Water Droplets in Crude Shale Oil Harbor an Active Heterotrophic Microbial Community Feeding on Both Oil and Necromass

Biodegradation
in oil reservoirs was generally thought to take
place at the oil–water-transition zone. In shale oil reservoirs,
however, there is no aquifer below the oil leg, and it is unclear
if biodegradation can take place. Here, we investigated the microbial
community in crude oil from a shale oil reservoir in China. We found
small water droplets trapped in the shale oil (approximately 2 ×
105 droplets/mL oil) originating from ancient groundwater.
These droplets contained over 100-fold greater cell density than the
production water from shale reservoirs and a high cell viability comparable
to those measured in groundwater. Most intact cells were mainly distributed
at the oil–water interface of the water droplets, whereas dead
cells were located in the water phase. Based on the determined cell
density, we conservatively extrapolate the global microbial population
in water droplets in oil to be 10–8 to 10–6 of the total microbial biomass in the subsurface. Our study revealed
a community of chemoheterotrophs in the water droplets that are most
likely feeding on dissolved hydrocarbons and extracellular substances
derived from biomass and detritus. The results indicate that small
water droplets in shale oil reservoirs harbor a complex and active
microbial community, contributing to biodegradation.