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In 2010 scientists from the Integrated Ocean Drilling Program (IODP) sailed into the South Pacific Gyre, a marine desert more barren than all but the most arid places on Earth. Near the center of the gyre is the Oceanic Pole of Inaccessibility—best known by fans of H. P. Lovecraft as the home of the be-tentacled Cthulhu—as well as the South Pacific garbage patch, where microplastic particles accumulate. At times the closest people are astronauts passing above on the International Space Station.
Although ocean currents swirl around it, within the gyre the water stills. Few nutrients enter, and life struggles. Here it takes at least one million years for a meter of marine “snow”—the corpses, poo and dust that transfer energy from the light-rich upper layers to the depths—to pile up on the bottom. It is the least productive patch of water on the planet.
Through this seawater, the IODP team lowered kilometers of pipe from a derrick more than 60 meters high. Twelve thrusters held the entire rig just so in the heaving seas. Once the pipe hit bottom, a drill plunged down to 75 meters into pelagic clay and calcareous nannofossil ooze at multiple different sites.
By the time the cores of sediment were raised to the surface, the tubes contained up to 100 million years of Earth history. What the team wanted to know was how long and in what state microbes trapped in this milieu could survive in an almost completely empty oceanic refrigerator. As expected, the sediment samples did not contain many bacterial cells to begin with: just 100 to 3,000 per cubic centimeter, which is 10 to 10 million times less than at equivalent depths in more productive waters.
But when the scientists fed those cells in the laboratory, something unexpected occurred. One hundred million years of starvation might have made the cells microbial “zombies”—alive but incapable of growth or able to grow but not at a rate humans could measure. In the early 2000s, a few bacteria isolated from marine sediments up to 35 million years old were coaxed to grow in culture, but the experiment was not designed to gauge their growth. A 2017 study of oxygen-free coal-bed microbes (which were deposited on land 12 million to 16 million years ago but later submerged by the ocean) found that the resurrected subjects did grow, only extremely slowly. Their doubling times were on the order of months to a century, some of the slowest ever directly measured.
But in this study, which was published in July 2020 in Nature Communications, up to 99 percent of the microbes that were fed quickly “woke up,” ate and got to work doing what bacteria do. Within 68 days of incubation, they had increased their numbers up to 10,000-fold, doubling about every five days. Their progeny contained specially labeled isotopes of carbon and nitrogen that could have been obtained only by eating the food the scientists offered.
It is worth pausing to consider the meaning of these results. Seventy percent of Earth’s surface is covered by marine sediment, whose microbial residents represent up to half of all microbial biomass on the planet. In this experiment, cells that may have settled to the bottom of the ocean when plesiosaurs swam overhead awoke and multiplied. Four geologic periods had ground by, but these microbes, protected from cosmic rays by a thick coat of ocean and sediment, persisted. When dredged up and offered a bite, they carried on as if nothing unusual had happened.
In a sense, it hadn’t. If it feels like forever since the pandemic began, imagine being starved in the dark for 100 million years. The dense sediment, which approaches something like flourless chocolate cake, has an estimated pore size of 0.02 micron. Given that a typical bacterium is a few microns across, you can see the challenges inherent to migrating in search of food or even hoping some blunders into you.
Some bacteria, many of which are anaerobic, make structures called endospores that are fortified and metabolically inactive, seemingly to allow bacteria to endure harsh conditions. Spores have often been suggested as a vehicle for superannuated bacteria. Yet when the scientists identified the gyre-sediment cells by probing their DNA, they saw that spore-forming types were relatively absent. The majority of the revived bacteria, it turned out, breathed oxygen.
An even more surprising find was a thriving population of light-harvesting bacteria discovered in one sample 557 days into its incubation. Called Chroococcidiopsis, this cyanobacterium has a reputation for survival so formidable it is being considered for terraforming Mars. In addition to being able to live under translucent rocks in dry, cold, salty, and radiation-drenched places, it has the unusual ability to capitalize on red light. How this photosynthetic microbe managed to reproduce in a dark lab chamber remains a mystery.
To be sure, although the sediment in which the cells were trapped was up to 100 million years old, the age of individual cells remains uncertain. Some are possibly descendants of the original community and therefore much younger. But reproduction is costly, and given the conditions, it seems likely to be rare. Putting it all together—the tight quarters, the lack of spores, and the rapid reanimation—the authors of the Nature Communications paper think that the microbes in this impoverished sediment have been alive but idling the entire time.
Scientists have flirted with the idea that individual bacteria may survive without reproducing for many millions of years. (That will remain unknown until we develop techniques that can date microbes, not just sediment.) A few years ago, when I wrote about bacteria that were resurrected from Paleozoic coal for my Scientific American blog, I speculated that under certain highly constrained but possibly abundant conditions, bacteria may be effectively immortal. Time does appear to take a toll: the oldest cells from the Cretaceous seafloor sediment multiplied about half as fast as their more spry brethren that had been down there “only” a few million years. Still, mounting evidence suggests that we may be sitting atop a planet that is full of living fossils that are literally that—both fossils and alive.
The people who love dinosaurs (and to be fair, who among us aren’t dinosaur people?) have their museums filled with bones and teeth and tracks. The plant people have their petrified forests and fossil fronds. But the microbe people have something even better: our dinosaurs aren’t dead.
