What are archaea?

Microorganisms seem to have a bad reputation a lot of the time. Many people think of them as causing infection and disease, except perhaps for those good ones that live inside us and help us digest food, among other things.

But the world of microorganisms is much more vast, varied and vital to the ongoing function of our planet than any of us can imagine. Microorganisms are involved in ecological processes like taking CO₂ out of the atmosphere or recycling waste materials and nutrients. Many microbial species are still undiscovered, but there’s one group in particular that scientists know comparatively little about: the archaea.

Archaea: a domain of living things

To understand what makes archaea special, we need to remember that life on Earth can be organised into three major groups, or ‘domains’: eukarya, bacteria, and archaea. All archaea and bacteria are microbial species (living things too small to see with the naked eye) and represent a vast number of different evolutionary lineages. In eukarya, you’ll find animals, plants, fungi and some other organisms called protists. Some of these eukaryotic groups contain microbial species, too.

Bacteria and archaea may seem pretty similar, but there are some major differences between the two groups. The structure of their cells is different: they’re made of slightly different compounds and components, containing fundamentally different genetic material. Archaea can also generate energy differently and have unique ecological roles to play, such as being responsible for producing biological methane—something no eukaryotes or bacteria can do.

These differences may not seem like a big deal to most people—why, then, are they in different groups? By comparing the genomes of different organisms and studying the rate at which genetic changes occur over time, scientists can trace the evolutionary histories of living things and estimate when each group formed a new branch of the tree of life. The molecular and genetic differences between archaea and other living things are profound and ancient enough to warrant an entirely separate domain.

It's extremely acidic and full of heavy metals, but this doesn't bother the many species of microbes (including archaea) that live in Spain's Rio Tinto river. Image adapted from: Carol Stoker, NASA; CC0

Archaea are famous for their love of living in extreme environments. If it’s super hot (more than 100° Celsius), freezing, acidic, alkaline, salty, deep in the ocean, even bombarded by gamma or UV radiation, there’s probably life there, and that life is probably archaeal species.

Of course, it’s worth remembering that while these conditions seem inhospitable for us, they’re perfectly normal for archaea. They’re also normal for numerous non-archaeal species, too, but archaea get a lot of the fame for it.

This is partly what makes archaea so difficult for scientists to study: when their ‘normal’ is so ‘extreme’ for us (and vice versa), it’s pretty tough to study archaea in a lab or access them in their natural environments. However, scientists are slowly learning more, helped by new techniques and technologies that make it easier to discover these species in the first place. Methods such as metagenomics allow for the study of genetic material without the need to grow cultures of a particular species in a lab, allowing researchers to study the genetic blueprints of more microbes than ever before. 

Hydrothermal vents on the ocean floor, where the surrounding water can reach over 300° Celsius, are home sweet home for some archaeal species. Image adapted from: NOAA Photo Library; CC BY 2.0

Archaea are generally pretty friendly. A lot of archaea live in mutualistic relationships with other living things, meaning they provide some kind of benefit to another species and get something good in return. For example, the vast numbers of methanogens (archaea that produce methane as a by-product) that live in the human digestive system help to get rid of excess hydrogen by utilising it to produce energy. This hydrogen is a waste product produced by the bacteria that help break down the food we eat, so getting rid of the excess means bacteria can do their job more effectively and efficiently. It’s a delicate balance, though—the presence of archaea in the human gastro-intestinal tract may also be associated with disease in some cases.

They’re also very resourceful. Many forms of archaea can utilise totally inorganic forms of matter—hydrogen, carbon dioxide or ammonia for example—to generate organic matter themselves. Most other living things require at least some kind of organic material to generate energy, so archaea occupy a unique place in the global food web in this regard.

Archaea may also give us a glimpse into how to look for life beyond Earth. We now know that there are so many environmental conditions—regardless of how extreme they may appear to be—that are capable of supporting life, so we can widen the boundaries of our search for life on other planets (like Mars, perhaps).

The very cold and ultra-salty Deep Lake in East Antarctica is home to haloarchaea. Image adapted from: Ricardo Cavicchioli (used with permission)

Haloarchaea, for example, are known for surviving in super-salty conditions with very little water and are capable of surviving in a state of near-starvation for a very long time—as in, potentially millions of years at a time. Even exposure to high levels of UV radiation doesn't bother them. This raises the possibility that other living things might be able to exist in similarly salty, rocky, dry places on other planets, meteorites or moons.

So, what's out there? Are there archaea-like living things on other planets? We still have so much to discover about the world of archaea here on Earth, but as they continue to challenge and broaden our very definitions of where life can thrive, it's an exciting time for new biological discoveries.

This article has been reviewed by the following expert: Professor Ricardo Cavicchioli Professor of Biotechnology and Biomolecular Sciences, UNSW Sydney