Look and learn, sloths: the microbes deep beneath the Pacific ocean take inactivity to new heights. They are so slow on the uptake of nutrients from their environment that they barely classify as alive. Their very existence could help define the limit between life and death.
Paradoxically, though, they may also be among the oldest living organisms on Earth.
Everything happens slowly in the North Pacific gyre, one of the five largest ocean gyres in the world. Sand and mud washing off the continents rarely finds its way there, so the seafloor accumulates sediment at a sluggish rate. The clay just 30 metres below the seafloor was deposited 86 million years ago, almost 20 million years before Tyrannosaurus rex graced the Earth.
That clay contains so little energy in the form of nutrients that it should be incapable of supporting a living community. Microbes have been found in other, only slightly more energy-rich communities below the seafloor, though.
In a bid to hone in on the lower energy limits for life, Hans R?y at Aarhus University in Denmark probed the clays below the North Pacific gyre. Under the microscope, he found a community made up of bacteria and single-celled organisms called archaea in vanishingly small numbers.
"There are only 1000 tiny cells in 1 cubic centimetre of sediment, so finding just one is literally like hunting for a needle in a haystack."
Lower limit for life
The microbes rely on oxygen, carbon and other nutrients in their deep environment to live, but R?y's team found that carbon is so limited that the cells respire oxygen 10,000 times slower than bacteria in lab-grown cultures.
R?y thinks the microbial community is so sparse, and the metabolic rates so low, that the nutrient levels probably represent the bare minimum required to keep cellular enzymes and DNA working. "It looks like we have reached the absolute lower limit for the metabolism of cells," he says.
Yuki Morono at the Japan Agency for Marine-Earth Science and Technology in Nankoku, Japan, recently studied similar low-energy microbial communities below the Pacific seafloor near Japan. Under a microscope, he says, the microbes show few signs of life. "They appear to be dead by our time scale."
But when Morono's team treated the cells to what he calls a "luxury meal" of glucose and other nutrients, most of them absorbed some food ? suggesting that they are, in fact, alive. "Their lives are just very slow compared with ours," he says.
Extreme life spans
Because of their remarkably slow metabolic rates, individual cells may have extremely long life spans, says R?y. The cells Morono's team examined looked intact, yet it would take each of them hundreds or thousands of years to generate enough energy to go through cell division and produce daughter cells. That means some of Morono's cells could be thousands of years old.
R?y says his cells could be older still. Elsewhere on Earth, life is primarily concerned with building up enough energy to fuel reproduction. In extremely energy-poor communities, though, reproduction makes less sense because it creates new rivals that also need to feed. "If you can just barely meet your energy requirement, then it is suicide to divide into two," he says. He thinks it makes more sense for the cells to use the energy they gather to repair cellular molecules that have been damaged over centuries of use instead of fuelling cell division.
With communities like this, getting enough cells to learn more about them is a challenge. "You can make pure-culture studies with organisms with a doubling time up to a couple of weeks, maybe a month. Beyond that, you simply do not get dense enough cultures [to study] within a reasonable time," says R?y. "It is safe to say that we do not know anything about the adaptations to low energy life that these organisms might have ? only the thermodynamic limits which constrains them."
Journal references: R?y et al: Science, DOI: 10.1126/science.1219424; Morono et al: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1107763108
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