The ocean depths are home to myriad species of microbes, mostly hard to see but including spaghetti-like bacteria that form whitish mats the size of Greece on the floor of the Pacific, scientists said.
The survey, part of a 10-year Census of Marine Life, turned up hosts of unknown microbes, tiny zooplankton, crustaceans, worms, burrowers and larvae, some of them looking like extras in a science fiction movie and underpinning all life in the seas.
"In no other realm of ocean life has the magnitude of Census discovery been as extensive as in the world of microbes," said Mitch Sogin of the Marine Biological Laboratory in Woods Hole, Massachusetts, head of the marine microbe census.
The census estimated there were a mind-boggling "nonillion" - or 1,000,000,000,000,000,000,000,000,000,000 (30 zeroes) - individual microbial cells in the oceans, weighing as much as 240 billion African elephants, the biggest land animal.
Getting a better idea of microbes, the "hidden majority" making up 50 to 90 per cent of biomass in the seas, will give a benchmark for understanding future shifts in the oceans, perhaps linked to climate change or pollution.
Among the biggest masses of life on the planet are carpets on the seabed formed by giant multi-cellular bacteria that look like thin strands of spaghetti. They feed on hydrogen sulphide in oxygen-starved waters in a band off Peru and Chile.
"Fishermen sometimes can't lift nets from the bottom because they have more bacteria than shrimp," Victor Gallardo, vice chair of the Census Scientific Steering Committee, said. "We've measured them up to a kilo per square metre."
The census said they carpeted an area the size of Greece - about 130,000 square kilometres or the size of the US state of Alabama. Toxic to humans, the bacteria are food for shrimp or worms and so underpin rich Pacific fish stocks.
The bacteria had also been found in oxygen-poor waters off Panama, Ecuador, Namibia and Mexico as well as in "dead zones" under some salmon farms.
They were similar to ecosystems on earth that thrived from 2.5 billion to 650 million years ago.