Diagram of carbon dioxide sequestration, showing the supercritical CO2 front as it moves out into an underground water reservoir. (Credit: Djuna Gulliver)

The threat of ballooning carbon dioxide levels in the atmosphere puts us between a rock and a hard place, which is exactly where some people propose the gas should go.

Carbon sequestration – capturing CO₂ and storing it in underground reservoirs – presents an attractive solution to the greenhouse problem. How microbial communities exposed to the influx of CO₂ respond, however, remains to be seen.

Sequestering CO₂ requires hot, pressurized conditions, which turn the gas into something between a gas and a liquid: a supercritical fluid. Supercritical CO₂ is deadly to many organisms, and is widely used as a sterilizing agent. But some microbes may be able to withstand the deadly substance. They’re found in deep underground, in pools of oil or saltwater, and they may be the key to keeping CO₂ safely trapped. The research is still in its early stages, however.

“At this point, we’re asking a fundamental question: Will microbes be active?” Janelle Thompson, a civil and environmental engineer at MIT, asked.

In other words, would the trapped CO₂ kill off the microbes making biological processes irrelevant in sequestration? Or could the microbes play a role in the trapping process by helping seal in the CO₂, or perhaps even reacting with it to transform it into minerals?

It appears some microbes can survive, at least for a while, according to research by Djuna Gulliver, a graduate student in civil engineering at Carnegie Mellon University. Gulliver, who works with the Department of Energy’s National Energy Technology Laboratory, presented a poster on the effects of carbon sequestration on microbial ecology Friday morning at the American Geophysical Union’s Fall Meeting.

Gulliver obtained microbe samples drilled from a natural underground water reservoir and potential sequestration site in the Wellington Oil Field in Kansas. To examine the effect of different CO₂ levels on the microbes, she exposed them to gas mixtures at 40 degrees Celsius (104 degrees Fahrenheit) with CO₂ pressures ranging from 0 to 136 atmospheres. Then she watched how the bacterial communities evolved over about two months.

Salt-tolerant bacteria ended up surviving best, Gulliver found. But at the highest CO₂ pressures, even these bacteria died out.

Why did the salt-loving bacteria do so well? “I think they’re used to very low-nutrient systems,” Gulliver said. These bacteria aren’t expected to be as important for CO₂ sequestration as, say, bacteria that stick together on a surface, forming a biofilm. Such bacteria are a double-edged sword for carbon trapping, Gulliver said. They could be helpful by sealing up cracks in underground CO₂ reservoirs where the gas might escape. Or they could clog up the pipe used for pumping in CO₂, rendering it unusable.

Gulliver is hoping to do a longer study because bacteria grow extremely slowly underground. An improved understanding of how microbes interact with sequestered CO_2, she said, could have important commercial potential as industries grapple with managing their carbon footprints.

-Tanya Lewis is a science communication graduate student at UC Santa Cruz.