Lifeforms living in sediment at the bottom of the ocean get their energy from chemicals produced from irradiated seawater. Although this sounds awfully like Godzilla's backstory, the radiation, in this case, has been there long before humans thought of testing nuclear weapons in the Pacific. The discovery may greatly widen the places we search for extraterrestrial life.
Most life on Earth gets its energy, directly or indirectly, from sunlight – either through photosynthesis, eating something that photosynthesizes, or eating something that eats something... you get the idea. Where sunlight isn't available, life sometimes still finds a way, for example feeding off the abundant heat and molecules around hydrothermal vents on the bottom of the ocean. More recently, we have learned there is surprisingly abundant life in rocks miles beneath the sea bed.
When drilling into the mud at the bottom of the ocean, Dr Justine Sauvage of the University of Gothenburg found that hydrogen is the main source of energy for microbes quite close to the surface. Previous discoveries involved rocks kilometers down. The unexpected part of this is that we know plenty of oceanic life falls to the seafloor and gets consumed there – it was anticipated that just a little further down, any lifeforms would be living on organic scraps from the water column, not relying on something entirely different.
Instead, Sauvage and co-authors report in Nature Communications that sediment-dwellers get most of their energy from hydrogen and oxidant molecules. These form when alpha and gamma particles released by radioactive isotopes such as uranium and thorium break water's molecular bonds.
Co-author Professor Steven D'Hondt of the University of Rhode Island said in a statement that radiation-produced hydrogen molecules are surprisingly abundant. "The marine sediment actually amplifies the production of these usable chemicals. If you have the same amount of irradiation in pure water and in wet sediment, you get a lot more hydrogen from wet sediment. The sediment makes the production of hydrogen much more effective."
By irradiating tubes of sediment collected at different spots on the deep ocean, comparing them with seawater and distilled water, Sauvage found the same amount of radiation could release up to 27 times more hydrogen in sediment compared to in pure water. The team is still working to explain why, but they think minerals in the sediment enhance the water-splitting activity. If true, this could prove very interesting to scientists working on finding better catalysts for water electrolysis, a quest that, if successful could lead to a clean energy revolution.
Whether or not the work has industrial applications, it's certainly something astrobiologists will want to pay attention to. “This work provides an important new perspective on the availability of resources that subsurface microbial communities can use to sustain themselves,” Sauvage said. “This is fundamental to understand life on Earth and to constrain the habitability of other planetary bodies,”
Mars is on our mind at the moment, but the work could be just as important for somewhere like Europa, indicating a source of energy at the bottom of its vast global ocean.
It's still not an abundant source of energy, however. The authors find 50-100 times as much energy rains down from above as is available from hydrogen. However, with most organic matter getting consumed in the sea bed's top few centimeters, the areas below depend on more exotic energy sources.
