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Projects: LIFE Experiment: Shuttle & Phobos

Archaea: the natural extremophiles

by Amir Alexander

Click to enlarge > The Dead Sea, home of Haloarcula marismortui

Along with the samples of bacteria and eukaryotea, LIFE will also carry three species of archaea. This domain of life was discovered and defined in the 1970's (in part by LIFE science team member George Fox), when new organisms were detected in places where no life was thought exist. Like bacteria, archaeons (as archaea organisms are called) are single-cell organisms, and like bacteria they are prokaryotes – meaning that the cells do not contain a well defined "organelles" such as a nucleus, mitochondria, etc. But, they are sufficiently different from bacteria to be considered a separate branch of life.

Haloarcula marismortui
Since many archaeons are extremophiles that thrive under conditions that would destroy other organisms, scientists are particularly interested to see how they will fare on the long voyage through space. One of these archaeons, known as Haloarcula marismortui, is halophilic, meaning that it thrives in extremely salty environments. Not coincidentally it was discovered in one of the saltiest bodies of water on Earth, the Dead Sea between Israel and Jordan ("marismortui" is Latin for "of the Dead Sea"). The Dead Sea is appropriately named because it is nearly ten times saltier than the open ocean, and is therefore devoid of macroscopic life such as fish and seaweeds. Only rare microorganisms such as Haloarcula marismortui make their home in this exceptionally saline environment.

Now while radiation resistance is obviously a highly desirable trait for an organisms hitching a ride through space, it is not so clear why scientists would be interested in an organism that seems to enjoy high salinity. After all, the LIFE organisms are unlikely to encounter any salt deposits on their journey through space. The reason, explained Warmflash, has little to do with conditions the organisms will encounter on the way to Mars, and much to do with conditions that prevailed on Mars itself billions of years ago.

If Mars had water on its surface at some point in the distant past, it was in all likelihood a very salty brew. The reason, said Warmflash, is that such a mix is the only kind of water that would remain liquid under the planet's low-pressure atmosphere. This view is further supported by evidence from Martian meteorites -- rocks that were blasted off the Red Planet by some ancient calamity, and after spending uncounted years in space ultimately landed on Earth. One family of Mars meteorites is called "Nakhlites" after the largest of them, which landed in Nakhla, Egypt, and it contains high levels of chlorides and sodium. This strongly suggests that Nakhlites were formed in a high-salinity environment, possibly in the seas or lakes of ancient Mars.  Salts have also been found by the Mars Exploration Rovers.

Click to enlarge > The Oceans of Mars Mars as it may have appeared billions of years ago, with an ocean covering a large portion of its surface. If life existed on Mars in the past, it likely populated such oceans. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Now if life did exist on Mars at some point, then it likely would have populated these same Martian seas, and would therefore be halophilic – just like Haloarcula marismortui. Could salt-loving organisms from Mars survive the long trip to Earth? We don't know, but seeing how well our Earthly salt-loving archaeon does on the trip just might provide scientists with the beginnings of an answer.

Methanothermobacter wolfeii
Haloarcula marismortui, then, was selected for the exclusive club of LIFE organisms not because of its ability to survive conditions in space, but rather because it may have been suitable to survive on Mars – at least at some point in the past. Much the same can be said of the section for the trip of another member of Archaea, designated Methanothermobacter wolfeii. This organism belongs to a family of "methanogens" – archaeons that produce methane gas and are common in animal and human intestines as well as in raw sewage.

Recent observations from Earth as well as by the Mars Express orbiter have detected trace amounts of methane in the Martian atmosphere. The presence of methane is surprising, because it is an unstable gas that would not survive for long in the atmosphere. This strongly suggests that something on Mars is producing methane, and one possibility is that this "something" could be methane producing organisms similar to Methanothermobacter wolfeii. We do not, of course, know whether our archaeon could survive in the Martian environment, not to mention the brutal conditions of the long journey between the planets. But just as was the case with its salt-loving relative, the fortunes of this methanogen on a 34 month journey in space just might shed some on the question.

Click to enlarge > Pyrococcus furiosus This archaeon, whose name means "rushing fireberry," is a heat-loving extemophile that thrives in temperatures of 100 degress Celsius.

Pyrococcus furiosus
The third member of Archaea included among the LIFE specimens is another extremophile, Pyrococcus furiosus, known for its great fondness for heat. This archaeon was discovered in 1986 in volcanically heated marine sediments off the coast of Italy, and it thrives in temperatures between 70 and over 100 degrees Celsius (160 and 220 degrees Fahrenheit). Now overheating is not really a problem in interstellar space, nor is it likely to be a concern on surface of Phobos. There is, however, a certain risk that a malfunction will cause the LIFE biomodule to overheat when it reenters Earth's atmosphere at the end of the journey. In that case, Warmflash explained, Pyrococcus furiosus will serve as a kind of temperature control. If it is the only organism to survive, this will indicate that the cause of the others' demise was not conditions in interplanetary space but simply overheating.

Read more about the bacteria and eukaryotea in the LIFE biomodule.