Biofilm activity, ammonia removal and cell growth of the heterotrophic nitrifier, Acinetobacter sp., facilitated by exogenous N-acyl-homoserine lactones

Abstract

In the present study, the heterotrophic nitrification-aerobic denitrification strain, Acinetobacter sp. JQ1004, was treated with three typical N-acyl-homoserine lactone (AHL) molecules (C6-HSL, C8-HSL, and 3-oxo-C10-HSL) during the nitrogen removal process. The effects of AHLs on biofilm formation, flocculation, extracellular polymeric substance characteristics, and nitrogen removal were investigated. Findings revealed that low concentrations of these three AHLs could promote ammonia removal and cell growth as follows: C8-HSL > C6-HSL > 3-oxo-C10-HSL, whereas high concentrations suppressed nitrogen removal. Transcript levels of the amoA gene in the heterotrophic nitrification process were detected by real-time PCR, indicating that the addition of each AHL with 10 nmol L^-1 could stimulate expression of amoA. Notably, the addition of C6-HSL at relative lowly concentrations significantly accelerated biofilm formation and self-aggregation of strain JQ1004. Many microbial-flocs were observed between cells using scanning electron microscopy when strains were dosed with 10 nmol L^-1 C6-HSL. Excitation emission matrix spectra revealed that the addition of C6-HSL and C8-HSL at 10 nmol L^-1 did not change the components and structures of the extracellular polymeric substance (EPS), but the fluorescence intensity of various components increased substantially. However, the addition of 3-oxo-C10-HSL reduced the fluorescence strength of EPS, which had no remarkable effect on biofilm formation, self-aggregation, and nitrogen removal of the strain.

Fig. 1. Effects of short chain AHLs on nitrogen removal and cell growth by strain JQ1004.

Fig. 2. Growth curves of strain JQ1004 treated with different AHLs using ammonia as nitrogen source.

Fig. 3. Effect of different AHLs on biofilm formation.

Fig. 4. The EPS content of the strain JQ1004 in response to different AHLs with concentrations of 10, 30, 50 nmol L⁻¹.

Fig. 5. EEM distribution of Acinetobacter sp. EPS in the presence of different AHLs with concentration of 10 nmol L⁻¹.

Fig. 6. The flocculability and hydrophobicity of strain JQ1004 in response to different AHLs.

Fig. 7. The characteristics of strain JQ1004 observed under scanning electron microscopy (×20 000): (a) without any AHLs; (b) treated with 10 nmol L⁻¹ C6-HSL.

Fig. 8. Different expression levels of amoA gene in the presence of different AHLs with the concentration of 10 nmol L⁻¹. Data are mean values for three independent experiments.