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. [In Photos: Archaea Turn Great Salt Lake Pink]
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.
Since A. fulgidus is an early-Earth organism, the researchers suspect that perchlorate was also around at that time and that the ability to metabolize it was part of an adaptation to all sorts of highly toxic chemicals, many of which are oxidizers. An oxidizer takes electrons away from other molecules. Such chemicals tend to be fairly toxic to many microbes because they disrupt their metabolisms or cell walls.
"The use of perchlorate by early ancestral microbes might thus have been one of the first entries of highly oxidative compounds in the microbial metabolism, probably even before photosynthesis evolved," said Martin Liebensteiner, a doctoral student at the Wageningen University Laboratory of Microbiology in the Netherlands and lead author of the study, detailed this week in the journal Science.
Oxygen is another oxidizer (hence the name), and a highly reactive one at that. Before plants evolved, there wasn't any in the atmosphere. In fact, oxygen is so reactive that it can kill some types of Archaean life and many bacteria. Living things had to adapt to using such chemicals, or nothing else would have survived once plants' ancestors, cyanobacteria, started dumping oxygen into the air en masse. Humans' mitochondria are the legacy of that adaptation, which involved incorporating oxygen-using cells into other life forms, allowing them to tolerate the new atmosphere. The findings here might be suggesting other strategies for using oxidizing chemicals that were around before that happened.
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.
The sulfides (negatively charged sulfur atoms) react with the chlorite to make more highly oxidized sulfur compounds, like sulfate and chlorine, by separating the oxygen from the chlorine and adding oxygen atoms to the sulfide.
This has an added bonus for the tiny creature: It can generate energy by using the sulfur compounds, and using that energy makes more sulfide. As the sulfide gets "recycled," it can react with more chlorite molecules released from the reaction that break up the perchlorate.
"It seems as if A. fulgidus relies on the interaction of these abiotic and biotic reactions in order to grow with perchlorate," Liebensteiner wrote in an email to LiveScience.
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. [The 7 Harshest Environments on Earth]
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.
Liebensteiner said he didn't want to speculate too much about what this means for evolution billions of years ago, because the evidence isn’t yet sufficient. Other scientists, he noted, have shown that in places where perchlorates form naturally, such as deserts, perchlorate would tend to accumulate because perchlorate is relatively stable (i.e., absent the action of the enzyme in bacteria and archaeans, it doesn't react with anything without adding a lot of heat). But it hasn't stuck around.
"That's the point where people start getting thoughts that because of bacterial activity, [the perchlorate] didn't accumulate," Liebensteiner said.
And 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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