Distinct Mineral Weathering Behaviors of the Novel Mineral-Weathering Strains Rhizobium yantingense H66 and Rhizobium etli CFN42

Abstract

Bacteria play important roles in mineral weathering, soil formation, and element cycling. However, little is known about the interaction between silicate minerals and rhizobia. In this study, Rhizobium yantingense H66 (a novel mineral-weathering rhizobium) and Rhizobium etli CFN42 were compared with respect to potash feldspar weathering, mineral surface adsorption, and metabolic activity during the mineral weathering process. Strain H66 showed significantly higher Si, Al, and K mobilization from the mineral and higher ratios of cell numbers on the mineral surface to total cell numbers than strain CFN42. Although the two strains produced gluconic acid, strain H66 also produced acetic, malic, and succinic acids during mineral weathering in low- and high-glucose media. Notably, higher Si, Al, and K releases, higher ratios of cell numbers on the mineral surface to total cell numbers, and a higher production of organic acids by strain H66 were observed in the low-glucose medium than in the high-glucose medium. Scanning electron microscope analyses of the mineral surfaces and redundancy analysis showed stronger positive correlations between the mineral surface cell adsorption and mineral weathering, indicated by the dissolved Al and K concentrations. The results showed that the two rhizobia behaved differently with respect to mineral weathering. The results suggested that Rhizobium yantingense H66 promoted potash feldspar weathering through increased adsorption of cells to the mineral surface and through differences in glucose metabolism at low and high nutrient concentrations, especially at low nutrient concentrations.

Importance: This study reported the potash feldspar weathering, the cell adsorption capacity of the mineral surfaces, and the metabolic differences between the novel mineral-weathering Rhizobium yantingense H66 and Rhizobium etli CFN42 under different nutritional conditions. The results showed that Rhizobium yantingense H66 had a greater ability to weather the mineral in low- and high-glucose media, especially in the low-glucose medium. Furthermore, Rhizobium yantingense H66 promoted mineral weathering through the increased adsorption of cells to the mineral surface and through increased organic acid production. Our results allow us to better comprehend the roles of different rhizobia in silicate mineral weathering, element cycling, and soil formation in various soil environments, providing more insight into the geomicrobial contributions of rhizobia to these processes.

FIG 1

Changes in element (Si, Al, K) release into the culture medium and changes in pH during the weathering of potash feldspar in the presence of Rhizobium strains H66 and CFN42 at low (1 g liter⁻¹) and high (5 g liter⁻¹) glucose concentrations in BHm. Error bars represent ±1 standard deviation (n = 3).

FIG 2

SEM images of Rhizobium yantingense H66- and Rhizobium etli CFN42-inoculated minerals after 0, 3, and 15 days of incubation in low-glucose (1 g liter⁻¹) and high-glucose (5 g liter⁻¹) media. Very few cells were observed on the mineral surface due to the significantly lower mineral surface adsorption of strain CFN42 than strain H66 or to the possibility that the cells of strain CFN42 were loosely adsorbed onto the mineral surface and were removed from the surface when washed in phosphate buffer and prepared for scanning electron microscope analysis.

FIG 3

Changes in the cell numbers on the mineral surfaces and in the culture medium and the ratio of the cell numbers on the mineral surfaces to the total cell numbers on the mineral surfaces and in the culture medium during mineral weathering in the presence of strains H66 and CFN42 in low-glucose (1 g liter⁻¹) and high-glucose (5 g liter⁻¹) media. Error bars represent ±1 standard deviation (n = 3).

FIG 4

Changes in residual glucose and exopolysaccharide concentrations in the culture medium during potash feldspar weathering in the presence of strains H66 and CFN42 in the initial presence of low (1 g liter⁻¹) and high (5 g liter⁻¹) glucose concentrations. Error bars represent ±1 standard deviation (n = 3).

FIG 5

Changes in the concentrations of organic acids during mineral weathering in the presence of either strain H66 or CFN42 in low-glucose (1 g liter⁻¹) or high-glucose (5 g liter⁻¹) medium. Error bars represent ±1 standard deviation (n = 3).

FIG 6

RDA analysis of the relationship between the ratio of the cell numbers on the mineral surface to the total cell numbers (including bacterial cell numbers on the mineral surface and in the culture medium) and element (Si, Al, and K) releases during mineral weathering in the presence of strains H66 and CFN42 in low-glucose (1 g liter⁻¹) and high-glucose (5 g liter⁻¹) media.