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Introduction to Archaea
provided by St Andrews College, Scotland 


A Short History of the World

The earth condensed around 4.6 billion years ago.

It remained a ball of molten rock for a few hundred million years, before cooling sufficiently to allow water to condense and form oceans around 4.2 billion years ago.

Throughout this time and up to about 3 billion years there was heavy bombardment of the surface by meteors and comets, and irradiation by UV light. 

Not a nice place to live.

Oldest confirmed evidence of microbial life is around 3.5 billion years from the beginning, putting the start of life between 4 - 3.5 billion years -- soon after water condensed.

First fossils of photosynthesis bacteria observed at 3 billion years. Oxygen built up over a billion years, becoming abundant around 2 billion years ago.

Eukaryotes appeared around 2 billion years ago and multicellular life around 1 billion. Vertebrates at 400 million years, mammals at 240 and humans only 2 million years ago.

So Life has been around for most of the history of our planet, but for much of that time under very different circumstances: heat, irradiation, lack of oxygen.



The Temperature of Life

The world around us is inhabited by mesophiles: organisms adapted to live around the same temperature range as we do. Organisms that can live above about 50 C are called thermophiles.
Very few higher eukaryotes are thermophiles.

Organisms growing above about 75 C are called hyperthermophiles.

Most hyperthermophiles are archaea, and the most extreme are crenarchaea. The high-temp record, at 113 C, is held by Pyrolobus fumarii. This is only possible under pressure where the boiling point of water is raised.



However, some of the cold-loving organisms, "psychrophiles" are also crenarchaea, so they span a great range of temperatures.

Bear in mind that for an organism to be a hyperthermophile all the components of the cell - proteins, DNA etc, must be stable.

Three Great Domains

All cellular life on earth can be classified as belonging to one of three domains: 
Eukarya, Bacteria or Archaea

The first, Eukarya, includes humans and all organisms with a nucleus. Bacteria and archaea are unicellular prokaryotes (lacking a nucleus). They have many features in common, but are now generally recognized as constituting two fundamentally different domains of life. 

LUCA (the Last Universal Cellular Ancestor) is the progenitor cell from which all life has evolved (a sort of microbial Eve). 

There is a lot of debate about what LUCA was. 

We can be pretty certain that it was already a complex life form with hundreds of genes encoded by a DNA or RNA genome, a cell membrane and a protein complement for metabolism and information processing.

This tree is drawn to reflect the theory that the archaea and eukarya are more closely related to one another than either is to the bacteria (compare the last common ancestors denoted by the blue and red arrows). 

This still controversial theory arises from the analysis of genome sequencing projects over the last eight years, which shed light on the unexpected relationships between eukarya and archaea in the ways in which they process information (in other words DNA replication, transcription and translation). 

The archaea are split into two deep subdomains: euryarchaea and crenarchaea. The former includes the methanogens and halophiles, whilst the crenarchaea are some of the most hyperthermophilic organisms known, growing at up to 113C. 

Many archaea however are not extremophiles, and are found in temperate seawater and soil. It is now believed that archaea constitute a significant proportion of the biosphere.

Archaea are found in many harsh environments on the planet, and have adapted to live at extremes of temperature (from Polar ice to superheated water), pressure, pH ( from extreme acid pH 0 to alkaline pH 10) and salinity (saturated NaCl in salt pans and the Dead Sea). 

Archaea constitute a major portion of the biosphere, and are also present in soil, temperate sea water and even the human gut. To survive in extreme environments like boiling acid, every component of the cell must be adapted: for example proteins must be stable and functional at temperatures where "normal" proteins fall apart.

Recently, a privately held research firm, ArchaeaSolutions, Inc. has been able to develop commercial applications for archaea's use in decomposition of organic matter in wastewater treatment plants. Learn more about how ArchaeaSolutions, Inc., can reduce disposable biosolids, ammonia, nitrates and odor in wastewater.