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Like many other visitors to the La Brea tar pits, sisters Samantha and Katie Salazar watched a basketball-sized bubble emerge from dark, slimy gunk Sunday and wondered, why are the tar pits bubbly?
For years, educators at the Hancock Park site could only guess that methane gas was being released as the byproduct of oil creation 1,000 feet below the surface.
Researchers at UC Riverside have finally found the answer: Hardy bacteria embedded in the natural asphalt are eating away at the petroleum and burping up methane.
Of the bacteria the researchers isolated in tar pit samples, about 200 to 300 are previously unknown species.
"I was totally surprised, but totally delighted," said John Harris, a paleontologist who is chief curator at the Page Museum, where fossils from the tar pits are collected.
"The tar pits are worldfamous already for fossils, but this is another claim to fame," he said.
Since the early 20th century, scientists and volunteers scouring the tar pits have found bones, shells, trunks and leaves from some 600 kinds of animal and plants from the last great ice age; the fossils range in age from 11,000 to 40,000 years old.
The pits have yielded mammoths, saber-tooth cats, a condor-like bird known as Merriam's Teratorn and coastal redwood trees.
The site became the richest late-Pleistocene site in North America because of the forms of petroleum welling up from below the surface.
"Brea" means tar in Spanish, but the slick black liquid in the pits is more properly called heavy oil.
Lighter compounds evaporate off the heavy oil -- accounting for the gas station-type odor at the tar pits -- and natural asphalt is the heavier stuff left behind.
Natural asphalt is extremely sticky and acts as a superb preservative, Harris said, which is why the tar preserved so many fossils.
An inch of the natural asphalt was enough to trap animals the size of a horse, he said. The immobilized animals attracted other predators, which then got stuck as well.
Mostly, the excavation had focused on dead things, Harris said.
"We weren't looking for stuff living in it," he said.
Although scientists had previously found one living thing in the asphalt -- an oil fly that lays its larvae there -- no one had managed to extract bacteria.
This was difficult in the past because the asphalt "is normally so gooey, it's impossible to get anything out of it," said David Crowley, one of the UC Riverside scientists who found the new bacteria.
Crowley and Jong-Shik Kim, a postdoctoral research associate, realized that they could pour cold liquid nitrogen on the asphalt, crush it into a fine powder and extract bacterial DNA from the powdered form.
"We found some really great bacteria," Crowley said. "The types we found are all very specialized for life in extreme environments."
Living in the asphalt means living with no oxygen, almost no water and lots of toxic chemicals, he said.
Some of the newly found bacteria, from the genus Bacillus, are related to the bacteria that survive the cold conditions 50 miles above the surface of the Earth, where ultraviolet rays sterilize practically everything else.
Others, from the genus Rubrobacter, are related to bacteria that can withstand more than 10,000 times the radiation that would kill a person.
The bacteria found in the pits work as part of a community. They eat up the petroleum and make organic acids, such as acetic acid, the compound that creates the sour flavor in vinegar.
Other bacteria, which Crowley and Kim are still working to characterize, consume these acid byproducts and produce the methane that bubbles to the surface.
Because several types of bacteria were found, the study reinforces the idea among scientists that bacteria don't live in isolated colonies, but often live in groups with other species, said James Lake, a UCLA microbiologist who was not involved in the study.
It also raises questions about the age of the bacterial strain found in the tar pits, he said.
It "makes you wonder whether this is an old-growth forest or if it's a new one," he said.
Crowley and Kim are still working on discerning the ancestry of the bacteria they found in their chunk of asphalt 20 feet below the surface.
The ancestors of these bacteria probably lived in animals and plants that decomposed into oil millions of years ago. As the oil rose up through the soil, so did the bacteria.
The bacteria that Crowley and Kim found could be direct descendants of ancient bacteria. But they also might have more recently mated with younger bacteria in the loamy sand near the surface.
What makes these petroleum-eating bacteria interesting is their potential environmental application, Crowley said.
Their ability to break up complex hydrocarbons could help clean up oil spills or clear the holds of oil tankers.
Some of the species they discovered in the genus Pseudomonas, for example, could help degrade trichloroethylene, a solvent in dry-cleaning and metal degreasing that is a major groundwater contaminant, Crowley said.
"These are definitely keepers," he said.
The Salazar sisters, who were standing with their mother by a statue of a mammoth getting stuck in the tar pit, marveled that the muck was actually teeming with life. They didn't know that live bacteria were creating the bubbles.
"It's kind of creepy," said Samantha, 15.
Katie, 9, said the discovery was "amazing," and planned to include the new information in a school report she was writing on the tar pits.
"If we could put [the bacteria] to work somehow, that would be amazing," said mother Lisa Salazar.