March 16th, 2013 by James Ayre
The first direct evidence of life deep beneath the oceanic crust has now obtained by researchers. The oceanic crust covers about 60% of the globe, when you factor in depth and calculate total volume this means that this new ecosystem is the largest on the planet. And it is apparently home to a wide array of chemosynthetic organisms, amongst others.
The presence of such an enormous ecosystem that is entirely independent of sunlight is very interesting, and makes a string argument for the possibility of life being present on other planets.
“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.
For a long while, it was assumed by much of the scientific community that sunlight was a prerequisite for life on the Earth. This initial assumption began changing a couple of decades ago with the discovery of life at deep sea volcanic vents that seemed to be entirely independent of sunlight. These ecosystems were found to be based around the chemicals and minerals being released by the volcanic vents. The newly-found life that is present deep within the porous rock material of the oceanic crust takes this separation from the Sun even further.
“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.
“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.
Interestingly, while this ecosystem is likely based around hydrogen, there is actually a variety of different life forms found there. “The hydrogen-oxidizing 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.”
“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.
The basalt in question is over 3.5 million years old, and laboratory cultures clearly show that the DNA taken from the rock is from living creatures, not fossils. “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,” says 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.”
Image Credit: Jesper Rais, AU Communication
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