FAR FROM the sunlight that sustains life, the deep seabed appears sterile and desolate. However, its appearance belies a thriving bacterial ecosystem that can contain up to 45% of the world’s biomass of microbes. This ecosystem is powered by the so-called sea snow: a constant rain of small nutrient-rich particles that fall like manna from the ocean layers near the surface, where photosynthesis takes place.
However, not all snow is digestible. And the indigestible parts accumulate, layer by layer, burying while the insects do in the underlying layer. To see how these bacteria survive the mound, a team of researchers led by Morono Yuki of the Japanese Agency for Marine-Terrestrial Science and Technology and Steven D̵
For many microbes, burial is an immediate death sentence. Some, however, are unable to enter a dormant state, slowing their metabolism almost, but not entirely, to zero. They can remain in this state for considerable periods. But just how long has it been the subject of debate.
The samples that Dr. Morono and Dr. D’Hondt chose for the exam came from a place in the Pacific Ocean where the seabed is located approximately 6,000 meters below the surface. This made drilling a challenge. But the expedition was able to recover sediment cores that extended to the underlying rock, in some cases 100 meters thick. The oldest material of these nuclei dates back to 101.5 million years ago, in the middle of the Cretaceous period, the apogee, on the ground, of the dinosaurs.
Examination of the sediments showed that even the oldest still contained some bacteria. The question was: were these organisms alive or dead? To find out, the researchers incubated the samples, slowly feeding the carbon and nitrogen-rich compounds in order to convince any microbes still alive to remain inactive.
The results shocked Dr. Morono, “At first I was skeptical, but we found that up to 99.1% of the microbes in the sediments deposited 101.5 million years ago were still alive.” And there were also several. The team found representatives of phyla called Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria, all familiar to microbiologists. In a sample (admitted just 13.5 million years ago) they also discovered representatives of the archaeologists, a group of organisms that resemble bacteria under the microscope, but have a biochemistry so different that they are considered a separate domain of life.
Finding such living fossils since the Cretaceous is extraordinary. It is not possible to be certain, given the period of time spent, that they have not undergone any cell growth and division. But if they did, it would have been minimal given the lack of nutrients in the slime in which they were found. Nor are they likely to have migrated there from the upper strata. The slime in question was sealed with a bacteria-proof layer of kerine-like material called porcelain.
This discovery will therefore cast an interesting light on the evolution of bacteria on Earth. It will also lift the spirits of those who hope to find life elsewhere in the solar system. The sediments that will be examined for signs of biology Perseverance, an American rover on Mars that took off from Cape Canaveral, Florida on July 30, is 35 times older than those studied by Dr. Morono and Dr. D’Hondt, and has had no protective protection from environmental degradation resulting from being buried at the bottom of the sea under many meters of overload. But future rovers on Mars will pierce beneath the surface of the planet in layers that have had equivalent levels of protection from the rock above them, if not seawater. And a factor of 35, although large, does not seem insurmountable.■
This article appeared in the Science and Technology section of the paper edition under the title “The insects that time has forgotten”