Three quarters of all matter in the universe consists of hydrogen. The early Earth was also rich in hydrogen, thanks to intense geological and volcanic activity.
Just as stars burn hydrogen to produce heat and light through nuclear reactions, life emerged by extracting energy from this simple molecule through chemical reactions.
Some of these early life forms were archaea: a mysterious third life form that was not discovered until the 1970s. (The other two forms are bacteria and eukaryotes, the group that includes all animals, plants and fungi.)
We’ve studied thousands of species of archaea to understand how they have thrived on our ever-changing planet for billions of years. In their genetic blueprints we found instructions for producing special enzymes (called hydrogenases) to extract energy from hydrogen gas, allowing them to survive in some of the most punishing environments on Earth. Our latest research has been published in Cell and Nature Communications.
A life powered by hydrogen
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 that it would dissolve iron.
Here hydrogen is continuously formed by the geothermal processes in the earth’s crust. Archaea devour this hydrogen to repair their bodies and can sometimes even grow in otherwise lethal conditions.
We discovered that some archaea can even use the small amounts of hydrogen in the air as an additional food source. This ability would likely help them survive transport through the atmosphere from one hydrogen-rich hot source to another.
Survive in the dark
Many archaea are not found on the surface, but live a modest life far underground. Plants and animals cannot survive in this environment because there is no light or oxygen to sustain them.
Archaea have found a solution: they break down deeply buried organic material from plant or animal remains. They do this through a process called ‘hydrogen-forming fermentation’.
Just as yeasts convert sugar into carbon dioxide during the beer fermentation process, these dark-dwelling archaea convert organic material into hydrogen gas.
This process releases some energy, but only a small amount. To survive, some archaea form ultra-small cells to minimize their energy needs. Many are also parasites of other microbes, stealing organic matter to fuel their own growth.
Archaea makes methane
Many archaea live in extreme environments, but some find a warm home in animals.
In the animal intestine, many bacteria help digest food through hydrogen-forming fermentation. But a group of archaea known as methanogens eat hydrogen and exhale the powerful greenhouse gas: methane.
Methanogens are especially abundant and active in the intestines of livestock, which are responsible for about a third of human-induced methane emissions. We have also been working on ways to inhibit the activity of methanogens in the gut to reduce these emissions.
These same archaea are also responsible for methane emissions from many other sources, from termite mounds to thawing permafrost and even trees.
Learning from the hydrogen economy of Archaea
As our societies try to move away from fossil fuels, perhaps we can learn from Archaea’s hydrogen economy, which has been thriving for billions of years.
Much of the hydrogen on Earth is trapped in water. (It’s the H in H₂O.) To extract and work with the hydrogen, industries currently require expensive catalysts such as platinum. However, there are also biological hydrogen catalysts, enzymes called hydrogenases, that do not require noble metals and work under a wider range of conditions.
We discovered that some archaea make highly streamlined hydrogenases. These enzymes can form a basis for more efficient and economical hydrogen catalysts.
Hydrogen and the history of life
Perhaps hydrogen is a 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) emerged about two billion years ago, when an archaeal cell and a bacterial cell merged.
Why were they merged? The most widely accepted theory, known as ‘the hydrogen hypothesis’, suggests that the fusion of two cells allows them to exchange hydrogen gas more efficiently. A likely scenario is that the archaeal cell survived by making hydrogen, which the bacterial cell then ate to make its own energy.
Ultimately, this process led to the creation of all eukaryotes over a billion years of evolution. Most modern eukaryotes, including humans, have since lost the ability to use hydrogen.
But traces of the ancient archaea and bacteria still exist. The body of our cells comes from archaea, while the energy-producing organelles within the cells, called mitochondria, come from bacteria.
Hydrogen may be simple, but it has helped create much of the complexity on Earth.
(Authors:Pok Man Leung, Research Fellow in Microbiology, Monash University and Chris Greening, Professor of Microbiology, Monash University)
(Disclosure Statement: Chris Greening receives funding from the Australian Research Council, National Health & Medical Science Council, Australian Antarctic Division, Human Frontier Science Program and Wellcome Trust. Pok Man Leung does not work for, consult with, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment)
This article is republished from The Conversation under a Creative Commons license. Read the original article.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
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