"Instead of relying on organic carbon, which is a very scarce resource in the cave, they use the energy in nitrogen-containing compounds like ammonia and nitrite to convert carbon dioxide from the air into biomass," she said.
To reveal the cave's hidden microbial communities, the researchers swabbed stalactites and other formations hanging from the ceiling of Kartchner Caverns for DNA analysis. The genes found in these samples were used to reconstruct bacteria and archaea — single-celled microorganisms without a cell nucleus — that live in the limestone recesses.
Earlier studies indicated that stalactites act as islands for cave microbes, meaning there is little mixing between populations of microorganisms on different cave formations.
From their DNA analysis, the researchers not only encountered a diverse range of organisms that make up a complex food web within the cave, they also stumbled on some microbes that were likely previously unknown to science.
"Twenty percent of the bacteria whose presence we inferred based on the DNA sequences were not similar enough to anything in the database for us to be able to identify them," Neilson said. "On one stalactite, we found a rare organism in a microbial group called SBR1093 that comprised about 10 percent of the population on that stalactite, but it represented less than 0.5 percent of the microbes on any of the others."
The organism's DNA sequence has only been found three times in history: in a type of sedimentary rock in the salty waters of Shark Bay in Australia; in a site contaminated with hydrocarbons in France; and in a sewage treatment plant in Brisbane, Australia, Neilson said.
"This suggests there are many microbes out there in the world that we know almost nothing about," she added. "The fact that these organisms showed up in contaminated soil could mean they might have potential for application such as environmental remediation."
Studying these types of microbes can help scientists understand their resilience in extreme environments, which could have applications in the search for life on other planets as well.
"When you think about exploring Mars, for example, and you look at all those clever strategies that microbes have evolved and tweaked over the past 4 billion years, I wouldn't be surprised if we found them elsewhere if we just keep looking," study principal investigator Raina Maier, a professor in the University of Arizona's department of soil, water and environmental science, said in a statement.
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This story originally appeared on LiveScience's OurAmazingPlanet.