Densely Populated Water Droplets in Heavy-Oil Seeps.

Area of Science:

• Microbiology
• Geochemistry
• Petroleum Science

Background:

• Microbial degradation of oil is traditionally associated with the oil-water transition zone (OWTZ).
• Recent findings suggest microbial life exists within water droplets dispersed in heavy oil.
• The prevalence of this phenomenon in oil reservoirs remains largely uninvestigated.

Purpose of the Study:

• To investigate if microbial life in dispersed water droplets is a common feature of heavy oil seeps.
• To characterize the microbial communities within these water droplets.
• To assess the viability and activity of these microbial populations.

Main Methods:

• Sampling of heavy oil from three distinct global locations: Pitch Lake (Trinidad and Tobago), La Brea Tar Pits (California, USA), and McKittrick oil field (California, USA).
• Analysis of water droplet characteristics, including size and cell density.
• Assessment of microbial viability using ATP content assays and LIVE/DEAD staining.
• Microbial community profiling via 16S rRNA gene amplicon sequencing.

Main Results:

• All three sampled heavy oil seeps contained dispersed water droplets with high microbial cell densities (up to 10^9 cells ml^-1).
• Microbial populations within the droplets were active and viable, with 60% membrane-intact cells and significant ATP concentrations.
• 16S rRNA sequencing revealed known anaerobic oil-degrading microorganisms and surprisingly similar microbial communities across geographically distant sites.

Conclusions:

• Small water inclusions in heavy oil serve as densely populated microhabitats for active microbial communities.
• The presence of similar microbial communities in water droplets across geographically separated oil seeps suggests this is a generic trait of biodegraded oil reservoirs.
• Microbial oil degradation may occur within these water pockets in oil legs, in addition to the OWTZ.