Discovery of crude oil 'eater' could revive oil fields

BEIJING, Dec. 30 (Xinhua) -- Previous studies found that petroleum hydrocarbon can be converted into methane by microbes, a process similar to traditional biogas fermentation requiring different types of bacteria and archaea.

An unexpected discovery of a new microbe by Chinese scientists, recently published in the journal Nature, proves that such conversion can be done by a single type of archaea alone. It paves the way for further exploitation of depleted oil fields, the China Science Daily reported.

CRUDE OIL EATERS

Previous studies found that the degradation of crude oil into methane needs the co-existence of two different types of microbes: hydrocarbon-degrading bacteria and methanogenic archaea.

Methane is a major component of natural gas and biogas and is often used as a fuel and starting material in chemical synthesis.

"There is close cooperation between them. If split apart, they can't do the job," said Li Meng, one of the correspondent authors of the paper and a professor with Archaeal Biology Center, Institute for Advanced Study, Shenzhen University.

Archaea have existed on Earth for approximately 3.5 billion years. They exist in extreme environments like hydrothermal vents on the seafloor, hot springs, and saline-alkali lakes. Methanogenic archaea are a family of anaerobic archaea that produce methane without oxygen, playing a significant role in biogas fermentation.

Previous studies found that methanogenic archaea produce methane in four ways -- acetic-acid fermentation, CO2 reduction, methyl cracking, and oxymethyl conversion. The raw materials are simple organics, mainly compounds with one or two carbon atoms.

"Textbooks told us methanogenic archaea can't directly 'eat' complex organics such as alkyl hydrocarbons with dozens of carbon atoms. And there was no study about microbes that directly degrade petroleum hydrocarbon into methane or CO2," said Cheng Lei, another correspondent author and a researcher with the Biogas Institute of China's Ministry of Agriculture and Rural Affairs.

However, hydrocarbon degradation by bacteria and archaea takes a long time and is unstable. It makes it challenging to develop for engineering applications.

The Biogas Institute has carried out fundamental research on anaerobic microbes for 40 years. It has preserved nearly 600 anaerobic microbe model species, while global research institutions have over 2,000.

Anaerobic microbes host abundant biological resources on Earth, but only less than 0.1 percent of them are known, said Zhou Zhuo from the Biogas Institute, the first author of the paper.

Most anaerobic microbes are "dark matters." Scientists know they exist, but don't know what they are, said Zhou.

UNEXPECTED DISCOVERY

The research team from the Biogas Institute has studied anaerobic degradation since 2005, but the work was challenging from the start.

"It often takes one or two years for anaerobic microbes to complete the growth cycle to degrade the petroleum hydrocarbon. The longest reported time was over 800 days," said Cheng Lei.

In 2019, they spotted a sample from an oil deposit with a growth cycle of two to three months.

"It's an unexpected discovery. The growth cycle was much shorter, so we were very interested," Cheng recalled. "We wanted to know what kind of microbe can convert petroleum hydrocarbon so quickly."

In 2019, scientists from the Max Planck Institute for Marine Microbiology of Germany put forward a hypothesis that a new type of archaea, named Ca. Methanoliparia, may have the ability to independently degrade long-chain alkyl hydrocarbons to produce methane. But there was no supporting evidence at the time.

Cheng Lei said they found the trace of these new archaea in the sample. But there were many microbes mixed, and they needed evidence to prove that the new archaea could do the job alone.

The team used a stable carbon isotope labeling test to confirm that the normal alkane added into the sample converted into methane and carbon dioxide.

Then they found Ca. Methanoliparia with the key intermediate metabolites in methane production from alkane degradation and confirmed its carbon metabolism pathway.

The result indicated that the new archaea could directly oxidize long-chain alkyl hydrocarbons without the help of bacteria, providing the fifth way to produce methane.

UNDERGROUND BIOGAS

In traditional oil field recoveries, the crude oil deep underground is driven by water pressure or chemicals and pumped to the surface. More than half of the deposits are challenging to recover by conventional technology and stay underground in depleted oil fields.

The head of the Biogas Institute, Wang Dengshan, believed this from-zero-to-one discovery lays a foundation for underground biogas development to revive depleted oil fields.

"We don't need to pump out the oil left underground. We can turn it into methane and collect it. It's equivalent to a massive underground biogas pool built in the crude oil deposit, on a scale of square kilometers," said Wang.

If the technology based on this research is applied to depleted oil fields in China, there will be hundreds of millions of tonnes of extra oil and gas for exploitation.

Compared with conventional petrochemistry, the biological conversion could directly produce methane without carbon emissions from crude oil exploitation, refining, and chemical processing, he said.

The new archaea may also have a further application prospect in synthetic biology.