Viruses are ubiquitous in many ecosystems, and play important roles in regulating structure and function of ecosystems. Oil reservoir is a deep underground extreme environment for crude oil storage, but a large number of microorganisms still inhabit. 

Microbial enhanced oil recovery technology is based on the directional regulation of functional microorganisms in oil reservoirs. Some studies have detected virus gene sequences in oil reservoir, but little is known about its abundance, diversity and ecological function in such environment. 

Zheng Liangcan and Prof. Zhang Ying from the Microbial Ecology and Technology Research Group of the Institute of Applied Ecology, Chinese Academy of Sciences, collected oil production water from two blocks of Daqing Oilfield where the long-term water flooding and microbial flooding techniques have been carried out, respectively.  

By using epifluorescence microscopy, the researchers detected the number of virus-like particles (VLPs) and bacteria, and found that the number of viruses in the oil reservoir environment was 6.6-7.9 times the number of bacteria. 

Using viral metagenomics analysis, they revealed the structure and diversity of virus communities at the family and species level in oil reservoir. In the production fluid samples of water flooding and microbial flooding, the annotated virus sequences accounted for 35.34% and 55.04% of the total sequence, respectively, implying the existence of a large number of unknown viruses. 

Caudovirales dominated the virus community, with relative abundance of 65-74%. Only 45.16% of the virus species were listed in the Virus-Host database, while the host information of the remaining viruses was unknown. 

The researchers found that the Shannon diversity index of virus community was higher in the production well of water flooding than that of microbial flooding. Compared to water flooding, they found even little difference in virus community structure among the production fluid samples of microbial flooding. 

Consistent with the decrease in bacterial community diversity, the injection of nutrient activator reduced the diversity of virus community composition in production wells carried out microbial flooding oil recovery, suggesting that more attention should be given to the phage-driven changes in microbial community in the study of microbial enhanced oil recovery. 

This ecological research of virus in oil reservoir broadens the research field of environmental virology, and helps the understanding of microbial ecological ecosystem of oil reservoirs and microbial interactions in such extreme underground environment. 

This study was published in Microorganisms under the title "Viral Abundance and Diversity of Production Fluids in Oil Reservoirs."  

This study is jointly supported by the National High-Tech Research and Development Program of China and the National Natural Science Foundation of China.