Microbiology, Lec 7.

Archaea (Ch. 22)

Let's start with a quick review of the universal phylogenetic tree. We have already discussed some of the structural and biochemical similarities among the 3 domains (Lecture 4) and this information is summarized in Table 22.1 of the book.

Major Groups of the Archaea.

Right now the Archaea are being broken up into 3 groups based on biochemical similarities and to a lesser extent on sequence data. The 3 groups are:

1) Methanogens,
2) Extreme Halophiles
3) Extreme Thermophiles

Please note however that there are thermophiles distributed throughout the phylogenetic tree of the Archaea and that the halophiles are closely related to some of the methanogens (see Figure 22.5).

Methanogens (= methane generators)

Methane (CH4) or "natural gas" is an odorless gas that is used widely today in industry and the home. Much of this gas was probably created over the eons by methanogens. Methane is also a greenhouse gas that is contributing to global warming because it is increasing in concentration in the atmosphere.

The methanogens are the most widely distributed of the Archaea and have been found almost everywhere that humans have looked. They are most abundant in anaerobic environments (guts, swamps, sediments etc.) About 30% of adult humans have high populations of methanogens in their guts (all of us have some methanogens inside) and this may explain why some folks emit more gas than others. Methane is odorless so the foul smell of some flatulence comes from microbial fermentations of organic compounds, especially proteins.

Table 22.2 shows some of the huge diversity of methanogens. Note that they come in all shapes and have a wide variety of cell-wall types - ranging from pseudopeptidoglycan ("pseudomurein" in Figure 2.4) to protein or glycoprotein cell walls. Also note that they all have "Methano" as part of their generic names. The % G+C of the methanogens ranges from 26 to 62, again indicating that this is a very diverse group.

The diversity of methanogenic metabolic types is given in Table 22.3. Please don't memorize these - I will point out the main types of reactions in lecture. We'll come back to the specifics of some of these reactions later in the course.

Figure 22.8 is a synopsis of role that methanogens play in the food web resulting from breakdown of complex molecules like cellulose in anaerobic environments. Note that they are just one part of complex anaerobic food webs.

These organisms require high concentrations of salt in order to live. Their optimal NaCl concentrations for growth range from 2 to 4.5 M (Table 22.4). For reference the salt concentration of sea water is about 0.5 M. Thus, they are found in habitats like the Great Salt Lake, Dead Sea and salterns (= evaporation basins for obtaining salt) etc. They also colonize some salted foods but no halophiles have been shown to cause food-borne illnesses.

The % G+C of the halophiles ranges from 60 to 71 and the best studied extreme halophile, Halobacterium, has a glycoprotein cell wall. One other unique aspect of some extreme halophiles is that they have a novel form of photosynthesis found in no other living organisms. They have a trans-membrane protein called bacteriorhodopsin (figure 9.15) which acts as a light driven proton pump (unlike regular photosynthesis, no electron transport is involved).

As noted above, thermophiles are distributed throughout the Archaea, but many of them group on a basal branch of the Archaeal tree (see Figure 22.5). Most of the extreme thermophiles have temperature optima for growth above 80 degrees C whereas most thermophilic Eubacteria have temperature optima of 70 degrees or lower (see Figure 22.11). This niche separation is further indication that the Archaea are adapted to environmental extremes similar to those that may have been present on earth when life evolved. It is also of ecological significance that the extreme thermophiles exists in environments that are too hot for photosynthesis to take place. Thus, it is not surprising that many of them have unique forms of metabolism that involve sulfur compounds etc.. (see Table 22.5).

Because of their high thermostability, enzymes from thermophiles are being used in many commercial applications. For example, proteases from moderate thermophiles are used in laundry detergents and DNA polymerases from extreme thermophiles are used in the polymerase chain reaction (PCR) and other molecular techniques that are carried out at high temperatures.