Silicate weathering in terrestrial ecosystems plays an important role in the formation of soil and soil nutrients, in neutralization of acid rain, and in the long-term drawdown of atmospheric CO₂. Many studies indicate that bacteria can significantly affect mineral dissolution by different ways, such as producing acids and metal-complexing ligands, changing redox conditions, or mediating the formation of secondary mineral phases. Exopolysaccharides are also reported to be related to mineral weathering. The probable mechanisms included chelating the structural elements, breaking chemical equilibrium by absorbing ions, forming biofilm and so on. However, there are little reports on the key genes involved in this process. So, this study focuses on the mineral-weathering genes.

A high effective mineral-weathering bacterium, strain M78, was isolated from the adjacent soil of purple siltstone in Yanting, Sichuan Province, China. Potash feldspar which is one of the main silicates in purple siltstone was used as the test mineral. The dissolved elements of Fe, Si and Al by this strain in two days turned out to be 0.45 mg L^-1, 0.38 mg L^-1 and 0.13 mg L^-1, respectively. The result of the 16S rDNA sequencing showed that strain M78 belonged to the species of Ensifer adhaerens. The production of exopolysaccharides of strain M78 was 0.82±0.01 mg mL^-1, which we presumed to be the main factor responsible for the mineral-weathering efficiency. We used transposon insertion method to screen for the genes involved in mineral weathering. A mutation library was built, and the mutants with significantly declined production of exopolysaccharides were the target of screening. We got three target mutants (T3, T6 and 10), the exopolysaccharides productions of which were 0.37±0.07 mg mL^-1, 0.31±0.01 mg mL^-1 and 0.29±0.06 mg mL^-1. The functional groups on the surface of exopolysaccharides were compared between the mutants and the wild type by fourier transform infrared spectroscopy. The result showed mutants T3 and T6 could produce different exopolysaccharides compared to wild type. The genome walking and genome-wide scanning were taken to locate the inserted genes. The results showed that the inserted genes of the mutants were found to be ctrA, yfiH and kspE coding cell cycle response regulator, polyphenol oxidase, capsule polysaccharide ABC transporter permease, respectively. According to the different characters between the mutants and the wild type, we suggest that ctrA, yfiH and kspE may be involved in the mineral-weathering through the process of synthesis and transportation of the exopolysaccharides in different ways.