Viruses are ubiquitous in many ecosystems, they 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 Institute of Applied Ecology of the Chinese Academy of Sciences collected water samples from two blocks of Daqing Oilfield, where the water flooding and microbial flooding oil recovery techniques have long been carried out, respectively.  

By using fluorescence 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 measured the structure and diversity of oil reservoir virus communities at the family and species level. 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.