## 4 Discussion
Conversely, the nutrient injection increased bacterial community richness but reduced the evenness of production well W226, which indicates that several specific bacterial populations, especially functional bacteria, were significantly enriched in well W226. According to an analysis of community profiles at the genus level, _Arobacter_, _Halomonas_, and _Thalassolituus_, which are associated with emulsification, lowering of interfacial tension, and lowering of oil viscosity, showed high increases in relative abundance. These capabilities may contribute to enhanced oil recovery. Among them, the abundance of _Arobacter_, a genus involved in polycyclic aromatic hydrocarbons (PAH) degradation, nitrate reduction, and biosurfactant production, increased to more than 70% after nutrient stimulation. Previous studies have shown that this genus is one of the three 'core' bacterial genera found in the water produced at eight oil reservoirs throughout northern China [14; 33; 34]. Furthermore, _Arobacter_ and _Pseudomonas_ were also dominant in both the production wells in Daqing Oil Field, after flooding with polymer or alkaline-surfactant-polymer solutions [35]. Several studies have indicated that some species of _Arobacter_ are capable of degrading oil and producing lipopeptide surfactants [36; 37]. In combination, these results suggest that _Arobacter_ spp. may contribute to enhanced oil recovery. The halophilic bacterium _Halomonas_ is involved in hydrocarbon and polycyclic aromatic hydrocarbon degradation and accounts for a large number of the bacterial communities that inhabit many oil reservoirs, including the alkaline-surfactant-polymer (ASP)-flooded blocks [35; 38; 39]. Moreover, the type species of _Thalassolituus_ spp.--_T. olevorans_ could produce biosurfactants and emulsify crude oil when cultivated with \(n\)-alkanes [40; 41]. In addition, the proportion of _Pseudomonas_ spp. after nutrient flooding also increased by 1.14 times in this study. _Pseudomonas_ is currently one of the most frequently detected genera in oil reservoirs worldwide [42; 43; 44]. _Pseudomonas_ populations have been applied widely in the reduction in oil/water tension and in enhancing oil recovery because of their ability to degrade alkanes of different chain lengths and to produce various lipopeptides and glycolipid surfactants. [PERSON] et al. investigated the microbial populations in low permeability reservoirs with different water cuts in Changqing Oil Field. The results showed that _Acinetobacter_ and _Pseudomonas_ spp. were the dominant functional bacterial genera for crude oil degradation. Furthermore, _Pseudomonas_ and _Halomonas_ spp. were the top two dominant groups in the reservoirs with the water cut of more than 80% [10]. Therefore, nutrient injection enriched functional bacteria associated with hydrocarbon degradation- and emulsification-capabilities in the oil well W226. This phenomenon is consistent with the finding that functional bacterial populations, such as HOB and NRB, flourished after nutrient flooding. The enrichment of these functional bacteria may improve the emulsification and viscosity of oil, and contribute to the improvement of oil recovery. This conclusion could also be confirmed by the variation in predicted functional genes in the production well W226 and control W202 (Figure 6). Compared with the functional gene profile of the W202 sample, some categories of metabolism, including energy metabolism and carbohydrate metabolism of both bacterial and archaea, were more abundant in the W226 sample. Furthermore, bacterial membrane transport of the environmental information processing category displayed higher percentages in W226. These increases in the abundance of microbial functional genes indicated the improvement of metabolic functions, such as biosurfactant production and hydrocarbon degradation, which may help enhance oil recovery. It is worth noting that the nutrient injection significantly reduced the population of SRB.