PARIS - Far beyond the caress of sunlight, micro-organisms are flourishing at great depths beneath the ocean floor, scientists reported on Wednesday.
US biologists looked for telltale scraps of genetic code in a core drilled deep into the sedimentary floor of the Pacific Ocean off Peru.
They were hunting for traces of messenger RNA (mRNA), which hints at the presence and even the identity of living cells.
The results revealed a vast ecosystem living at all the sub-sea depths that were tested, from five to 159 metres, says their study.
The microscopic critters included bacteria, primitive single-cell organisms called archaea, and fungi.
The mRNA signatures point to cell proliferation and even movement. Some of the proteins transcribed by the mRNA are for flagella, which are whip-like tails that help cells "swim" through fluid.
"The take-home story there is, if there's room to move, they move," said William Orsi of Woods Hole Oceanographic Institution (WHOI), which shared the research with the University of Delaware.
The paper is published in the journal Nature.
The invisible population is so huge that climate scientists may have to take it into account when they calculate Earth's "carbon budget" -- the amount of greenhouse gas that is emitted or absorbed by the biosphere and set against emissions made by humans.
"Cells are very abundant there, but they do not have high activity levels," said WHOI microbial ecologist Virginia Edgcomb.
"But it's a huge biosphere, and when you do the math, you see we're talking about a potentially significant contribution.
"Carbon is being turned over, and that has important implications for models of carbon and nitrogen cycling."