Abstract
Viruses are widely distributed in various ecosystems and have important impacts on microbial evolution, community structure and function and nutrient cycling in the environment. Viral abundance, diversity and distribution are important for a better understanding of ecosystem functioning and have often been investigated in marine, soil, and other environments. Though microbes have proven useful in oil recovery under extreme conditions, little is known about virus community dynamics in such systems. In this study, injection water and production fluids were sampled in two blocks of the Daqing oilfield limited company where water flooding and microbial flooding were continuously used to improve oil recovery. Virus-like particles (VLPs) and bacteria in these samples were extracted and enumerated with epifluorescence microscopy, and viromes of these samples were also sequenced with Illumina Hiseq PE150. The results showed that a large number of viruses existed in the oil reservoir, and VLPs abundance of production wells was 3.9 ± 0.7 × 108 mL−1 and virus to bacteria ratio (VBR) was 6.6 ± 1.1 during water flooding. Compared with water flooding, the production wells of microbial flooding had relative lower VLPs abundance (3.3 ± 0.3 × 108 mL−1) but higher VBR (7.9 ± 2.2). Assembled viral contigs were mapped to an in-house virus reference data separate from the GenBank non-redundant nucleotide (NT) database, and the sequences annotated as virus accounted for 35.34 and 55.04% of total sequences in samples of water flooding and microbial flooding, respectively. In water flooding, 7 and 6 viral families were identified in the injection and production wells, respectively. In microbial flooding, 6 viral families were identified in the injection and production wells. The total number of identified viral species in the injection well was higher than that in the production wells for both water flooding and microbial flooding. The Shannon diversity index was higher in the production well of water flooding than in the production well of microbial flooding. These results show that viruses are very abundant and diverse in the oil reservoir’s ecosystem, and future efforts are needed to reveal the potential function of viral communities in this extreme environment.
Highlights
Diverse microbial communities inhabit oil reservoirs and have critical roles in mediating ecological processes
The main purpose of studying the microbial ecology of oil reservoirs is to improve oil recovery rate, and the pilot—and full-scale of research efforts especially focus on the microbes, such as methanogenic archaea, fermentative bacteria, iron-reducing bacteria, and sulphate-reducing bacteria that have an important role in enhancing the oil recovery [23]
This study shows that viruses and bacteria were extremely abundant both in the injection and production wells during water and microbial flooding
Summary
Diverse microbial communities inhabit oil reservoirs and have critical roles in mediating ecological processes. Viruses could induce microbial cell lysis and release dissolved organic carbon and particulate organic carbon from inside host cells into the extracellular environment These resources may be reused by other surrounding microbes further facilitating biogeochemical cycles in the biosphere [7,8]. Microbial ecology in oil reservoirs mainly focuses on the relationship between injected and indigenous microbes, and the effects of injected nutrients and environmental factors on microbes and microbial community composition in the reservoir. It began in 1996, Voordouw [22] in the University of Calgary investigated the sulfur bacteria community in the oil fields of northern Canada through the most advanced molecular biological methods available at that time. Viruses in the special environment of oil reservoir may be one of the major causes for the dynamic changes of the microbial community within the reservoir
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