You'd think it would take a pretty big toolkit to prepare the Martian surface for human life. Not necessarily, at least at the level of a human habitat, according to Robert Richmond of NASA's Marshall Space Flight Center. In fact, the components may prove to be surprisingly, even microscopically, small in size. And although it is perhaps still too early for would-be planetary travelers to begin packing, the notion of terraforming Mars—altering its environment to allow for human habitation—is one that, for many scientists, has recently evolved from pure science fiction to theoretical possibility.
One significant reason for the surge in optimism is a series of discoveries suggesting that extremophilic organisms—those that thrive under extreme environmental conditions—may be uniquely equipped to serve as vectors of change on Mars's inhospitable surface. Among the most promising of those organisms is the bacterium Deinococcus radiodurans, which has been found in many types of soil and such unappealing spots as sewage systems and animal fecal material. Richmond, a radiation biologist at the Space Flight Center, along with Michael Daly of the Uniformed Services University of the Health Sciences and Rajagopalan Sridhar of Howard University Medical Center, has been testing the limits of D. radiodurans in the hopes of harnessing its unusual characteristics for just such a toolkit (Proceedings, International Society for Optical Engineering, v. 3755, pp. 210–222.)
It is widely accepted that current planetary conditions on the immediate surface of Mars eliminate the possibility of sustaining life as we know it: Low atmospheric pressure and surface temperature combined with relatively high levels of ultraviolet and ionizing radiation would appear effectively to prevent the long-term survival of organic life. In the 1970s, the Viking missions established that Martian soil contains high levels of certain metals and oxidizing species. To survive such a noxious environment, an organism must be highly resistant to oxidizing conditions.
Hence the excitement over D. radiodurans. Not only can this bug withstand extreme amounts of radiation (whence it receives its name), but it has proved quite resistant to the effects of peroxides and other oxidizers as well. And when subjected to desiccation, freeze-drying and exposure to solar-flux ultraviolet radiation, the organism fares extremely well. Its multiple resistances have led Richmond and his colleagues to term the bacterium a "polyextremophile." Although scientists have documented the existence of extremophiles living in isolated environments like deep-sea hot vents or hotsprings for decades, rarely, if ever, has an organism been found to withstand such a wide array of extreme conditions.
What makes the research so thrilling, says Richmond, is the "chance to actually uncover the utilities of the bacterium and not just isolate and classify it."
Those possible utilities have increased dramatically in number since the successful sequencing of the bacterium's genome in November 1999. Sequencing the DNA revealed that many copies of the genome are present in any given bacterial cell in register—all the bases making up the DNA sequence are lined up in the same way, and the sequence itself is full of repetitions. It has since been proposed by Daly and coworkers that D. radiodurans's durability is the product of an efficient and highly accurate repair system: If exposure to radiation damages one strand of DNA, another strand may serve as a template. This hypothesis provides an explanation for each of the microbe's resistances. Thanks to its efficient repair system, D. radiodurans can survive any number of extreme environments.
With a more thorough understanding of what causes D. radiodurans's multiple durabilities, scientists such as Daly are now working to genetically engineer the bacterium to perform work that people cannot. After all, Richmond says, "you must always think of the organism's utility in managing a habitat—you have to put the bug to work for you." Because it could successfully withstand the high levels of oxidants found on the Martian surface, D. radiodurans might be engineered to detoxify the soil. In Daly's lab, for example, he and his colleagues insert genes that code for an enzyme capable of oxidizing organic toluene, thereby rendering this toxic component of organic solvents harmless to humans. It may be possible to engineer a bug capable of reducing iron or manganese ions to their elemental forms, thus reducing the concentrations of noxious substances, and advancing one step closer to the creation of a habitable space.
In fact, NASA is considering launching probes to specific Martian sites. This allows consideration of the use of extremophile organisms such as D. radiodurans to begin microterraforming small surface areas. The bacteria could begin transforming the harsh and uninhabitable Martian terrain in such a future scenario into one capable of sustaining human life. At its most fantastic, terraforming involves the alteration of an entire planet's environment, but, realistically speaking, says Richmond, we can perhaps imagine modifying the oxygen and soil content of a small room, several cubic meters in area, in direct contact with the habitat.
When can we expect microterraforming to occur? Obviously, planetary biologists are still investigating the possibility that life might exist on Mars without human intervention, and they will want to be certain of its sterility before infecting it with engineered bacterial colonies. The notion of interplanetary contamination raises many difficult ethical questions, and although the NASA Planet Protection Committee has developed strict guidelines to prevent the contamination of other planets, it will most likely prove difficult to maintain such regulations if and when humans do land on Mars.
What D. radiodurans can provide is a microscopic (and therefore easily portable) factory—a kind of terraforming toolkit—from which any number of products potentially can be derived. Whether it is engineered to reduce metals, produce drugs for ailing astronauts or simply manufacture the polymers necessary for the production of thread, D. radiodurans, one of the world's oldest bacteria, may provide a means of expanding the limits of the human imagination beyond the written sci-fi page.—Rebecca Sloan Slotnick