The Earth's oceanic crust covers an enormous expanse, and is mostly buried beneath a thick layer of mud that cuts it off from the surface world. Scientists now document life deep within the oceanic crust that appears to be sustained by energy released from chemical reactions of rocks with water.
The core drill slides through a drill pipe, extending from the drill ship at the sea surface, through a water depth of 2.5 km and hundreds of metres of sediment, into the oceanic crust off the west coast of North America. Microbiologist Mark Lever is on board the Integrated Ocean Drilling Program's research vessel JOIDES Resolution to examine rock samples from the depths. The results of the studies he and his colleagues carried out are published today in the journal Science.
"We're providing the first direct evidence of life in the deeply buried oceanic crust. Our findings suggest that this spatially vast ecosystem is largely supported by chemosynthesis," says Dr Lever, at the time a PhD student at the University of North Carolina at Chapel Hill, USA, and now a scientist at the Center for Geomicrobiology at Aarhus University, Denmark.
Energy from reduced iron
We have learned that sunlight is a prerequisite for life on Earth. Photosynthetic organisms use sunlight to convert carbon dioxide into organic material that makes up the foundation of Earth's food chains. Life in the porous rock material in the oceanic crust is fundamentally different. Energy – and therefore life's driving force – derives from geochemical processes.
"There are small veins in the basaltic oceanic crust and water runs through them. The water probably reacts with reduced iron compounds, such as olivine, in the basalt and releases hydrogen. Microorganisms use the hydrogen as a source of energy to convert carbon dioxide into organic material," explains Dr Lever. "So far, evidence for life deep within oceanic crust was based on chemical and textural signatures in rocks, but direct proof was lacking", adds Dr Olivier Rouxel of the French IFREMER institute.
Our biosphere is extended
The oceanic crust covers 60 per cent of the Earth's surface. Taking the volume into consideration, this makes it the largest ecosystem on Earth. Since the 1970s, researchers have found local ecosystems, such as hot springs, which are sustained by chemical energy.
"The hot springs are mainly found along the edges of the continental plates, where the newly formed oceanic crust meets seawater. However, the bulk of oceanic crust is deeply buried under layers of mud and hundreds to thousands of kilometres away from the geologically active areas on the edges of continental plates. Until now, we've had no proof that there is life down there," says Dr Lever.
Even though this enormous ecosystem is probably mainly based on hydrogen, several different forms of life are found here. The hydrogen-oxidising microorganisms create organic material that forms the basis for other microorganisms in the basalt. Some organisms get their energy by producing methane or by reducing sulphate, while others get energy by breaking down organic carbon by means of fermentation.
Basalt is their home
Mark Lever is a specialist in sulphur-reducing and methane-producing organisms, and these were the organisms he also chose to examine among the samples taken from the oceanic crust. These organisms are able to use hydrogen as a source of energy, and are typically not found in seawater. Dr Lever had to make sure that no microorganisms had been introduced as contaminants during the drilling process, or transported from bottom seawater entering the basaltic veins.
"We collected rock samples 55 kilometres from the nearest outcrop where seawater is entering the basalt. Here the water in the basaltic veins has a chemical composition that differs fundamentally from seawater, for instance, it is devoid of oxygen produced by photosynthesis. The microorganisms we found are native to basalt," explains Dr Lever.
Active life or dead relics?
Dr Lever's basalt is 3.5 million years old, but laboratory cultures show that the DNA belonging to these organisms is not fossil. "It all began when I extracted DNA from the rock samples we had brought up. To my great surprise, I identified genes that are found in methane-producing microorganisms. We subsequently analysed the chemical signatures in the rock material, and our work with carbon isotopes provided clear evidence that the organic material did not derive from dead plankton introduced by seawater, but was formed within the oceanic crust. In addition, sulphur isotopes showed us that microbial cycling of sulphur had taken place in the same rocks. These could all have been fossil signatures of life, but we cultured microorganisms from basalt rocks in the laboratory and were able to measure microbial methane production," explains Dr Lever. Dr Jeff Alt of the University of Michigan at Ann Arbor adds that "Our work proves that microbes play an important role in basalt chemistry, and thereby influence ocean chemistry".
Chemosynthetic life plays a role
Mark Lever and his colleagues developed new sampling methods to avoid sampling microbial contaminants from seawater, which is often a major problem in explorations of the oceanic crust. The researchers work in an area of the world that is extremely hard to reach. As Dr Andreas Teske of the University of North Carolina at Chapel Hill expresses "this study would not have been possible without the close collaboration of microbiologists, geochemists and geologists from the US, Denmark, France, Germany, the UK and Japan – each team member going to the limits of what was technically possible. Such strong proof for life in the deep ocean crust has eluded scientists for a long time".
Exploring the oceanic crust is still a young science. However, the prospects are great.
"Life in the deeply buried oceanic crust is supported by energy-sources that are fundamentally different from the ones that support life in both the mud layers in the sea bed and the oceanic water column. It is possible that life based on chemosynthesis is found on other planets, where the chemical environment permits. Our continued studies will hopefully reveal whether this is the case, and also what role life in the oceanic crust plays in the overall carbon cycle on our own planet," says Dr Lever.
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More information: 'Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt' by Mark A. Lever, Olivier Rouxel, Jeffrey C. Alt, Nobumichi Shimizu, Shuhei Ono, Rosalind M. Coggon, Wayne C. Shanks, III, Laura Lapham, Marcus Elvert, Xavier Prieto-Mollar, Kai-Uwe Hinrichs, Fumio Inagaki, and Andreas Teske in Science, 15 March 2013.