Under the carbon cycle, microscopic algae at the sea surface suck up carbon dioxide from the atmosphere.

Many of these microscopic creatures, called prokaryotes, become infected by naturally-occurring marine viruses.

When they die, their carbon-rich remains gently sink to lower depths, where they are then cannibalistically gobbled up by other bacteria.

These prokaryotes in turn become a meal for a larger life form and so on, up the food chain.

Researchers long ago grasped that viruses on the sea surface play a Dr.-Jekyll-and-Mr.-Hyde role, killing biomass while at the same time sustaining it.

Now, though, evidence has emerged that these tiny bacterial pathogens also carry out unsung work at the ocean depths -- a dark, inhospitable, nutrient-poor place that counts as last great unexplored ecosystem on the planet.

Marine scientists led by Roberto Danovaro of Italy's Polytechnic University of Marche in Ancona sifted through samples of sediment hauled up from scores of sites from around the world, at depths ranging from 183 metres (595 feet) to a bone-crushing 4,603 metres (14,959 feet).

Team member Antonio Dell'Anno said the virus count was astonishingly high.

"We found surprising results," he told AFP in an interview.

"In the deep ocean, there is a strong interaction between viruses and prokaryotes, which helps sustain the deep-sea ecosystems independently of the nutrient inputs coming from the surface waters.

"It's a sort of self-sustaining mechanism, helping the ocean depths to overcome severe nutrient limitations."

The virus work has "huge implications" for understanding the ocean carbon cycle, he said.

It not only helps to sustain life at great depths, he said.

Beyond 1,000 metres (3,250 feet) or so, prokaryotes account for 90 percent of the total biomass.

Humans are among the indirect beneficiaries of the process, because the abyssal nutrients help sustain the seafood that ends up on our plates.

Another big question is what role viruses may play in the complex arithmetic of global warming.

The sea absorbs billions of tonnes of atmospheric CO2 each year, thus acting as a cushion for man-made emissions of heat-trapping greenhouse gases. The carbon detritus from prokaryotes killed by viruses on the ocean floor is mostly respired thus not effectively stored there forever.

Viruses are by far the most abundant "life form" in the oceans, according to the study, which appears in Thursday's issue of Nature, the London-based science weekly.

They number roughly 4 X 10 to the power of 30 -- a four following by 30 zeroes.

Globally, as much as 630 million tonnes of carbon are taken up each year by "viral shunt," when the remains of one microscopic organism, which has been killed by a virus, are later snapped up by another one.

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