HKU Bulletin September 2007 (Vol. 9 No. 1)

16 COVER STORY 17 To the Ends of the Earth S igns of life in Earth’s driest corners provide clues to what Mars may offer. Research involving the University’s scientists is helping the American space agency, NASA, to develop strategies to detect life on Mars, and overturning traditional views that hyper-arid deserts are lifeless. On field trips to Earth’s driest deserts, researchers have encountered microscopic life forms amid desolate conditions similar to Mars. These areas receive less than 25 millimetres of rain per year – and may get as little as 10 millimetres in 10 years. Nonetheless, colonies of bacteria cling to the undersides and insides of the desert rocks and have managed to survive up to 12,000 years, living on nothing but sunshine and miniscule amounts of water. These one-celled organisms have also survived desiccation – they can recover after completely drying out – and they were even able to withstand radiation levels equivalent to those on Mars in experiments, so long as they were shielded by rocks. “This research is not saying there is life on Mars, but it makes it more plausible that this is a useful model for those who are looking for life on Mars,” Dr Stephen Pointing, Assistant Professor of the School of Biological Sciences said. “They shouldn’t be sending little Rovers out to search the surface for life, they need to look under rocks instead.” Pointing has studied hyper-arid deserts in China, South America, Australia and Africa, including Yungay in the Atacama desert in Chile, the driest place on Earth. “Yungay was previously regarded as lifeless but there were these white quartz rocks on the surface and when we dug them up, we saw they were dark green below the surface. This was due to photosynthetic bacteria that had exploited tiny gains in moisture within this niche to survive,” he said. These bacteria have now been documented in most of the hyper-arid deserts visited, and although they are all related, it has been shown that they have evolved to become genetically distinct from each other as a result of isolation and adaptation to different climates. The findings are of interest not only to astrobiologists, who look for life on Earth that is similar to what could exist on other planets, but also those who seek to understand changes in our environment. “The abundance of these colonies could be a very good indicator of catastrophic environmental shifts from aridity to hyper- aridity. Arid conditions you can recover from, but hyper-arid ones you can’t,” Pointing said. Arid deserts have average rainfall of 25 – 200 millimetres per year and can support more life forms than hyper-arid ones. They are also less likely to produce the sand storms that plague some areas of China, whose land mass is 28 per cent desert. Assessing microbial colonies there could be a useful tool in monitoring the spread of hyper-desertification and other environmental changes, he said. The findings were recently published in two leading journals in their fields, Environmental Microbiology and Microbial Ecology , and are part of on-going work by Pointing and his Extremophiles Research Group into life in extreme environments. Although they are also investigating volcanoes, geothermal hot springs and hyper-saline lakes, it is the hyper-arid deserts, in particular the Atacama, that provide the starkest picture of what life might be like on another planet. “Most people think of deserts as having cacti and camels but you only find those in arid or semi-arid deserts. Where we go, there are no animals or plants. It’s really hard field work, there’s low oxygen levels, it’s cold, it’s very dry, the sun exposure is quite hard. The weirdest thing is the silence because there are no birds. Life is just hanging on in these places,” Pointing said. The Problems of Predicting the Future A lthough cl imate change fears have been hitting the headlines with increasing urgency in recent years we are still some way from establishing the natural variability in the system, according to a Postdoctoral Fellow of the University, Dr Adam Switzer. Switzer, who presented a paper at a HKU workshop on climate change, conceded that through the study of geological records “we know there is an incredible amount of variability in the system.” Research shows that climate change in the past has been surprisingly rapid at times, while there have also been long periods of reduced variability. “Then all of a sudden it will increase again. So there seems to be threshold levels and tipping points, but then when you look at all the climate models that are put forward most of them are based on smooth projections, often exponential projections. Some of them will add a bracket for potential high and low. But we still don’t have a good grasp of the natural variability,” said Switzer. “On one graph I presented at the workshop you have CO 2 , nitrogen, methane and temperatures all going up. But you need to understand that the baseline of that graph is not a straight line. And, it’s a horrible thought, but if the natural baseline has falling CO 2 , nitrogen and methane then the anthropogenic affect is actually worse than what we think it is.” The problem, according to Switzer, is the difficulty in establishing the natural sequence, as opposed to the anthropogenic effect. “One way of doing it is to look at changes in the geological record and try to figure out what the causes were for those changes. What sort of natural variability can you expect in all of these variables. And then put that as a bracket on your baseline, and then put on what you think is anthropogenic on top of that.” Switzer, who works with Professor Wyss Yim in the Department of Earth Sciences, insists it is important to learn from what the geological records tell us. “We have a good paleo-climatic record here, in terms of the Pearl River, and we are working on Pearl River Delta records which show changes in the vegetation, carbon flux and salinity. “Our recent research is mainly trying to figure out why glacial periods finish. It seems that when sea level is much lower and the climate much colder, muddy sediments are exposed to the air and release more greenhouse gases. Eventually these cause a tipping point in the other direction and trigger a warming period. “The global implications of greenhouse gas formation in muddy continental shelves are understudied and relatively unknown. So we are looking at production of greenhouse gases during lowstands of sea level. We are just starting to get a handle on the chemistry of what happens when these are released into the atmosphere. “So we need to be very careful when making predictions of what’s going to happen a hundred years from now because the earth only has to reach one of those tipping points and your predictions are thrown out the window.”