1) a “nonrigid” cell wall and unique cell membrane lipid, 2) substrate degradation that produces methane as a waste, and 3)
TABLE 3.1 Commonly Used Names for Methane-
forming Bacteria
Methanogenic bacteria
Methanogens
Methane-forming bacteria
Methane-producing bacteria
specialized coenzymes. The cell wall lacks muramic acid, and the cell membrane does not contains an ether lipid as its major constituent (Figure 3.5). Coenzymes that are unique to methane-forming bacteria are coenzyme M and the nickel-containing coenzymes F420 and F430. Coenzyme M is used to reduce carbon dioxide(CO2) to methane. The nickel-containing coenzymes are important hydrogen carriers in methane-forming bacteria.The coenzymes are metal laden organic acids that are incorporated into enzymes and allow the enzymes to work more efficiently. The coenzymes are components of energy-producing electron transfer systems that obtain energy for the bacterial cell and remove electrons from degraded substrate (Figure 3.6).
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| Figure 3.1 Common shapes of methane-forming bacterial cells. Commonly occurring shapes of methane-forming bacteria include rod or bacillus (a), curved rod (b), spiral (c), and coccus or spheri- cal (d). |
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| Figure 3.2 Common growth patterns of methane-forming bacterial cells. Commonly occurring growth patterns of methane-forming bacteria include an irregular cluster (a) and a filamentous chain (b). |
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| Figure 3.2 Common growth patterns of methane-forming bacterial cells. Commonly occurring growth patterns of methane-forming bacteria include an irregular cluster (a) and a filamentous chain (b). |
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| Figure 3.4 Location of methane-forming bacteria on the phylogenetic tree. The phylogenetic tree (the historical development of different life forms) contains old ( arachae ) life forms closest to the base of the tree, while new life forms closest to the end of the branches. The tree contains the domains Thermopiles, Archaea, Eubacteria (true bacteria), and the Eucarya (higher life forms). The methane- forming bacteria are found closest to the base of the tree. |
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| Figure 3.5 Cell wall of methane-forming bacteria. The cell wall of methane-forming bacteria (a) does not contain muramic acid, while the cell of other bacteria (b) contains varying amounts of muramic acid. |
