By University of California, Los Angeles
Since the discovery of the deep subseafloor biosphere in the mid-1990s, scientists have studied the conditions under which organisms thrive in this isolated and generally food-deprived environment and wondered which conditions set a limit to the existence of life.
In 2016, a group of international scientists set out to sea on board Japanese scientific drillship Chikyu to study the temperature limit of the deep subseafloor biosphere. Sediment samples were collected from a drill hole that cut through the geological subduction zone of the Nankai Trough off Japan.
At this site, temperature increases steeply with depth to reach 120C, a temperature suggested close to the limit for life, at 1,200 metres beneath the seafloor. To their surprise, the scientists found a very small, but very active microbial community thriving under these deep and hot conditions.
The scientists determined the number of cells in the sediment and measured their metabolic rates by highly sensitive radiotracer measurements of methane production and sulfate reduction. They discovered that the metabolic rates per cell were extraordinarily high for the deep biosphere. Published today in the journal Nature Communications, new findings about the samples collected in 2016 are shedding light on the survival strategies of organisms living in this harsh environment.
“We propose that the organisms are forced to maintain a high metabolic turnover, which approaches the activity of microbes living in surface sediments and in laboratory cultures, to provide the energy required to repair thermal cell damage,” said study lead author Felix Beulig from the University of Bayreuth.
Added study leader Tina Treude, UCLA professor of marine geomicrobiology: “The energy required to repair thermal damage to cellular components increases steeply with temperature, and most of this energy is likely necessary to counteract the continuous alteration of amino acids and loss of protein function.”
It is far from trivial to detect microbial metabolic activity in sediments with less than 500 cells per cubic centimetre of sediment, which is seven orders of magnitude lower than in the average surface sediment. “We worked under extremely controlled, sterile conditions and performed a large number of control experiments simultaneously with the sample incubations,” said Florian Schubert from the German Research Centre for Geosciences, who conducted these analyses as part of his PhD studies.