Soil remediation with a microbial community established on a carrier: strong hints for microbial communication during 1,2,4-Trichlorobenzene degradation

Sorption characteristics of N-acyl homserine lactones as signal molecules in natural soils based on the analysis of kinetics and isotherms

Quorum sensing, the communication between microorganisms, is mediated by specific diffusible signal molecules. Adsorption is an important process that influences the transport, transformation and bioavailability of N-acyl homoserine lactone (AHL) in complex natural environments such as soil. To examine the adsorption characteristics of N-hexanoyl, N-octanoyl, N-decanoyl and N-dodecanoyl homoserine lactones in soil, equilibrium and kinetic experiments were conducted in two types of soils (oxisol and alfisol) and monitored using Fourier-transform infrared spectroscopy (FTIR). A pseudo-second-order equation accurately described the sorption kinetics of AHLs in the two soils (R² ≥ 0.97, NSD ≤ 21.25%). The AHL sorption reached equilibrium within 24 h and 12 h for oxisol and alfisol, respectively. The sorption kinetics of AHLs adsorbed on the soils were fitted to the Boyd model, suggesting that film diffusion was the rate-limiting process. Partition played a more vital role than surface adsorption in the AHL adsorption process. The adsorption isotherms of AHLs could be described by the Langmuir and Freundlich equation (R² ≥ 0.98), indicating that the sorption process involved monolayer sorption and heterogeneous energetic distribution of active sites on the surfaces of the soils. The thermodynamic parameter, Gibbs free energy (ΔG), and a dimensionless parameter showed that the sorption of AHLs was mainly dominated by physical adsorption. Additionally, according to the FTIR data, the electrostatic forces and hydrogen bonding possibly influenced the adsorption of AHLs on the above mentioned two soils. The sorption characteristics of AHLs in soils correlated well with the molecular structure, solubility speciation and log P (n-octanol/water partition coefficient) of AHLs.

Sorption characteristics of N-acyl homoserine lactones (signal molecules) in natural soils.

N-acyl-homoserine lactone dynamics during biofilm formation of a 1,2,4-trichlorobenzene mineralizing community on clay

In Gram-negative bacteria, quorum sensing systems are based on the N-acyl-homoserine lactone (AHL) molecule. The objective of this study was to investigate the role of quorum sensing systems during biofilm formation by a microbial community while degrading the pollutant. Our model system included 1,2,4-trichlorobenzene (1,2,4-TCB) and its mineralizing Gram-negative bacterial community to investigate the relationships between AHL dynamics, cell growth and pollutant degradation. Biomineralization of 1,2,4-TCB was monitored for both the planktonic bacterial community with and without sterile clay particles in liquid cultures. The bacterial growth and production of AHLs were quantified by fluorescent in situ hybridization and immunoassay analysis, respectively. A rapid production of AHLs which occurred coincided with the biofilm formation and the increase of mineralization rate of 1,2,4-TCB in liquid cultures. There is a positive correlation between the cell density of Bodertella on the clay particles and mineralization rate of 1,2,4-TCB. 3-oxo-C_12:1-HSL appears to be the dominant AHL with the highest intensity and rapidly degraded by the bacterial community via two main consecutive reactions (lactone hydrolysis and decarboxylic reaction). These findings suggest that the integrated AHLs and their degraded products play a crucial role in biofilm formation and biomineralization of 1,2,4-TCB in culture.

Enhanced degradation of isoproturon in an agricultural soil by a Sphingomonas sp. strain and a microbial consortium

Isoproturon (IPU) degradation in an agricultural soil inoculated with an isolated IPU-degrader strain (Sphingomonas sp. strain AK1, IS) or a microbial consortium (MC) harboring this strain, with or without carrier material, were investigated in soil microcosm experiments during 46 days. Effect of the carrier material and inoculation size on IPU-degradation efficacy of the inoculants were studied. Mineralization, extractable residues and non-extractable residues of ¹⁴C-labeled IPU were analyzed. The low IPU mineralization in untreated soil (7.0%) was enhanced to different extents by inoculation of IS (17.4%-46.0%) or MC (58.9%-67.5%). Concentrations of IPU residues in soils amended with MC (0.002-0.095 μg g dry soil^-1) were significantly lower than in soils amended with IS (0.02-0.67 μg g dry soil^-1) and approximately 10 times lower than in the uninoculated soil (0.06-0.80 μg g dry soil^-1). Less extractable residues and non-extractable residues were detected in soil with higher IPU mineralization. Inoculation size (as indicated by the volume of liquid cultures or by the number of carrier particles) determined the IPU-removal efficacy of IS in soil, but this effect was less pronounced for MC. The low sorption of IPU to soil and the decreasing IPU-mineralizing rates suggested incapability of IS to establish the IPU-mineralizing function in the soil. The thorough removal of IPU and persistent IPU-mineralizing activity of soil inoculated with MC indicated a high persistence of IPU-metabolic trait. Our results showed that microbial consortia might be more efficient than single degrader strains to enhance clean-up of organic chemicals in soil.

A New Acyl-homoserine Lactone Molecule Generated by Nitrobacter winogradskyi

It is crucial to reveal the regulatory mechanism of nitrification to understand nitrogen conversion in agricultural systems and wastewater treatment. In this study, the nwiI gene of Nitrobacter winogradskyi was confirmed to be a homoserine lactone synthase by heterologous expression in Escherichia coli that synthesized several acyl-homoserine lactone signals with 7 to 11 carbon acyl groups. A novel signal, 7, 8-trans-N-(decanoyl) homoserine lactone (C10:1-HSL), was identified in both N. winogradskyi and the recombined E. coli. Furthermore, this novel signal also triggered variances in the nitrification rate and the level of transcripts for the genes involved in the nitrification process. These results indicate that quorum sensing may have a potential role in regulating nitrogen metabolism.