As the world turns to green hydrogen and other renewable energy sources, scientists have discovered that archaea – the third form of life after bacteria and eukaryotes – have been producing energy using hydrogen gas and ‘ultraminimal’ enzymes for billions of years.
Specifically, the international team of researchers found that at least nine phyla of archaea, a domain of single-celled organisms that lack membrane-bound internal structures, produce hydrogen gas using enzymes thought to exist only in two other life forms .
Archaea, they found, not only have the smallest hydrogen-using enzymes compared to bacteria and eukaryotes, but their hydrogen consumption and production enzymes are also the most complex yet characterized.
Small and powerful, these enzymes apparently allowed archaea to survive and thrive in some of Earth’s most hostile environments with little or no oxygen.
“Humans have only recently begun to think about using hydrogen as an energy source, but archaea have been doing it for a billion years,” says Pok Man Leung, a microbiologist at Monash University in Australia, who co-led the study. .
“Biotechnologists now have the opportunity to take inspiration from these archaea to produce hydrogen industrially.”
Hydrogen is the most abundant element in the Universe and is used globally to make fertilizers and other chemicals, treat metals, process food and refine fuels.
But the future of hydrogen lies in energy storage and the production of steel, which can be produced with zero emissions if renewable energy is used to turn materials such as water into hydrogen gas.
ORGANISMS produce and release hydrogen gas (H2) for entirely different purposes, primarily to dispose of excess electrons produced during fermentation, a process where organisms extract energy from carbohydrates such as sugars without oxygen.
Enzymes used to consume or produce H2 are called hydrogenases, and they were first observed comprehensively on the tree of life just eight years ago. Since then, the number of known microbial species has exploded, especially archaea, which hide in extreme environments such as hot springs, volcanoes and deep sea trenches.
However, most archaea are known only from the parts of their genetic code found in these environments, and many have not been cultured in the laboratory because it is too difficult to do so.
So Monash University microbiologist Chris Greening and his colleagues looked for the gene that encodes part of a fast-acting type of hydrogenase. [FeFe] hydrogenases, in more than 2300 archaeal species groups listed in a global database.
They then charged Google’s AlphaFold2 to predict the structure of the encoded enzymes and expressed those enzymes in E. coli bacteria, to check that those genes were actually functional and produced hydrogenases capable of catalyzing hydrogen reactions in their surrogate host.
“Our discovery brings us one step closer to understanding how this crucial process created all eukaryotes, including humans,” says Leung.
Eukaryotes are organisms whose cells contain a nucleus and membrane-bound organelles such as mitochondria and other useful cellular factories.
All eukaryotes are thought to have evolved from the fusion of an anaerobic archaea and a bacterium that it ingested billions of years ago. A second endosymbiosis, much later, then gave birth to the ancestor of plants, with chloroplasts.
Greening, Leung and their colleagues found the genetic instructions for [FeFe] hydrogenases in nine archaeal phyla and confirmed that they are indeed active in those microorganisms—making it three of three domains of life that use these types of enzymes to produce hydrogen.
But unlike bacteria and eukaryotes, further analysis showed that archaea assemble “remarkable hybrid complexes” for their hydrogen production needs, joining two types of hydrogenases together.
“These findings reveal novel metabolic adaptations of archaea, simplified H2 catalysts for biotechnological development and a surprisingly intertwined evolutionary history between the two H.2-metabolizing enzymes”, writes the team in their paper.
However, many of the cataloged archaea genomes analyzed in this study are incomplete, and who knows how many more species have yet to be discovered.
It is more than likely that archaea harbor other ingenious ways of creating energy that we have yet to discover.
The research was published in Cell.