Nitrogen elimination by a novel salt-tolerant bacterium Alcaligenes aquatilis DN-1: Characteristics and resistance mechanism of the strain

Abstract

Nitrogen (N) removal from saline wastewater has emerged as a key concern to safeguard environment. High salinity can impede the growth of bacteria responsible for nitrogen removal. The identification of salt-tolerant bacteria capable of nitrogen removal is thus important for wastewater treatment. In this study, a novel strain, Alcaligenes aquatilis DN-1, capable of aerobic-heterotrophic nitrogen removal (AHNR) was isolated from wastewater. The strain DN-1 showed 100 %, 99.97 %, and 85.24 % efficiencies for removing ammonium, nitrite, and nitrate, respectively. Particularly, strain DN-1 exhibited excellent ammonium removal efficiency under 0–6 % salinity. A maximum nitrogen removal rate of 98.92 % was achieved at NaCl concentration of 6 %. Results of transcriptomic analysis indicated that up-regulation of gdhA, gltB and gltD genes was observed, which were involved in the assimilation pathway of ammonium removal. Ammonium removal pathway from hypersaline wastewater by strain DN-1 was accomplished mainly through assimilation. Genes involved in synthesis of ectoine and 5-hydroxyectoine, i.e., ectA, ectB, ectC and ectD, showed significantly high levels of transcriptional expressions, indicating that strain DN-1 balanced osmotic pressure by accumulating ectoine and 5-hydroxyectoine to withstand salinity-induced osmoadaptation. Meanwhile, genes associated with oxidative stress (trxB, trxC, dsbA, dsbB, dsbC and dsbD) were also upregulated to some extent. Genes involved in osmoadaptation and oxidative stress cooperated with ammonium metabolism pathways to counteract salt stress. The study indicated that the isolatedstrain for was highly efficient in eliminating ammonium from wastewater. The findings further offered valuable insights into the adaptation mechanisms of Alcaligenes aquatilis DN-1 in salt wastewater.