A vast carpet of underwater microbes that covers an area as big as Greece has been discovered on the seabed off the west coast of South America. Scientists believe the microbes could be directly descended from some of the earliest life forms to have evolved on Earth.

The discovery is part of a series of astonishing finds made since 2000 as part of the decade-long Census of Marine Life, an international project by more than 2,000 scientists from 80 countries to explore the largely unknown life which inhabits the oceans.

The "microbial mat" lives in a deep layer of seawater that is deprived of both light and oxygen and seems to have survived by "eating" hydrogen sulphide and "breathing" nitrates. It could represent a present-day community of organisms descended from primitive microbes which first evolved about 3 billion years ago, when there was no oxygen on the planet.

Scientists said they were taken aback by the spectacle when the first images of the microbial mat appeared on the television screens from video cameras on board a submersible robot, which had been lowered into the deep ocean to explore the continental shelf off the coasts of Chile and Peru.

"It was like a big carpet of white grass with filaments sticking out and waving in the water, said Victor Gallardo, a Chilean scientist on the expedition. "It looks like a carpet lying on the seabed, separating the underlying sediment from the overlying water," he added.

Initial tests showed the microbial mat is composed of a community of micro-organisms adapted to growing under extreme hypoxia, when there is little or no oxygen. It is the same kind of conditions that existed before the evolution of the first photosynthetic algae, which were able to convert carbon dioxide into oxygen.

"The microbes in the mat play around with very simple stuff, such as hydrogen sulphide and nitrates. They use nitrates in the water as we would use oxygen and they take the sulphide as their food," Dr Gallardo said.

Scientists estimate that the mat extends over vast areas of the seabed in this region of the ocean, covering a territory roughly the size of Greece. Explorers have found them off the central and northern parts of both Chile and Peru, and they have also been detected in sulphur-rich waters off the Galapagos islands, Ecuador and Panama.

The largest filaments of the mats are about half the width of a human hair and are composed of individual microbial cells organised into long multicellular strands that are white because of a build-up of sulphate salts in the living tissue. The bacteria within the mats are some of the biggest known.

"For most of the time in the history of the world, most of the ocean was anoxic so these species probably dominated the planet for hundreds of millions of years," said Professor Ron O'Dor, chief scientist on the Census of Marine Life and a marine biologist at Dalhousie University in Halifax, Canada.

"The DNA in these micro-organisms has probably been alive for longer than anything else on the planet," he added.

The Census of Marine Life, which publishes its final report in October, has discovered that there are many more species of microbe living in the oceans than previously thought – in 2000 there was thought to be about 20,000 species of marine microbe but now the number is nearer to 20 million, Professor O'Dor said.

Marine biologists working on the Census have estimated that the total weight of marine microbial life – the biomass – is equivalent to 240 billion African elephants. This is about 35 "elephants" of marine microbes per person living in the world.

"In no other realm of ocean life has the magnitude of Census discovery been as extensive as in the world of microbes. Scientists are discovering and describing an astonishing new world of marine microbial diversity and abundance, distribution patterns and seasonal changes," said Mitch Sogin, the leader of the International Census of Marine Microbes.

"The extreme diversity and curious distribution of deep-sea microbes are among the great mysteries of nature, and begs the question about the evolution of life on Earth," he added.

In the 1950s marine biologists estimated that there were about 100,000 microbial cells inhabiting a single litre of seawater. More recent estimates suggest the true number is nearer to 1 billion microbes per litre.

Samples taken from the deep seabed have revealed an astonishing variety of life. "Far from being a lifeless desert, the deep sea rivals such highly diverse ecosystems as tropical rainforests and coral reefs," said Pedro Marinez Arbizu, one of the scientists working on the Census.