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It's life, Martinus, but not as we know it

2011; Wiley; Volume: 9; Issue: 1 Linguagem: Inglês

10.1890/1540-9295-9.1.84

1540-9309

Adrian Burton,

Space Science and Extraterrestrial Life

On the dark side of the Moon there is a crater that goes by the name of Beijerinck. Few people have ever heard of it, or of the man after whom it is named, Martinus Willem Beijerinck (1851–1931). Darwin, Newton, Einstein, Pasteur, Fleming…all household names. But Beijerinck? Relative obscurity is hardly justice for the man who was the first to filter out a virus and develop enrichment culture for isolating bacteria, and who did pioneering work on the nitrogen and sulfur cycles. The isolation of Rhizobium bacteria from the root nodules of leguminous plants, the description of nitrogen fixation and denitrification, the isolation of sulfate-producing Thiobacillus, and the identification of hydrogen sulfide produced by Aerobacter are all among Beijerinck's achievements. So it is fitting that in this Special Issue of Frontiers, and on the 80th anniversary of his death, we tip our hats to the man who helped us get started down the road to understanding how interacting biogeochemical cycles keep life running on this planet...and perhaps on others as well. Martinus Willem Beijerinck. Fittingly, “Martinus” in Latin means “of Mars”, a planet that is still holding out on us regarding whether it is, or ever was, home to any form of life. NASA missions have tantalized us with the possibility of finding Martian life, past or present, but with a focus that may have saddened – or perhaps maddened – Beijerinck (who by all accounts was a man of irascible disposition). We have been informed that Mars is now a deathly cold place with little liquid water, although it once had rivers that eroded its red-rock landscape, and must once have been warmer if water flowed there. But how often do we hear that Mars appears to be awfully low on nitrogen, a vital ingredient for life as we know it? Or that, if water ever did exist in great supply, it would have increased the planet's atmospheric pressure, making a water cycle more likely on a once warmer Mars? How often do we hear what a Martian nitrogen cycle – some means of getting that nitrogen into living things – might be like? Nitrogen is a major missing piece in the Martian jigsaw puzzle. It makes up 78% of Earth's atmosphere, but less than 3% of that of Mars, and considering that Mars' atmosphere is about 100 times thinner than Earth's, that's not a lot of nitrogen. We also have yet to find a nitrate in the Martian regolith. But that doesn't necessarily mean they are not there. “The ratio of nitrogen isotopes in the Martian atmosphere suggests that most nitrogen was lost into space over geological time, but it is also possible that life sequestered it”, says Kenneth Nealson, Professor of Geobiology at the University of Southern California (Los Angeles). “It might be present as fixed nitrogen in subsurface niches. If life existed on Mars, but became extinct, then vestiges of sedimentary fixed nitrogen might still be detectable. Alternatively, life may have never evolved, leaving, at best, abiotic nitrate deposits. ‘Follow the nitrogen’ is a mantra that makes sense when searching for life on Mars.” Yet even if Mars once had sufficient nitrogen, liquid water, warmth, and the other essentials for life (and encouragingly, NASA's Phoenix Mars Lander has shown that some vital components, such as soluble salts of magnesium, sodium, and potassium, do exist there), was there enough time for life to evolve before the atmosphere turned hostile? And if so, did it have time to spawn the extremophiles that would be necessary to perpetuate it? “Extremophiles on Earth probably evolved from organisms that developed in places where life was easier than [in any environment] on Mars now”, says extremophile researcher Jacek Wierzchos (Spanish Research Council, Madrid, Spain). “Although life is possible under unbelievable conditions, these must remain stable over long periods of time. This is essential for the transition from fragile extremoresistance to sturdy extremophilia.” Martian life may therefore have been in two desperate races against the clock: one to get started, the other to adapt to the huge changes in Martian global chemistry that made it the place it is today. It may all seem a bit unlikely, but we have collected Martian meteorites (bits knocked off Mars by various impacts) from Antarctica that show possible relicts of magnetotactic bacteria. Their true identity remains controversial, but could these be the remains of little Martians? One has to wonder what Beijerinck would have made of possible magnetotactic bacteria on a cold, dry, apparently nitrogen-poor planet. How would he have filtered those bacteria from out of the rocks and enriched them in culture? How would he have explained the biogeochemical interplays that made them tick? Beijerinck will never know whether there is or ever was life on Mars, but his pioneering work here on Earth has given us, or maybe our descendants, a chance to know the answer. In his own words, “Fortunate are those who are starting now”.