London, August 28 : Marine biologists have determined that the lack of animals on the ocean floor is a result of trillions of deep-sea viruses that control the world's deepest food chain.
Roberto Danovaro of the Polytechnic University of Marche in Italy said that the viruses keep the inhabitants of the sea bed very small - nothing bigger than a microbe.
This also keeps carbon and nutrients trapped at the bottom of the seas.
According to a report in New Scientist, Danovaro's team collected dozens of samples of sediment from sites around the world.
Everywhere they looked, the top centimeter of sediment contained large quantities of viruses. The average gram of sediment contained 1 billion viruses, which is the equivalent of 8 trillion viruses per square meter of ocean floor.
This was true whether the sediment had been sampled from relatively shallow continental shelf, just a few hundred meters beneath the surface, or from the deep abyss up to 6 kilometers down.
What's more, the sediment viruses were busily infecting tiny microbes. These simple life forms, known as prokaryotes, are one of the lowest rungs in the food chain.
Usually, the nutrients and carbon contained in prokaryotes are used by the larger organisms that eat them.
But something very different happens when prokaryotes are infected by viruses: the viruses burst the prokaryotes open and release their carbon and nutrients into the water column.
This is known as the "viral shunt".
Nutrients are shunted back down the food chain to be taken up by the remaining, as yet uninfected microbes - allowing them to produce more microbes and host more viruses.
Danovaro found that the deeper he sampled sediment, the more active the viruses were.
According to the researchers, their results shed light on an unsolved paradox of the deep ocean.
In 1998, a study showed that between 30 per cent and 45 per cent of the carbon contained in microbes on Earth is found in the top 10 centimeters of deep-sea sediments.
Yet other studies have shown that this resource is hardly used by the animals that inhabit the abyss.
"The huge nitrogen and phosphorus-rich prokaryotic biomass represents a potentially enormous and high quality food source for benthic consumers in deep-sea ecosystems," according to Danovaro and colleagues.
The new study found that for all the samples, the more prokaryotes in the sediment, the more viruses there were.
The team concluded that the viruses and the prokaryotes effectively feed off each other, at the expense of larger organisms that have to make do with other sources of energy.