Rock samples collected at the base of glaciers in Canada, Norway, Greenland and Antarctica have helped resolve a longstanding mystery: what were the energy sources that supported life in the distant geological past, when the earth was covered with ice?
The microorganisms in subglacial habitats may have taken energy from hydrogen molecules during the harsh Neoproterozoic glaciations, 750 to 580 million years ago, according to a new study in Nature Geoscience. This hydrogen may have been the key to their survival, the authors found.
During the Neoproterozoic glaciations, ice sheets covered the world for millions of years. These ice sheets gave this period its common name, “Snowball Earth.” These environments, previously considered inhospitable to life, have been found to sustain diverse ecosystems over millennia. Subglacial environments lack carbon and light, which usually serve as energy sources for life. In well-lit environments, organisms can use light to produce organic molecules that are used by organisms higher up in the food chain. Darker environments also have organic matter, often from decaying organisms which provide energy for organisms living within them. But in subglacial environments, organic matter is quickly depleted.
“A wide diversity of microbes inhabit vast ‘wetland’ areas beneath ice sheets and many glaciers but life certainly isn’t easy for them,” Jon Telling, lead author of the paper, said in a press release. “They have to contend with cold temperatures, high pressures from overlying ice, dwindling food supplies as washed-in soils and vegetation are consumed, and constant crushing as rocks embedded in glacier beds are ground against bedrock or sediment.”
Telling and his team reproduced the conditions at the bottom of glaciers in their laboratory to better understand how life survived in these subglacial conditions. They found that some combinations of minerals and physical conditions led to hydrogen being released from rocks. Microorganisms grab hydrogen molecules, split the bond between the two hydrogen atoms in each molecule and use the energy in the bond for the biological activity, allowing them them to live and to reproduce.
Tests were conducted with six different types of silicate rocks from glacier sites in Canada, Norway, Greenland and Antarctica and scientists regulated experimental variables like the grain size, water content and temperature. The rocks were crushed much as they would be under a glacier, and each type was found to produce hydrogen under the proper conditions. However, calcite, the rock the researchers tested as a control, did not produce hydrogen. This result confirmed the importance of silicate rock in the survival of microorganisms.
Though the environment in the laboratory is constructed to simulate the natural conditions, there are still many differences between experiment and natural conditions. For instance, the experiment condition is rather stable while the natural conditions vary significantly. The authors pointed out that these differences and other factors are likely to lead to an underestimation of the amount of hydrogen that would be produced in natural environments. In particular, they suggest that the short duration of the experiment and the possibility of escaping gasses during the experiments would add to such an underestimation.
The study indicates that hydrogen generated through mashing rocks that can provide a mechanism in support of continued microbial metabolism. The authors note that other researchers have proposed methane as the energy source used by prehistoric microorganisms, but they show that hydrogen could have been far more abundant, and would have been available for longer periods. This hydrogen could have supported food webs in subglacial refugia, in which organic matter produced by bacteria would have provided energy for eukaryotes–organisms whose cells have the nuclei which bacterial cells lack.
Though the ancient eukaryotes were tiny, simple organisms, they are of great importance because they are the ancestors of modern multicellular organisms, from sponges and jellyfish to worms and insects to fish, reptiles birds and mammals. It is remarkable to imagine that they survived planetary glaciations that lasted millions of years by consuming organic matter produced by bacteria—and that these bacteria survived by drawing on the energy in hydrogen molecules released from rocks crushed by ice sheets.