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Scientists: ancient Mars microbes might still live

Aug. 27, 2007
Courtesy PNAS
and World Science staff

Some mi­crobes can live for more than half a mil­lion years by re­pair­ing their DNA—so if Mars ev­er har­bored si­m­i­lar life forms in its wet­ter past, they too might live on, sci­en­tists say.

Re­search­ers an­nounced the find­ings in this week’s early on­line issue of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces. 

The Au­reum Cha­os re­gion on Mars, in an im­age from the Eu­ro­pe­an Space Agen­cy’s Mars Ex­press space­craft.
The find­ings might al­so apply to cer­tain oth­er So­lar Sys­tem bod­ies such as Jupiter’s moon Eu­ro­pa, they said. There, an icy shell co­vers what sci­en­tists think may be a liq­uid wa­ter ocean. Mars has very little known liq­uid wa­ter—es­sen­tial to life as we know it—but it’s be­lieved to have had much more long ago.

It’s al­so known that bac­te­ria can sur­vive for mil­len­nia, en­cap­su­lat­ed in ice, sed­i­ments and oth­er ma­te­ri­als. But for how long they live has been un­clear, as well as how they do it. The lead­ing idea was that they go dor­mant to sur­vive.

But such dor­man­cy usu­ally stops me­tab­o­lism, in­clud­ing DNA re­pair. With­out con­stant main­te­nance the ge­nome is vul­ner­a­ble to chem­i­cal re­ac­tions that dam­age the DNA. Even­tu­ally the ge­nome ac­cu­mu­lates so much dam­age that the cell can’t re­pro­duce.

In the new work, Sar­ah Stew­art John­son at the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy in Cam­bridge, Mass. and col­leagues col­lect­ed bac­te­ri­al sam­ples un­dis­turbed for more than half a mil­lion years in the fro­zen grounds of Si­be­ria and Can­a­da.

They an­a­lyzed the DNA of the an­cient cells, and also de­tected met­a­bol­ic ac­ti­vity. The key to the longe­vity of these bac­te­ria is con­tin­u­ous DNA re­pair, they wrote. They added that si­m­i­lar per­ma­frost en­vi­ron­ments around the world may har­bor spe­cies of vi­a­ble bac­te­ria adapted to past en­vi­ron­ments. An ear­li­er study pub­lished in the jour­nal’s Aug. 14 is­sue al­so pro­posed that as glaciers melt due to glob­al warm­ing, they might re­lease an­cient bac­te­ria that be­come ac­tive again.

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Some microbes can live for more than half a million years by repairing their DNA—which means that if Mars ever harbored similar life forms in a wetter past, they too might live on, scientists say. Researchers announced the findings in a study in this week’s early online edition of the research journal pnas. The findings might also apply to certain other Solar System bodies such as Jupiter’s moon Europa, they said. There, an icy shell covers what scientists think may be a liquid water ocean. Mars has only tiny amounts of known liquid water, but researchers believe it had much more long ago. It’s also known that bacteria can survive for millennia, encapsulated in ice, sediments and other materials. But for how long has been unclear, as well as how they do it. The leading idea was that the cells go dormant to survive. But such dormancy usually stops metabolism, including DNA repair. Without constant maintenance the genome is vulnerable to chemical reactions that damage the DNA. Eventually the genome accumulates so much damage that the cell can’t reproduce, with fatal results. In the new work, Sarah Stewart Johnson at the Massachusetts Institute of Technology in Cambridge, Mass. and colleagues collected bacterial samples undisturbed for more than half a million years in the frozen grounds of Siberia and Canada. The authors analyzed the DNA of the ancient cells and detected metabolic activity. The key to the longevity of these bacteria is continuous DNA repair, they wrote. They added that similar permafrost environments around the world may harbor species of viable bacteria adapted to past environments. An earlier study published in the journal’s Aug. 14 issue also proposed that as glaciers melt due to global warming, they might release ancient bacteria that become active again.