The lactonase BxdA mediates metabolic specialisation of maize root bacteria to benzoxazinoids.

Root exudates contain specialised metabolites that shape the plant's root microbiome. How host-specific microbes cope with these bioactive compounds, and how this ability affects root microbiomes, remains largely unknown. We investigated how maize root bacteria metabolise benzoxazinoids, the main specialised metabolites of maize. Diverse and abundant bacteria metabolised the major compound in the maize rhizosphere MBOA (6-methoxybenzoxazolin-2(3H)-one) and formed AMPO (2-amino-7-methoxy-phenoxazin-3-one). AMPO forming bacteria were enriched in the rhizosphere of benzoxazinoid-producing maize and could use MBOA as carbon source. We identified a gene cluster associated with AMPO formation in microbacteria. The first gene in this cluster, bxdA encodes a lactonase that converts MBOA to AMPO in vitro. A deletion mutant of the homologous bxdA genes in the genus Sphingobium, did not form AMPO nor was it able to use MBOA as a carbon source. BxdA was identified in different genera of maize root bacteria. Here we show that plant-specialised metabolites select for metabolisation-competent root bacteria. BxdA represents a benzoxazinoid metabolisation gene whose carriers successfully colonize the maize rhizosphere and thereby shape the plant's chemical environmental footprint

Characterization of Residual Oil Distribution in Sandstone by NMR: A Microscopic View on Oil Recovery by Miscible CO2 Flooding

AbstractMiscible CO2 flooding gains rising popularity due to high displacement efficiency and potential carbon negativity. It is well documented that pore-level heterogeneity is one of the dominant mechanisms responsible for the variation in oil recovery efficiencies of miscible CO2 flooding. However, owing to a lack of understanding of miscible CO2 displacement behaviors at pore level, quantitative analysis of the mechanisms for the influence of pore-level heterogeneity on the oil recovery efficiency of miscible CO2 flooding remains problematical. Recently, NMR has been widely used to investigate microscopic residual oil distribution and pore-level displacement behaviors of various agents. Nevertheless, current NMR-based methods for determining microscopic residual oil distribution require the use of fluorinated oil, Mn2+ solution, or heavy water for eliminating the mutual interference of oil and water NMR signals. The differences in properties of reservoir fluids and additional agents could give rise to the change in displacement characteristics and consequently affect the analysis results on pore-level displacement behaviors. In this study, a method to determine microscopic residual oil distribution in sandstone during gas floodings based on NMR T2 spectrum without additional agents except reservoir fluids is proposed. Using the method, we can avoid the possible changes in the displacement characteristics caused by additional agents. Miscible CO2 flooding experiments are conducted with ultra-low and extra-low permeability cores. The T2 spectra of cores before and after flooding are measured using a 23 MHz low-field NMR instrument. Residual oil distributions in cores are quantitatively characterized using the proposed method, and then, oil recovery efficiencies from different-sized pore throats are evaluated. It is found that for both cores about 60% of the residual oil is distributed in pore throats with <0.26 μm radius. Oil residing in pore throats with >0.03 μm radius is displaceable for miscible CO2 flooding, which indicates that the thickness of the raffinate layer in the pore should be less than 0.015 μm. More than 30% of the residual oil is distributed in unswept areas, and thus, the sweep efficiency improvement is expected to effectively enhance the oil recovery of miscible CO2 flooding. The sweep efficiency is negatively affected by the pore-size heterogeneity. For the ultra-low and extra-low permeability cores, the standard deviation of pore size is 0.39 and 1.15, respectively, and correspondingly, the sweep efficiency is 84.33% and 72.84%, respectively. In the core, the oil recovery efficiencies from swept pore throats with <3 μm radius are similar and exceed 50%. Pore throats with >3 μm radius can form a preferred flow path, which will significantly reduce the oil displacement efficiency from swept pore throats with <3 μm radius. The findings of this study can help for better understanding of the microscopic CO2 miscible displacement behaviors and the mechanisms for the influence of pore-level heterogeneity on the oil recovery efficiency of miscible CO2 flooding