When does life get too hot? Researchers report new insights into the limits of earthly existence: They have sounded out the temperature limits of microbial life in the increasingly hot depths under the sea floor. Analyzes of drill cores from the extreme habitat suggest that some hyperthermophilic bacteria can still exist there at temperatures of up to 120 degrees Celsius.
When we hear the term biosphere, we think of the colorful diversity of the creatures that live on the earth’s surface and in the oceans. However, in the last few decades, studies have shown that there is a huge part of the biosphere that remains largely hidden from us: There are enormous amounts of microbes down to depths of several kilometers beneath the earth’s surface and especially in the sedimentary layers under the oceanic floors. However, this habitat is still mysterious. An open question is where its limits are and what determines them. In addition to the availability of nutrients and pressure, a limiting factor could be temperature. Because the increasing proximity to the middle of the earth makes the hot interior of our planet noticeable.
So-called hyperthermophilic bacteria and archaea are already known from some of the world’s hot habitats, and they can even cope with temperatures of around 120 degrees Celsius. But when does it get too hot for the microbial life forms in the subsurface of the ocean floor? An international research team with the participation of the MARUM – Center for Marine Environmental Sciences at the University of Bremen has addressed this question. The scientists examined samples from drill cores. “Only a few scientific boreholes have so far reached depths at which the sediments are hotter than 30 degrees Celsius,” explains study leader Kai-Uwe Hinrichs from MARUM. “That is why the aim of the T-Limit expedition was to explore new scientific territory with a kilometer-deep borehole in sediments up to 120 degrees Celsius – and we succeeded,” says the scientist.
Drilling deep under the seabed
As he and his colleagues explain, temperatures of 120 degrees Celsius can usually only be found at great depths – around 4,000 meters below the sea floor. In order not to have to drill so deeply, the researchers therefore selected a sampling location where these temperatures are already present at a shallower depth: the deep-sea drilling vessel Chikyu extracted drill cores in the Nankai Trench off Japan. There the experts had to overcome a water depth of 4.8 kilometers, but 120 degrees Celsius could be reached with a bore of just 1180 meters underground, the scientists report. This enabled them to obtain material from different temperature ranges.
In order to be able to prove the microbial life in the samples, some of them were brought by helicopter to clean room laboratories in Japan. There they were examined with the most modern techniques of molecular biology in order to make the life forms visible or to prove their metabolic processes. “Without some of the research on land, in a high-quality and controllable research environment, the goal of the expedition would not have been achievable,” says co-author Yuki Morono of the Kochi Institute for Core Research of the Japanese Agency for Marine Geosearch and Technology (JAMSTEC) .
Some like it hot
As the team reports, a surprising drop and subsequent increase emerged in the analysis results of the samples from the various temperature ranges: “Amazingly, the microbial population density already collapses at a temperature of around 45 degrees,” says co-author Fumio Inagaki from JAMSTEC. “And then we were able to detect cells and microbial activity again in deeper, even warmer zones – up to a temperature of 120 degrees Celsius”. As the scientists also report, methane is produced in the populated zones up to a temperature range of 80 degrees through biological metabolic processes. In the even hotter areas of up to 120 degrees Celsius, the researchers were then able to demonstrate that the increased cell concentrations are due to acetate-degrading, hyperthermophilic microbes.
Another result was: In areas with over 45 degrees Celsius, populated zones alternated with those in which the researchers could not detect any microbes. “This result is fascinating – accordingly there are extensive, seemingly almost lifeless depth intervals in the hot ocean floor,” says Inagaki. The investigations also showed: While the concentration of vegetative cells decreases sharply at temperatures above 45 degrees and only reaches a level of less than 100 cells per cubic centimeter of sediment, the concentration of “dormant” cells increases rapidly and reaches a maximum at 85 degrees Celsius. These so-called endospores are cells of certain types of bacteria that are reactivated and can switch to living mode as soon as circumstances permit. “Some specialists can adapt to difficult conditions and are able to remain in a kind of deep sleep over geological time periods,” says Inagaki.
In conclusion, co-author Verena Heuer from MARUM says about the results: “They show that deadly limits and chances of survival lie close together at the lower edge of the biosphere. We didn’t expect that, ”emphasizes the scientist. However, the depths under the sea floor remain mysterious, because the insights are still samples from special areas. But the field of research will continue to develop: “Every borehole opens a window to new knowledge,” says Heuer. “With each expedition, technical and analytical methods are further developed, each time people with diverse experiences and new ideas come together to answer a scientific question together. And that’s what makes it so fascinating, ”says the scientist.
Source: MARUM, specialist article: Science, doi: 10.1126 / science.abd7934