DNA Secrets of a Salty Survivor

DiRuggiero and her research group have begun revealing this DNA-repair machinery in a recent series of experiments funded by NASA's Exploration Systems Mission Directorate.

In some experiments, they exposed Halobacterium cells to beams of intense UV radiation. "We used UV-C at 254 nm, which is the most lethal UV wavelength," says DiRuggiero. Most microbes, like E. coli that lives in the human gut, would have been completely exterminated, yet 80% of the Halobacterium cells survived. Indeed, they went on living and reproducing just fine.

In other experiments, the researchers used a vacuum chamber at NASA's Goddard Space Flight Center to expose cells of Halobacterium to a space-like vacuum (1 millitorr). Here, living in very salty water proved to be Halobacterium's saving grace: as the vacuum caused the water to evaporate away, the salt was left behind, forming salt crystals. The tiny cells of Halobacterium became trapped inside these crystals, along with a bit of entrapped water.

"The salt crystal is like a little house in which the cells are protecting themselves from additional desiccation," DiRuggiero explains. The cells can live in a semi-dormant state within the crystals for a long time. When dissolved back into water, the cells spring to life again, repair all the damage to their DNA caused by the partial desiccation, and go right on living.

Some scientists even claim to have found living cells of Halobacterium encased in salt deposits that are 250 million years old. (see journal references below) The claim is controversial, but if true, it could have some profound implications for the hunt for microbial life on Mars. Evidence from the Mars Exploration Rovers, Spirit and Opportunity, announced in March suggests that the Martian surface once had pools of salty water, which slowly evaporated away.

Image credit: Albert Lau A repair enzyme correcting an error in a DNA molecule. The enzyme is on the right in orange and green, and part of the double-helix-shaped DNA is on the left in blue.

"So if microbial life evolved on Mars and then the water evaporated, and if the microbes are trapped in salt crystals, they could still be there, and still viable. Given the data that we have from Earth, that's entirely possible," Kish says.