Studying the diverse metabolic pathways of archaea, such as methane production and hydrogen-producing fermentation, could inform strategies to reduce greenhouse gas emissions
Three-quarters of the matter in the universe is made up of hydrogen. The young Earth was also hydrogen-rich, thanks to intense geological and volcanic activity. Just as stars burn hydrogen to produce heat and light through nuclear reactions, life arose by extracting energy from this simple molecule through chemical reactions.
Some of these earliest life forms were archaea: a mysterious third form of life only discovered in the 1970s. (The other two forms are bacteria and eukaryotes, the group that includes all animals, plants, and fungi.) We have studied thousands of species of archaea to understand how they have thrived for billions of years on our constantly changing planet.
In their genetic blueprint we found instructions for making special enzymes (called hydrogenases) to extract energy from hydrogen gas, allowing them to survive in some of the harshest environments on Earth. Our latest research is published in Cell and Nature Communications. Better decisions start with better information.
Hydrogen-powered life
Archaea are found in places where no other life can survive. For example, some thrive in boiling hot springs where the water is so acidic it can dissolve iron. Here, hydrogen is constantly created by geothermal processes in the Earth's crust. Archaea eat this hydrogen to repair their bodies and grow, sometimes even in deadly conditions. We found that some archaea can also use small amounts of hydrogen present in the air as an additional food source. This ability would likely help them survive transport through the atmosphere from one hydrogen-rich hot spring to another.
Surviving in the Dark
Many archaea are not found on the surface, but live a humble life far underground. Plants and animals cannot survive in this environment because there is no light or oxygen to keep them alive. Archaea have found a solution: they break down deeply buried organic matter from plant or animal remains. They do this through a process called “hydrogen-forming fermentation.” Just as yeast converts sugar to make carbon dioxide in the process of beer fermentation, these dark-dwelling archaea convert organic matter to make hydrogen gas. This process releases some energy, but very little. To survive, some archaea form ultra-small cells to reduce their energy requirements. Many are also parasites of other microbes, stealing organic matter to fuel their growth.
Methane production by archaea
Many archaea live in extreme environments, but some find warm homes in animals. In the gut of animals, many bacteria help digest food through hydrogen-producing fermentation. But one group of archaea known as methanogens eats hydrogen and spits out the potent greenhouse gas: methane. Methanogens are particularly abundant and active in the gut of cattle, which are responsible for about one-third of human-caused methane emissions. We are also working on ways to inhibit the activity of gut methanogens to reduce these emissions. These are the same archaea that are also responsible for methane emissions from many other sources, from termite mounds to melting permafrost and even trees.
Learning from the hydrogen economy of Archaea
As our society tries to move away from fossil fuels, we can learn from the hydrogen economy of archaea, which has flourished for billions of years. Most of the Earth's hydrogen is bound in water. (It's the H in H2O.) To extract hydrogen and work with it, industries currently need expensive catalysts such as platinum. However, there are also biological hydrogen catalysts, called hydrogenases, that do not require precious metals and work under a wide range of conditions. We have found that some archaea make highly tuned hydrogenases. These enzymes could form the basis of more efficient and economical hydrogen catalysts.
Hydrogen and the history of life
Perhaps hydrogen is the key to our future energy. But it's worth mentioning that hydrogen also helps explain our past. The first eukaryotes (the ancestors of all animals, plants and fungi) evolved about two billion years ago, when an archaeal cell and a bacterial cell merged together. Why did they merge? The most widely accepted theory, known as the “hydrogen hypothesis,” suggests that the merger of two cells allowed them to exchange hydrogen gas more efficiently. One possible scenario is that the archaeal cell survived by making hydrogen, which the bacterial cell consumed to make its own energy. Eventually, this process gave rise to all eukaryotes over a billion years of evolution.
Most modern eukaryotes, including humans, have lost the ability to use hydrogen. But traces of ancient archaea and bacteria still exist. The body of our cells comes from archaea, while the energy-producing organelles inside cells called mitochondria are derived from bacteria. Hydrogen may be simple, but it has helped create a lot of complexity on Earth.
(Pok Man Leung, Monash University, and Chris Greening, Monash University)