Microbes living in tar pits show oil can be degraded from within
Petroleum remains a critical commodity for a range of industries. Cost and availability influence business decisions daily, while shortages can have highly detrimental knock-on effects across the whole economy. Microbial biodegradation – the process by which organic substances are broken down into smaller compounds by living microbial organisms – plays a key role in influencing the quality of oil. Current studies have suggested that this process takes place mostly at the oil-water transition zone in oil reservoirs. Above this zone is predominantly oil, while below is predominantly water. “Oil consists of a vast mixture of different compounds,” explains EcOILogy project coordinator Rainer Meckenstock from the University of Duisburg-Essen, Germany. “Some compounds such as alkanes (organic compounds that consist entirely of single-bonded carbon and hydrogen atoms) are less stable and will be degraded by microbes before more stable aromatic compounds.” When alkanes and smaller molecules are degraded, the viscosity of oil changes. The resulting ‘heavier’ oil is of less economic value and is also harder to extract from the subsurface. When oil is heavily degraded, it turns into cheaper tar or bitumen, which is polished into asphalt and used to pave roads.
Analysing water droplets
The inspiration for the EcOILogy project, which was supported by the European Research Council (ERC), came after a visit to the world’s biggest natural tar lake, the Pitch Lake in Trinidad and Tobago. “All that degraded tar made me think about whether small water droplets enclosed in the oil might enable microorganisms to degrade the oil at different levels, and not only at the oil-water transition zone,” adds Meckenstock. From this visit, and other visits to natural oil seeps in the United States, Meckenstock collected small water droplet samples dispersed in the oil. After analysis, these were found to contain complex microbial communities. “We found these water droplets with microorganisms in every oil seep,” says Meckenstock. “This indicates that this is a generic feature. The microbes were alive and active and present in surprisingly high densities.” The DNA of these microbes was next sequenced, and the composition of microbial communities analysed. Each droplet was found to constitute a mini-ecosystem, totally isolated from the environment. Meckenstock and his team also developed a new method to assess degradation rates. They uncovered extremely long time frames, millions of years, for a litre of oil to degrade.
Understanding oil degradation
The results of the EcOILogy project could be highly interesting for the global petroleum industry. “We know that microbial degradation is key to the quality of oil,” says Meckenstock. “However, our results suggest that oil biodegradation does not take place only at the oil-water transition zone in the oil reservoir. Microorganisms can also thrive in small water droplets and degrade the oil from within.” This new concept suggests that the extent of degradation is connected to overall water content of the oil reservoir. Meckenstock and his team plan to continue to study how these mini-ecosystems in water droplets are composed, and how essential functions such as nutrient recycling occur. “Our aim is to be able to explain how microorganisms and complex microbial communities can survive under such extreme conditions in oil reservoirs,” he notes. “They have to contend with saturated concentrations of oil, heat and disconnection from nutrients.”
Keywords
EcOILogy, oil, petroleum, tar, alkanes, reservoir, tar, bitumen, microbial
