Where are the limits of life? Up to what temperatures is it possible to live under the seabed – and how can organisms survive in extreme conditions? To answer these questions, scientists took samples from a depth of 1.2 kilometers below the seabed and found that some microbes can survive as high as 120 degrees Celsius. The key to this is the incredibly high metabolism that allows the microorganisms to quickly repair any thermal damage.
Life can thrive even deep beneath the seabed: although pressure and temperature increase with increasing depth, there is a lack of oxygen and a decline in organic material as a food source, some organisms have adapted to these very conditions. But where is the survival limit? And what strategies are microorganisms in the deep biosphere using to deal with hostile conditions?
samples from the deep
In search of answers to these questions, a team led by Felix Beulig of Aarhus University in Denmark examined sediment cores that had been collected from a depth of up to 1.2 kilometers below the seabed in the Nankai Trench off the coast of Japan. At this point, the Philippine plate slides under the Eurasian plate. Due to the tectonics of the plates, pressure and temperature increase particularly rapidly with depth, so that temperatures of up to 120 degrees Celsius are 1.2 kilometers below the seabed. Since most proteins are denatured at temperatures as low as around 50 degrees Celsius, higher temperatures are considered a particular challenge to the life of microbes.
“Despite the high temperatures, we were able to detect microbial cells in almost the entire pellet column, albeit at extremely low concentrations of less than 500 cells per cubic centimeter in sludge layers above 50 degrees Celsius,” the researchers reported. To gain more detailed information on the survival strategies of the microbes found, scientists analyzed their metabolism. “Our results show that microorganisms that break down sulphate and produce methane are active in sludge millions of years old at extreme temperatures,” say the researchers.
Active life in extreme conditions
Using highly sensitive measuring devices, scientists measured the metabolic rate of microorganisms from the hot deep in the laboratory – and found a surprising result: although other microorganisms that occur in the deep seabed are known to have a very slow metabolism, Beulig and his colleagues contributed to the discovery the amazingly high metabolic rate in the microorganisms tested. Despite living in extreme conditions with little biomass available, they were almost as active as the microbes found in habitable surface sediments.
“We suspect that the organisms are forced to maintain a high metabolic rate to provide the energy needed to repair thermal cell damage,” says Beulig. Co-author Tina Treude of the University of California Los Angeles explains: “The energy required to repair thermal damage to cellular components increases dramatically with temperature, and most of that energy is likely required to maintain continued containment of amino acid alteration and loss of protein function.”
Acetate as an energy supplier
Acetate, the salt of acetic acid, apparently serves as an energy source for cells. “Acetate, a small organic molecule that is also found in vinegar, is particularly interesting as a potential food source,” says co-author Verena Heuer of the Bremen Marine Science Center. “Acetate reaches concentrations in excess of ten millimoles per liter in the sediment pore water, which is exceptionally high for marine sediment.” It is likely produced at this depth by thermochemical processes in which biomass is converted into energy-rich molecules.
Due to the small number of cells per cubic centimeter of sediment, scientists conducted numerous control experiments to ensure that the measured processes were in fact biological reactions and that they were not caused by contamination of the samples. In addition, they calculated the expected degradation time of sulphate in the sludge and compared it with the experimental results. “Given that we compare two very different methodological approaches, operating on the day and million-year time scales respectively, the agreement between the experimental speed determination and the calculated decay time is encouraging,” says co-author Arthur Spivack of the University of Rhode Island.
Temperature limit still unknown
Until now, scientists have failed to establish which microbes were involved. The samples contained too little material for DNA analysis. They had not yet found the temperature limit they were actually looking for. “We need to go back and dig deeper,” says co-author Yuki Morono of the Japan Marine Sciences and Technology Agency. “The ultimate boundaries of the biosphere inside the Earth remain unknown. As this project has shown, the boundary is somewhere in the oceanic crust beneath sediment. In the future, it will be investigated by scientific marine drilling. ‘
“The fact that life with a high metabolic rate thrives in the sea depths at such high temperatures stimulates our imagination as to how life can evolve or survive in similar environments on planets beyond Earth,” adds Beulig’s colleague Bo Barker Jørgensen.
Source: Felix Beulig (University of Aarhus, Denmark) et al., Nature Communications, doi: 10.1038 / s41467-021-27802-7