An ancient form of life can use an ingredient in rocket fuel for energy, suggesting creatures with this odd ability are more diverse than anyone thought. The new discovery might offer insight into the history of life on the early Earth, and the evolution of metabolisms like ours that use reactive chemicals like oxygen. Called Archaeoglobus fulgidus, today the microbe lives in extreme environments, such as extremely hot hydrothermal vents. It’s a member of the Archaea, one of the three domains of life. (The other domains are bacteria, or prokaryotes, and creatures with cells that have nuclei, or eukaryotes, which include humans and other multicellular life.) Archaeans are some of the oldest life forms on Earth, thought to have appeared at least 2.7 billion years ago – and they are possibly much older than that. They often live in environments that don’t have oxygen or are otherwise inhospitable to many other living things. A group of Dutch researchers found that A. fulgidus metabolizes perchlorate, a chlorine atom connected to four oxygen atoms. Moreover, the microbe does so in a different way than known Archaea or bacteria do ― A. fulgidus is missing one of the enzymes other bacteria use to break down perchlorate.
Toxic Earth
When combined with potassium, perchlorate is used as an ingredient in fireworks and, when combined with ammonium, as an ingredient in rocket fuel. But it also occurs naturally, in deserts such as the Atacama in Chile, and may have been more plentiful on early Earth and even on Mars. Recently, the Curiosity rover found possible evidence of perchlorates in Rocknest ― a patch of sand in Mars’ Gale Crater ― suggesting the compound may exist all over the Red Planet.
Microbe’s perchlorate-eating ways
Other bacteria that can breathe and eat perchlorates use a two-step process involving specialized enzymes that turn perchlorate into chlorite ― which has two, rather than four, oxygen atoms ― and then separate the chlorite into chlorine and oxygen.
A. fulgidus doesn’t do that, Liebensteiner and his colleagues found. Whereas it uses an enzyme similar to that of known bacteria to perform the first step, it doesn’t have the enzyme that breaks up the chlorite. Instead, A. fulgidus’ metabolism uses sulfur compounds called sulfides, in a reaction that isn’t controlled by any enzyme but occurs naturally between the two sets of chemicals.
One other feature of A. fulgidus is that it lives in hot, high-pressure environments without oxygen. The creature was discovered in an underwater volcanic vent and is happy at temperatures near the boiling point of water, between 140 and 203 degrees Fahrenheit (60 to 95 degrees Celsius). That’s a lot like the conditions on Earth more than 2.5 billion years ago, when the planet’s atmosphere had no oxygen because plants hadn’t yet evolved. In addition, volcanic activity was much more intense.
Robert Nerenberg, an associate professor of environmental engineering who has studied perchlorate-metabolizing bacteria, noted that A. fulgidus only metabolizes perchlorate when it is in an environment where only sulfur is present. The research team did that in order to remove any oxygen from the environment, but the interesting thing, Nerenberg said, is that in the presence of chlorates the bacteria metabolize those instead of perchlorates. (Chlorate is perchlorate with one less oxygen atom). So A. fulgidus’ “preference” may not be for perchlorate.
The question, he said, is why any creature — bacteria or archaean — would retain an ability to metabolize perchlorate after billions of years when it might not need to. “Usually certain genes just sort of stop working after a while if there’s no selective pressure for them,” he said. “There has to be some benefit.” What that is, though, is a bit of a mystery.
The fact that A. fulgidus has a pathway for breaking down perchlorate that is similar to known bacteria, but lacking one enzyme suggests that, at a minimum, there are several ways to evolve perchlorate metabolism — either spontaneously or via gene transfer, which can happen among single-celled life forms.
More work is needed to see if this same kind of metabolism occurs in other Archaeans, and even in bacteria. “It definitely means that [A. fulgidus] is probably more diverse than people thought,” he said.
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