Microbial life can exist across a wide spectrum of conditions – far greater than us rather impressive but limited multicellular vertebrates. Whenever there is any discussion of extra-terrestrial life or evolutionarily biology, someone chips in, in rather a jolly fashion, that extra-terrestrial life is likely to be microbial.
We find microbes in so many places – from arctic sub-zero temperatures to hydrothermal springs. We’ve identified microorganisms that can live at temperatures of 270C, respiring using oxygen like us. Others living at hundreds of degrees use weird and wonderful gases as a respiratory substrate. Many think mitochondria (once free living microorganisms) made aerobic respiration possible in a time when oxygen was on the rise. Other atmospheric gases were decreasing, pushing forward the concentration of atmospheric oxygen – it is only the ingenuity of microorganisms that make more complex organisms possible.
One might marvel that such an incredibly diversity exists within microorganisms alone and consider it strange that a group of organisms so indistinct from our perspective can exist in such different conditions and I’m sure we’ve all had the teeth grinding moment when the layman describes extremophiles as ‘bacteria that can love extreme climates’. Whilst Chemosynthetic bacteria live in climates you may reasonably expect to find Archaea, many people seem to treat not only these, but all microorganisms as one.
Microorganisms now comprise over 90% of the known species of all living organisms.
It’s expected that any given gram of soil contains about 2000 species of bacteria alone that haven’t been formally identified. If there are this many distinct species of bacteria, how much variation can we expect to see across microorganisms in all 6 kingdoms of life?
What really falls under the term ‘microorganism’?
Well, entirely avoiding the debate as to the extant state of viruses and assuming them non-living (if this is verbose it might be as well to say “Virus can’t be held to be microorganisms as they aren’t truly alive”): microorganisms include any organism that is microscopic, too small to be seen by the naked eye, but visible under a microscope. This includes all prokaryotes (bacteria and Archaea), fungi, protozoa and some microscopic plants and animals. Some organisms which we might consider indistinct could in terms of genes and phenotype differ much more than humans and dogs, let alone our most recent common ancestor with the Rhesus’ Monkey. Fungi, which we most often think of as plant like mushrooms are the spore bearing bodies of particular types of microorganisms, are eukaryotes more similar to animal cells than to bacteria.
But the greatest malefaction of misconception is committed against not the Fungi which can be unicellular or multicellular, or against the micro animals, which we might reasonably recall things like dust mites, but against Archaea. Outside of a scientific community, references to Archaea are often met with blank stares, confused looks and the even more heart-breaking “Oh, aren’t they a type of Bacteria?” Is this such a travesty?
Archaea, nearly named archaeabacteria by Dr Woese was initially thought to be very similar to bacteria yet the name was never accepted.
The community at the time, including Dr Woese found it to be a misnomer which may lead people to believe they were a type of bacteria. Indeed from a macroscopic perspective they have one overriding similar quality. They are invisible. They are too small for us to see, so it’s easy to coat them all with the same brush. So what differences can we understand as biologists?
| Characteristic | Bacteria | Archaea | Eukaryotes |
| Membrane bound organelles? | No | No | Yes |
| Multicellular? | No | No | Yes |
| Sexual reproduction? | No | No | Yes |
| Membrane lipids? | Ester linked | Ether linked | Ester linked |
| DNA wrapped around histones? | No | Yes | Yes |
| Transcription factors required? | No | Yes | Yes |
| Initiator tRNA? | Formylmethionine | Methionine | Methionine |
| Introns present? | No | Yes | Yes |
Well firstly how are they similar?
Being prokaryotic neither bacteria nor Archaea have a distinct nucleus. They have no membrane bound organelles (this means no Golgi apparatus, no mitochondria). They are single celled organisms. Like bacteria they reproduce solely asexually, meaning the daughter cells are identical to the parent cells.
So are microbiologists just being fussy allocating a whole domain for these seemingly bacterial non-bacteria?
Well, no. They are distinctly different, for example in translation and transcription they have enzymes more reminiscent of eukaryotes. Perhaps more obviously significant is the fact that you can find Archaea in countless devastating environments that would as soon kill bacteria such as E. coli and even humans. Examples of such drastic conditions are extreme heat, pressure, acidity, salinity and yet Archaea survive and function normally. They all have weird and wonderful mechanisms. For example, our famous friend Pyrococcus furiosus, from which we get the DNA polymerase used in PCR, has an optimum functioning temperature conditions of 100 °C. This is due in part to ether linkages between isoprene chains allowing transmembrane phospholipids to form in comparison to branched glycerol based phospholipids held together by ester bonds in non-Archaeal organisms. Finally, someone without the slightest appreciation for the academic study of science can appreciate the fact that no archaea has ever been found to be pathogenic.
We can reasonably conclude that all the Archaea we’ve ever seen are not trying to kill us.
Not all Archaea are extremophiles, but they’ve certainly demonstrated a remarkable capacity to survive almost anywhere.
As it would be peculiar and incorrect to consider animals without taking into account the insects that make up most of the known species, it would be a peculiar and unhelpful approach to microbiology to consider everything ‘bacteria like things’ and Fungi. Microorganisms may seem easy to compartmentalise and forget about unless we’re in the class room. This is not possible as microorganisms live in such close proximity to us; not only do they wreak havoc on us whilst we live, they also eat us once we’re dead. On a more cheerful note, they also provide the nitrates that we build ourselves with, are essential for our health.
More stunningly, there are 10 times more microorganisms in and around our body compared to our total number of cells. Therefore, this truly seems a very odd thing to be ignorant about.
Dale Calridge
Image courtesy of Chook via Flickr
