Image: Phil Butterworth
Archaea: The case of the 'weird bacteria'
These tiny single-celled organisms were once thought to be just ‘weird bacteria’, but by the 1970s it had been realised that it was more than ‘weird’ – these organisms needed to be in a group of their own to create a third major branch of the tree of life.
There are two basic types of living organisms – prokaryotes and eukaryotes. The former has no central nucleus, so the DNA is free-floating, while the latter does have a nucleus. Bacteria are prokaryotes, while plants, animals (including humans), fungi and algae are eukaryotes.
Archaea are prokaryotes and single celled microbes similar to bacteria. The ‘weirdness’ comes in the analysis of the RNA polymerase and ribosomal proteins, which Dr Carl Woese of the University of Illinois realised were closer to eukaryotes than prokaryotes. He came to this conclusion by studying the 16S RNA gene.
Around a decade ago it became generally accepted that archaea should have their own kingdom – the other two being bacteria and eukaryotes. Because archaea are like bacteria in having no nucleus, but genetically more akin to eukaryotes, they are often depicted half way between the two other branches on the tree of life. The archaea kingdom is split into four categories – methanogens (typically making methane in the stomachs of ruminants like cows), halophiles (salt-loving), thermophiles (heat loving) and psychrophiles (cold loving). They are all very small in size and difficult to culture in the laboratory because their natural environments are hard to replicate, hampering scientific study.
Also in the 1970s, studies done in the Yellowstone National Park and later in the depth of our oceans on the voyage of Alvin, showed that life is not confined to the narrow life envelope we live in. Extremophiles live quite happily in very high and very low temperatures, in desiccating hypersaline and radioactive environments, at very high pressures far away from sunlight deep in the rocks or in the ocean, and in very acidic or very alkaline conditions.
Many of these extremophiles (but not all) are archaea. In fact some planetary scientists believe that if life is found elsewhere in the solar system, that it will be archaea. Even though this was known prior to the only biology mission to Mars, the Viking landers in 1976, the tests may not have been sensitive to any archaea lurking just beneath the highly radiated surface of the red planet. The only other landers touched down nearly three decades later in 2004, the Mars Exploration Rovers, Spirit and Opportunity. They were sent to explore the geological circumstances for life, rather than trying to directly detect past or present biology. It’ll be at least another one or two decades before such testing is carried out.