Three-quarters of all matter in the universe is made up of hydrogen. The young 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 arose by extracting energy from this simple molecule through chemical reactions.
Some of these early life forms were archaea: a mysterious third life form discovered only 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 on our ever-changing planet for billions of years. In their genetic blueprints, we found instructions to produce 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 is published in Cell and Nature Communications.
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 constantly formed from geothermal processes in the earth’s crust. Archaea eat this hydrogen to repair their bodies and sometimes even grow in otherwise deadly conditions.
We found that some archaea can even use the tiny amount 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.
Survival in the dark
Many archaea are not found on the surface, but live humble lives deep 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 matter from plant or animal remains. It does so through a process called “hydrogen-forming fermentation.”
Just as yeast converts sugar into carbon dioxide in the beer fermentation process, these dark archaea convert organic matter into hydrogen gas.
This process releases some energy, but only a little. 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.
Methane-producing archaea
Many archaea live in extreme environments, but some find a warm home in animals.
In the animal gut, many bacteria help digest food by fermenting it to produce hydrogen. But a group of archaea known as methanogens eat hydrogen and exhale a powerful greenhouse gas: methane.
Methanogens are particularly abundant and active in the guts of cattle, which are responsible for about one-third of human-caused methane emissions. We have also been working on ways to suppress the activity of gut methanogens 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.
Lessons from the hydrogen economy of archaea
As our societies try to move away from fossil fuels, we may be able to learn from the hydrogen economy of archaea that has thrived for billions of years.
Much of Earth’s hydrogen is bound up in water. (It’s the H in Hâ‚‚O.) Industry currently needs expensive catalysts like platinum to extract and work with hydrogen. However, there are also biological hydrogen catalysts, enzymes called hydrogenases, that do not require precious metals and work under a wider range of conditions.
We found that some archaea produce highly efficient hydrogenases. These enzymes can form the basis for 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 noting 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.
Why did they team up? The most widely held theory, known as the “hydrogen hypothesis,” proposes the merging of the two cells to allow them to more efficiently exchange hydrogen gas. A likely scenario is that the archaeal cell survived by creating hydrogen, which the bacterial cell then ate to generate its own energy.
Eventually, this process gave rise to all eukaryotes over billions of years of evolution. Most modern eukaryotes, including humans, have since 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 come from bacteria.
Hydrogen may be simple, but it helped create much of the complexity on Earth.
(authors:Pok Man Leung, Research Fellow in Microbiology, Monash University and Chris Greening, Professor, Microbiology, Monash University)
(Disclosure Statement: Chris Greening receives funding from the Australian Research Council, the National Health and Medical Science Council, the Australian Antarctic Division, the Human Frontier Science Program and the 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 not disclosed any relevant relationships other than his 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 source.)
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