Life on Earth consists of many millions of species, divided into ever smaller numbers when you zoom out to Genus, Family, Order, Class, Phylum and Kingdom – of which there are six.

When you divide in Domains, there are just three: Eukaryota, Bacteria and Archaea.

To sort of get an idea where Biology stands: ‘we’ didn’t even know whether the one third of life on this planet had direct interaction with the other two thirds. But there’s been a giant leap for life science – albeit one that crawls and squirms, rather than walk step by step:

Say hello to Ophryotrocha labronica – or ‘Archivory’ worm, of the Dorvilleids family. It is the first creature from ‘the other’ two thirds of life that we now know likes to dine on living things from the first domain. Consider that a food chain revolution. (But let’s hope it doesn’t eat them all – we’ll be needing these nice methane-consuming Archaea for a little time longer.) Credit: Andrew Thurber, Oregon State University.

Earth’s three walks of life: Eukarya, Bacteria – and Archaea

Eukarya includes anything from you and your goldfish, to the Aspergillus niger fungus you have currently growing on that old piece of bread behind your fridge [you really ought to take a better look].

Bacteria is of course a self-explaining Domain name. Perhaps you like this perspective: although you belong to the Domain of Eukarya, 10 times as many cells in your body belong to the Domain of Bacteria [don’t start eating antibacterial soap – you couldn’t live without them].

Archaea actually do not form the third domain, but rather the first. This a group of single-celled microorganisms with a distinctly different evolutionary history.

Archaea may be the oldest too. Although their body structure leaves no fossils, biochemical components in rock formations are seen as evidence of their presence. The oldest possible traces of Archaean life are found on Greenland, in Precambrian sediments dating back 3.8 billion years. [Somehow the genesis of life on Earth took far less time than the evolutionary transition from single-celled to complex life, which makes you wonder which is the biggest wonder of the two…]

Although in size and appearance Archaea sometimes seem to resemble Bacteria, some of their genes show greater similarity to Eukaryota. Sometimes they have very different biochemistry though, which allows them to dine on very different food sources than other life forms. Archaea for instance bring energy and elements derived from lifeless stuff like ammonia, metal ions and even hydrogen and methane gas (back) into the wider biosphere.

Although… do they? Biology did not provide the answer to that question until last week. For all we knew there could have been life on Earth, and life on Earth, two completely separate things coincidentally sharing a planet.

But that was of course down to a simple lack of research. A group of scientists of Oregon State University now write in ISME Journal [‘Multidisciplinary Journal of Microbial Biology’ – Nature group] that they have found evidence that Archaea can be part of metazoan diets.

Indeed, they discovered that the worm at the top of this story [which at this level of biology is closely related to us humans – we’re not completely sure but we think we can even distinguish a face] in turn dines on innocent and helpless Archaean prey. Although it does not depend on such food sources, the worm can complete its own lifecycle on a strict diet, excluding plant, animal or bacterial life forms:

“In the laboratory, we demonstrated that a dorvilleid polychaete, Ophryotrocha labronica, can complete its life cycle on two strains of Euryarchaeota with the same growth rate as when fed bacterial and eukaryotic food. Archaea were therefore confirmed as a digestible and nutritious food source sufficient to sustain metazoan populations.”

The term ‘archivory’ was coined by the researchers, to describe the trophic process of Archaea consumption by heterotrophic metazoans.

© Rolf Schuttenhelm | www.bitsofscience.org