Understanding microbial community in oil reservoirs is crucial to application of microbial enhanced oil recovery (MEOR) technology. Much effort has been directed toward more insights into the microbial community in oil field, resulting in the development of culture dependent and independent techniques. Culture-dependent methods for enhancement of microbial culturability and culture-independent methods for analyzing microbial communities are reviewed in an attempt to better understand the recent progress in methodological researches on the oil field microbial communities.
Microbial community is analyzed conventionally by culture-dependent approaches which cultivate, enrich and isolate microbial cells, identify and count them, then figure out the microbial structure from the data of the cultivated microbial cells. However, the cultivating nutrients and conditions afforded in laboratory are too simplified and somehow far different from those in the environments where microorganisms live. Consequently, majority of microorganisms can not grow and be culturable. In addition, microorganisms grow at different speeds on the same media and under the same conditions, leading to different enrichment and increase of the microbial population. The relative amounts of the cultivated microorganisms are therefore different from the real ratio of the microorganisms in the environments. As a consequence, the pattern of microbial community structure obtained by the conventional culture-dependent methods is not true and not helpful for directing the development and application of MEOR.
Two strategies, the increasing microbial culturability and culture-independent methods, are therefore developed and applied for getting more culturable microbial isolates and more insights into the real microbial community structure and functions. Increasing microbial culturability is to cultivate microorganisms with novel methods and under novel conditions. These novel cultivating conditions include: "oligotrophic" media instead of "rich" nutrients, adding signal molecules, novel electronic donating and accepting chemicals to media, etc.. The novel cultivating methods include dilution culture, high-through culturing, diffusion-growth chamber, cell encapsulation, sequence-guiding isolation techniques, etc. which can mimic environmental conditions and lead to more microorganisms cultivated. Although the novel cultivating techniques and conditions can make much more microorganisms culturable, they can not figure out the overall pattern of microbial community structure, the culture-independent approaches are therefore absolutely necessary for analyzing oil reservoir microbial community. Culture-independent approaches are based on the analyses of the functional genes, such as mcrA gene, and 16S rRNA and its gene (16S rDNA). The analyzing techniques contain FISH, T-RFLP, DGGE, clone library and sequencing etc.. Many researches with the culture-independent methods have showed the power of these methods and got many new ideas on the structure and functions of the microbial community in oil reservoirs. Moreover, it is a trend to apply both culture-dependent and culture-independent approaches to understand the functions of microorganisms for MEOR and structure of microbial community. With the microbial strains obtained from oil reservoirs and the knowledge of oil reservoir microbial community, MEOR can be developed and applied feasibly and reliably by manipulating the microbial community in the oil reservoirs.