4:00 PM - 5:00 PM
About 5 decades ago we started to realize that life is not restricted to Earth’s surface but extends to the deep interior, its sediments and rocks. This so-called deep biosphere is a world of microorganisms capable of a variety of metabolic reactions to gain energy. With improving sensibility of analytical methods and experimental techniques and with every new location studied, we realize the enormous ability of life to adapt to extreme environmental conditions, such as high or low temperature, or the sparsity of food. With increasing depth of marine sediments, substrates are more and more depleted and the energy yield of metabolic reactions ceases. However, until now we were not able to identify the vertical extend of the sub-seafloor biosphere. In 2012, Integrated Ocean Drilling Program (IODP) Expedition 337, embarked on the world’s largest scientific drilling vessels, the Japanese CHIKYU and recovered the deepest submarine core samples in the history of IODP by drilling down to 2.5 km below the seabed. The sequence of drill cores comprised a series of Miocene to Oligocene coal beds which are thought to support microbial life by providing an energy source. Microbial cells were found down to the deepest recovered sediments and DNA analysis revealed that ancient forest soil microbes were buried long ago could still be found in the deeply buried coal beds. By applying a very sensitive method using a radioactive tracer, we were able to measure active microbial sulfate reduction down to the deepest core samples. Sulfate reduction is one of the oldest known microbial metabolisms on Earth and occurs commonly in surface-near marine sediments, reducing and depleting seawater sulfate that has diffused into the sediment within centimeters to a few meters of sediment depth. The surprising finding of microorganisms capable of sulfate reduction in < 20-Ma old and 2.5-km deep sediments impressively underlines the ability of microbes for long-term survival. It might be that these microbes had already entered a dormant stage of life and were awaken by our incubation experiments. However, isotopic analysis of sulfur fractions in the sediments suggest that active microbial sulfate reduction must have been going on for long after burial while utilizing trace amounts of remaining sulfate in the sediment.