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Back when dinosaurs ruled the Earth, some hardy bacteria took up residence in the bottom of the Pacific Ocean. Eighty-six million years later, they are still there. And the new study says they are living the most Spartan lifestyle known on this planet.

NPR's Richard Harris has that story.

RICHARD HARRIS, BYLINE: Out in the middle of the ocean there's a place called the Pacific Gyre where almost nothing reaches the seafloor. Nutrients from the world's rivers don't get out that far. Most plankton that die in the water dissolve long before they can reach the seafloor far below.

Hans Roy says it's a rare day indeed when even a single particle lands in any given spot on the bottom.

HANS ROY: If you imagine that a grain of sediment falls on the surface, it will take a thousand years before the next grain will sit on top of it.

HARRIS: As a result, it has taken millions of years for a thin layer of sediment to form.

Roy, from Aarhus University in Denmark, was part of an expedition in 2009 to sample that ancient sediment. And amazingly enough, he found living bacteria buried in that clay. Amazing because there are almost no nutrients down there for them to feed on.

ROY: They left the surface 86 million years ago with one lunch box, and they're still eating out of it. It's like they're splitting a pie and they just keep splitting in half, and in half and in half, but nobody ever eats the last crumble. It's quite remarkable.

HARRIS: Roy says in the latest Science magazine that those bacteria may have the world's slowest metabolism, with barely enough oxygen and nutrients to keep them alive. He talks about his own food intake by way of comparison.

ROY: I weigh 140 pounds and I eat a few pounds of food every day. So it will take me a month or two to eat my own weight. These organisms will take a thousand years to eat their own weight.

HARRIS: Roy can't say exactly how old the individual bacteria he studies are. They may have been reproducing extremely slowly since the time of the dinosaurs. Or the individuals could be millions of years old, rebuilding themselves just fast enough to repair the inevitable damage of aging.

In any case, these microscopic life forms have lifecycles that defy human intuition.

ROY: That's so much slower than our own, that in our eyes it looks like suspended animation. This is pretty much like if you would stand up and look at a tree to see if it grows at all, you won't see anything because you're just looking at the wrong time scale.

HARRIS: But if you could stand there a hundred years, you would see something.

ROY: Definitely, but my contract runs until October.

(SOUNDBITE OF LAUGHTER)

HARRIS: Happily, nature has run that experiment on the seafloor. And one reason scientists are interested in this extreme lifestyle is because it provides clues about the absolute minimum conditions required to sustain life.

Andreas Teske, at the University of North Carolina in Chapel Hill, says that's useful for people looking beyond our planet for signs of life.

ANDREAS TESKE: We would like to know how far down can we go with respect to energy supply for life. So we have to look at the most difficult places on Earth. And the deep subsurface is certainly one of these most difficult, and at the same time - just by volume, by space, by extent - one of the most dominant places on Earth.

HARRIS: So these bacteria are likely quite abundant. And Hans Roy from Denmark says they're very likely to be here long after we're gone.

ROY: These organisms have no clue that we're even around. They could be sitting down there for 100 million years, the whole surface could be one scorched desert, and they still wouldn't know it.

HARRIS: On the other hand, we, presumably, have more fun.

Richard Harris, NPR News.

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