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Caves are often dark, damp, and remote. While they lack the nutrients and energy sources that sustain life in other ecosystems, they still host a diverse array of bacteria and archaea. But how do these microorganisms acquire enough energy to thrive? A team of researchers from Australia and Europe investigated this intriguing question by examining Australian caves.
Previous studies identified that microorganisms in nutrient-poor soils can harness energy from the atmosphere through trace gases, including hydrogen, carbon monoxide, and methane. These gases are present in minute quantities, classified as trace gases. Microbes possess specific proteins that can accept electrons from these gas molecules, enabling them to utilize these gases as energy sources, such as hydrogenase, dehydrogenase, or monooxygenase, fueling their metabolic processes.
The Australian research team hypothesized that cave-dwelling microbes may be using trace gases for survival. To test this, they studied four ventilated caves in southeastern Australia. The researchers collected sediment samples at four points along a horizontal line that extended from the cave entrance to 25 meters (approximately 80 feet) deep inside the cave, resulting in a total of 94 sediment samples.
The team treated the sediment samples with specific chemicals to extract microbial DNA, using it to identify both the abundance and diversity of microorganisms present. They found multiple groups of microorganisms throughout the cave, including Actinobacteria, Proteobacteria, Acidobacteria, Chloroflexota, and Thermoproteota. Notably, the density and diversity of microbes were significantly higher near the cave entrance, with three times more microorganisms in those regions compared to further inside.
The team utilized gene sequencing to analyze the microbial DNA for genes linked to trace gas consumption. Results revealed that 54% of cave microorganisms carried genes coding for proteins involved in utilizing trace gases like hydrogenases, dehydrogenases, and monooxygenases.
To assess the generality of their findings, the researchers searched existing data on microbial populations from 12 other ventilated caves worldwide. They discovered that genes for trace gas consumption were similarly prevalent among other cave microorganisms, concluding that trace gases might significantly support microbial life and activity in caves.
Next, the researchers measured gas concentrations within the caves. They deployed static magnetic flux chambers to collect atmospheric gas samples at four points along the sampling line, capturing 25 milliliters (about 1 ounce) of gas each time. Using a gas chromatograph, they analyzed the samples and found that the concentrations of hydrogen, carbon monoxide, and methane were approximately four times higher near the cave entrance compared to deeper areas. This suggests that microorganisms might be metabolizing these trace gases for energy.
To validate their findings further, they constructed a static magnetic flux chamber in the lab, incubating cave sediment with hydrogen, carbon monoxide, and methane at natural concentration levels. They confirmed that microbes also consumed trace gases in controlled conditions.
Finally, the researchers explored how these cave microbes obtained organic carbon. They conducted carbon isotope analysis, focusing on carbon-12 and carbon-13 ratios, which can vary based on microbial metabolic processes. Using an isotope ratio mass spectrometer, they determined that cave bacteria had a lower percentage of carbon-13, indicating their reliance on trace gases to generate carbon within the cave ecosystem.
The researchers concluded that atmospheric trace gases serve as a crucial energy source for microbial communities in caves, fostering a diverse array of microorganisms. They recommended that future studies examine how climatic changes, such as fluctuations in temperature and precipitation, might influence the use of atmospheric trace gases by cave-dwelling microorganisms.
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Source: sciworthy.com
