Science Spin January 2009

Life beneath the oceans

By Tom Kennedy

Like the flat Earth, that particular view has been discarded as utterly false, and, in fact, to trace the foundations of life, we need to look quite deep below the surface. Instead of a seven metre bio barrier, we now know that some microbes are even at home in rocky sediments below 800 metres. The diversity of the deep biosphere may well equal the microbial richness of soil, once considered the most fertile material on Earth. Until recently the existence of the deep biosphere was unknown, yet it may possibly make up one third of all living matter on Earth.

It might strike us as strange that microbes existing under conditions more severe than those found on Mars, can be so important, but without them life as we know it, would not have come into existence, and the Earth could certainly be a different, unrecognisible, place.

Recently, the geoscientist, Judith McKensie, was in Ireland to explain how a long running international programme of ocean floor exploration has revolutionised our understanding of the deep biosphere. As Professor of Earth System Science in Zurich, Judith is involved in the Integrated Ocean Drilling Programe (IODP). This involves drilling deep into the ocean floor, and as she explained, at first when cores were being examined, the last thing scientists expected to detect was any evidence of life. Now, scientists realise that rocks and microbes are seldom far apart, and Judith said that over the past decade her own research has shifted from geochemistry to geo-microbiology. Many minerals, such as iron ores, oil, and rock formations such as chalk, are biological in origin, yet, until quite recently, as Judith observed, "we never considered the importance of the microbes."

One of the reasons for this, she explained, is that the microbes were well hidden, so even if geologists had suspected that they were present, techniques for detection were simply not available. Advances in electron microscopy, and fluorescent markers changed all that, and the high concentration of life took everyone by surprise. A one gram sample of sub-surface rock could turn out to be supporting millions of microbes, and unlike those on the Earth's oxygen rich surface, these would be anerobic, drawing their energy from inorganic sources.

Recovering those microbes for further study has become an essential part of the IODP. As every core is drawn up from the depths, its surface is scanned for signs of life, and as Judith explained, no where on Earth seems to be too hostile for microbes.

The drilling programme, she said, is global, and in Europe seventeen states are in a consortium with Canada operating under the International Ocean Drilling Programme. Ireland is involved, as is Switzerland. "You might ask why Switzerland?" said Judith, but as she put it, "no one is not linked to the oceans." In Ireland, the link might be more obvious, but in every country the oceans are part of the past, and they have a major influence on global climate.

The ocean floor is far from uniform, and a number of different drilling vessels are deployed in what Judith referred to as mission specific platforms. The Americans, she explained, provide a 'non-riser' that can drill in deep waters where there is no danger of blow-out from intense underground pressure, while the Japanese vessel, like those used in oil exploration, is designed to deal with such a situation. In polar regions, ice-breakers are sent in with the drill ships, and Judith said the international team of scientists had been amused last year to hear about the Russians planting a flag on the North Pole. "Sorry," she said, "we got there first, and in 2004 put down a drill string, so you could say that ridge now belongs to the ocean drilling programme."

The drill ships have also been at work off the Irish coast, and during "Expedition 307" in 2005 carbonate mounds, some as high as the Eiffel Tower were investigated, and once again, the bio connection was significant. "It seems that microbial activities are producing methane inside these mounds," said Judith.

Coming to life

From 1983 to 2003 geochemists on the drilling programme had been noting with interest the depletion of sulphates in seawater. "Sometimes this happened at 10, sometimes 30, and sometimes 100 metres," Judith said, "and below this we began to get the production of methane." For the geochemists this was a clear case of sulphate reduction but as Judith explained, it took a while for scientists to realise that bacteria were so active in chewing up the sulphates, and that distinct sulphate methane transition zones, involving a close symbiotic relationship between bacteria and the more ancient archaea microbes exist.

Because the microbes were invisible to the naked eye their role had not been obvious, but the development of fluorescent stains, that attach themselves to DNA, finally convinced the drillers that life continued to thrive well below the seven and a half metre depth. However, the fact that microbes are so active and so extensive, even in the frozen depths, took everyone by surprise. In a way, the greening of our planet had masked the original inhabitants, but the archea are still very much with us, and as Judith observed, they are still doing what they have been doing for billions of years. Observing microbes at work now helps to explain what we see from the past, and Judith mentioned dolomite as an example. Under the electron microscope, living microbes have been seen to produce tiny crystals of magnesium carbonate, dolomite, and, in clinging together in colonies, cells have been surrounding themselves with solid coatings for the last 3.5 billion years.

Given that life could emerge in such a hostile world, and continue to thrive so deep into the ocean floor, makes Judith wonder if the same process might be at work elesewhere. "I think the most excting thing these days," she remarked, "would be to be a Mars sedimentologist."

Dumping at depth

Plans to dump carbon dioxide or store radioactive wastes at depth are often based on the assumption that the upper crust of the Earth is dead. As Judith explained, deep drilling shows that this is not the case. Carbon dioxide is currently being pumped down into oil fields and abandoned coal mines, but as Judith said, "we have to ask ourselves how is the deep biosphere going to handle this?" Strange things can happen at depth. Spaces are opened up, and microbial life reacts, as happened at Potsdam in Germany, where carbon dioxide was pumped in, and lots of iron came out.

The deep biosphere, explained Judith, has always had an enormous influence on the Earth's atmosphere, and currently it is thought to store 30 per cent, or more, of the world's carbon dioxide.

Slow growth

Life in the deep biosphere is a lot different from life on the surface, and to find out what these micro-organisms are like and how they survive, samples are brought back for culturing under similar conditions in the laboratory. As Judith explained, this is not an easy task. Matching extreme conditions is one thing, and cells can be kept alive, but promoting growth is a problem. Normally, cells divide about once a day, and once a week would be regarded as extremely slow. "Down in the deep biosphere," said Judith, "this might be once in 300, or even once in every 3,000 years."

Compared to the human timescale, this is almost geologically slow, but those cells are alive. On close examination, said Judith, the different stages of mitosis cell division can be seen. However, that extremely slow rate of cell division makes it impossible to bulk up the DNA, frustrating attempts to extract potentially valuable genes.