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 emerged by extracting energy from this simple molecule via chemical reactions.
Some of these early life forms were archaea: an enigmatic 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 studied thousands of archaeal species to understand how they thrived for billions of years on our ever-changing planet. In their genetic blueprints, we found instructions for producing special enzymes (called hydrogenases) to harvest energy from hydrogen gas, allowing them to survive in some of the harshest environments on the planet. Our latest research is 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 from geothermal processes in the earth’s crust. Archaea devour this hydrogen to repair their bodies and sometimes even grow in otherwise deadly conditions.
We discovered that some archaea can even use the tiny 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 spring to another.
Survive in the dark
Many archaea are not found on the surface, but live humble lives far underground. Plants and animals cannot survive in this environment because there is no light or oxygen to feed them.
Archaea have found a solution: they decompose organic matter deeply buried in plant or animal remains. They do this through a process called “hydrogen-forming fermentation.”
Just as in the beer fermentation process, yeast converts sugar to produce carbon dioxide, these dark-dwelling archaea convert organic matter to produce hydrogen gas.
This process releases 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 producing methane
Many archaea live in extreme environments, but some find a warm home among animals.
In the intestines of animals, many bacteria help digest food through hydrogen-producing fermentation. But a group of archaea known as methanogens eat hydrogen and exhale the powerful greenhouse gas: methane.
Methanogens are particularly abundant and active in the intestines of cattle, which are responsible for about a third of human-caused methane emissions. We have also been working on ways to inhibit the activity of intestinal 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.
Learning from the hydrogen economy of archaea
As our societies attempt to move away from fossil fuels, we may be able to learn lessons from the hydrogen economy of archaea, which thrived for billions of years.
A large part of the Earth’s hydrogen is immobilized in water. (That’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, enzymes called hydrogenases, which do not require precious metals and work under a wider range of conditions.
We found that some archaea produce highly streamlined hydrogenases. These enzymes can provide a basis for more efficient and economical hydrogen catalysts.
Hydrogen and the history of life
Hydrogen may be 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.
Why did they merge? The most widely accepted theory, known as the “hydrogen hypothesis,” suggests that the fusion of two cells would allow them to exchange hydrogen gas more efficiently. A likely scenario is that the archaeal cell survives by producing hydrogen, which the bacterial cell then eats to produce its own energy.
Eventually, this process gave rise to all eukaryotes over a billion 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 bodies of our cells come 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 Microbiology, Monash University and Chris Greening, Professor of Microbiology, Monash University)
(Disclosure Statement: Chris Greening receives funding from the Australian Research Council, National Health and Medical Sciences Council, Australian Antarctic Division, Human Frontier Science Program and the Wellcome Trust. Pok Man Leung does not work for, consult, 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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