Bacteria alive and thriving at depth.

Bacteria alive and thriving at depth

In recent years, scientists have found bacteria, a far down as 1,150 feet, in wells that penetrate deeply buried aquifers -- porous layers of rock that hold underground water. Such finds have forced hydrologists to question their traditional belief that deep aquifers were void of life. But it was not clear whether these bacteria were native residents of the aquifers or just contaminants from the world above, living solely within the wells. Moreover, no one had established how the bacteria were affecting their environment, if at all.

Experiments are now demonstrating for the first time that bacteria are indigenous to deep aquifers and that they actively change the chemistry of the groundwater, reports a group of hydrologists and microbiologists.

"The bacteria do a lot. They are probably one of the most important processes in determining groundwater chemistry," says Francis H. Chapelle of the U.S. Geological Survey (USGS) in Columbia, S.C., who conducted the experiments along with USGS colleague Peter B. McMahon, James T. Morris of the University of South Carolina in Columbia and Joseph L. Zelibor Jr. of the University of Maryland in College Park.

In a set of experiments near Hilton Head, S.C., Chapelle and his colleagues drilled more than 100 feet down into an aquifer and pulled up sediment cores, from which they isolated bacteria that were attached to the particles of sediment. In the laboratory, the researchers incubated the bacteria and demonstrated that the organisms metabolically produced carbon dioxide under both aerobic and anaerobic conditions, according to a report in the February GEOLOGY. Groundwater typically contains dissolved carbon dioxide gas.

The scientists could relate this laboratory-produced gas, by means of a peculiar isotopic signature, to that found dissolved in water from the aquifers. Earlier experiments had shown that the carbon in water from this particular aquifer was abnormally rich in carbon-13, a heavy isotope of carbon. Chapelle's group found that the carbon generated in the lab also contained high levels of carbon-13. He suggests that the bacteria, which feed on organic molecules, have a metabolism that selects this heavy isotope when producing carbon dioxide.

The gas can greatly affect the chemistry of the groundwater in the aquifer, says Chapelle. Dissolved carbon dioxide acidifies water and helps it eat away the limestone rocks of the aquifer. Such activity will enlarge the pores in the rock, enabling water to flow more freely through the aquifer and increasing the amount of water the aquifer can hold.

According to microbiologist Derek Lovley, the recent study "shows the potential for aerobic microbial metabolism to affect the geochemistry through CO2 production." However, Lovley, from the USGS in Reston, Va., says production rates measured in the lab were much higher than they would be in the ground. More work is required to understand what factor is limiting the bacteria in the natural environment.

None of the bacteria studied by Chapelle is infectious, and most would not significantly affect the drinking quality of water in the deep aquifers. However, some bacteria can make water less suitable for human use. Certain species reduce iron, making the metal more soluble in water. In tap water, iron can produce stains and an unpleasant taste.

Hydrologists have detected signs of bacteria in deep aquifers in many locations, says Glenn Patterson of the USGS in Columbia. Some bacteria are known to feed on toxic chemicals, and researchers are now exploring how bacteria may aid in cleaning up chemical spills.