Pitch Lake in Trinidad teems with microbial life in spite of the highly toxic environment. (Dirk Schulze-Makuch,Washington State University/ABC)

Researchers have found life in a toxic lake of liquid asphalt, a discovery that challenges notions of what conditions are needed for life.

The scarcity of water and oxygen in the lake make it the closest thing on Earth to the hydrocarbon seas of Titan, one of Saturn's moons.

The environment could also resemble conditions that might have occurred on a primordial Earth, when life first began.

Biologists found evidence of microbial life in Pitch Lake, a naturally occurring pond of warm pitch, reeking of methane and hydrogen sulphide, on the Caribbean island of Trinidad, near La Brea. The liquid asphalt also includes traces of toxic heavy metals.

The lake has been mined for asphalt for centuries, but now, scientists say there are bacteria living in the bubbling, sticky tar.

Dirk Schulze-Makuch of Washington State University and colleagues took samples of pitch from the lake, froze them and pulverized them. They then analyzed the powder for DNA.

They found evidence of tens of millions of living cells in every gram of pitch from the lake. The scientists published their research in the physics blog arXiv.org.

Biologists were able to determine what kind of life they'd found by analyzing the DNA chemically.

"We're able to generate sequences that represent a molecular marker, known as the small sub-unit ribosomal RNA gene, that all living things share," said Steven Hallam of the University of British Columbia, in an interview with CBC Radio's Quirks & Quarks.

The researchers found signs of bacteria and archaea, two types of single-celled microorganisms.

Hallam said the living organisms in Pitch Lake aren't like others found on Earth.

"They're using the residual energy of things like hydrogen sulphide … or methane as a source of electrons," he said. "They're respiring things like nitrate or sulphate. They're making a living in ways that are very unfamiliar to us."

One environment that's similar to the liquid asphalt pond is the recently discovered hydrocarbon seas on Titan, one of Saturn's moons, so the discovery is opening debate about what conditions are required for life on other worlds.

The pitch lake could also tell us something about how life began on this planet.

"One of the reasons to study environments like pitch lake is to look backward in time," said Hallam. "It's not just to think about life on other planets, but it's to think about what life might have been like on our planet at the early stages, when life was coming into being.

"One could imagine that the conditions in this environment might resemble some primordial state on the planet."

With files from the Australian Broadcasting Corporation