Evidence of specialized bromate-reducing bacteria in a hollow fiber membrane biofilm reactor

Transcriptome analysis provides new insights into the tolerance and aerobic reduction of Shewanella decolorationis Ni1-3 to bromate

As a possible human carcinogen, bromate is easily formed in drinking water and wastewater treatments using advanced oxidation technology. Microbial reduction is a promising method to remove bromate, but little is known about aerobic bromate reduction as well as the molecular mechanism of tolerance and reduction to bromate in bacteria. Herein, bromate reduction by isolate under aerobic conditions was reported for the first time. Shewanella decolorationis Ni1-3, isolated from an activated sludge recently, was identified to reduce bromate to bromide under both aerobic and anaerobic conditions. RNA-Seq together with differential gene expression analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis was performed to identify that bromate triggered the expression of genes for oxidative stress protection (e.g., ohr, msrQ, dsbC, gpo, gorA, and gst), DNA damage repair (e.g., dprA, parA, and recJ), and sulfur metabolism (e.g., cysH, cysK, and cysP). However, the genes for lactate utilization (e.g., lldF and dld), nitrate reduction (e.g., napA and narG), and dissimilatory metal reduction (e.g., mtrC and omcA) were down-regulated in the presence of bromate. The results contribute to revealing the molecular mechanism of resistance and reduction in S. decolorationis Ni1-3 to bromate under aerobic conditions and clarifying the biogeochemical cycle of bromine. KEY POINTS: • Aerobic bromate reduction by pure culture was observed for the first time • Strain Ni1-3 effectively reduced bromate under both aerobic and anaerobic conditions • ROS and SOS response genes were strongly induced in the presence of bromate.

Fate and reduction of bromate formed in advanced water treatment ozonation systems: A critical review

Disinfection in water treatment and reclamation systems eliminates the potential health risks associated with waterborne pathogens, however it may produce disinfection by-products (DBPs) harmful to human health. Potentially carcinogenic bromate is a DBP formed during the ozonation of bromide-containing waters. To mitigate the problem of bromate formation, different physical/chemical or biological reduction methods of bromate have been investigated. Until now, adsorption-based physical method has proven to be more effective than chemical methods in potable water treatment. Though several studies on biological reduction methods have been carried out in a variety of bioreactor systems, such as in biologically active carbon filters and denitrifying bioreactors, the microbiological mechanisms or biochemical pathways of bromate minimization have not been clearly determined to date. Genetic analysis could provide a broader picture of microorganisms involved in bromate reduction which might show cometabolic or respiratory pathways, and affirm the synergy functions between different contributing groups. The hypothesis established from the diffusion coefficients of different electron donor and acceptors, illustrates that some microorganisms preferring bromate over oxygen contain specific enzymes which lower the activation energy required for bromate reduction. In addition, considering microbial bromate reduction as an effective treatment strategy; field scale investigations are required to observe quantitative correlations of various influencing parameters such as pH, ozone dose, additives or constituents such as ammonia, hydrogen peroxide, and/or chloramine, dissolved organic carbon levels, dissolved oxygen gradient within biofilm, and empty bed contact time on bromate removal or reduction.

Denitrifying microbial community with the ability to bromate reduction in a rotating biofilm-electrode reactor

In this study, the microbial community for bromate reduction in a rotating biofilm-electrode reactor (RBER) was investigated. Continuous experiment demonstrated that the bromate reduction by an auto-hydrogenotrophic microbial community was inhibited by high concentration nitrate (50mg/L). The bacterial diversity of RBER were examined through the analyse of 16S rRNA gene sequences of clone libraries. The results showed that the bromate-reducing bacteria were phylogenetically diverse at the phylum level, representing the Firmicutes, Proteobacteria, Bacteroidetes and Actinobacteria. The relative abundances of these microbial community represented 99.1% of all phylum in the biofilms when bromate served as the sole electron acceptor. Meanwhile, the Bacillus strains became the largest phylotype and represented about 37% of the total bacteria in the biofilm, indicating that the genus Bacillus played the key role in the auto-hydrogenotrophic process. Moreover, three new bacterial genera, Exiguobacterium, Arthrobacter and Chlorobium appeared with the respective relative abundance being about 7.37%, 1.81%, and 0.52%, which might be the bromate-specific reducing bacteria.

Bromate Reduction by Rhodococcus sp. Br-6 in the Presence of Multiple Redox Mediators

A bromate (BrO₃^-)-reducing bacterium, designated Rhodococcus sp. strain Br-6, was isolated from soil. The strain reduced 250 μM bromate completely within 4 days under growth conditions transitioning from aerobic to anaerobic conditions, while no reduction was observed under aerobic and anaerobic growth conditions. Bromate was reduced to bromide (Br^-) stoichiometrically, and acetate was required as an electron donor. Interestingly, bromate reduction by strain Br-6 was significantly dependent on both ferric iron and a redox dye 2,6-dichloroindophenol (DCIP). Cell free extract of strain Br-6 showed a dicumarol-sensitive diaphorase activity, which catalyzes the reduction of DCIP in the presence of NADH. Following abiotic experiments showed that the reduced form of DCIP was reoxidized by ferric iron, and that the resulting ferrous iron reduced bromate abiotically. Furthermore, activity staining of the cell free extract revealed that one of diaphorase isoforms possessed a bromate-reducing activity. Our results demonstrate that strain Br-6 utilizes multiple redox mediators, that is, DCIP and ferric iron, for bromate reduction. Since the apparent rate of bromate reduction by this strain (60 μM day^-1) was 3 orders of magnitude higher than that of known bromate-reducing bacteria, it could be applicable to removal of this probable human carcinogen from drinking water.

Characterization of bromate-reducing bacterial isolates and their potential for drinking water treatment

The objective of the current study was to isolate and characterize several bromate-reducing bacteria and to examine their potential for bioaugmentation to a drinking water treatment process. Fifteen bromate-reducing bacteria were isolated from three sources. According to 16S rRNA gene sequencing, the bromate-reducing bacteria are phylogenetically diverse, representing the Actinobacteria, Bacteroidetes, Firmicutes, and α-, β-, and γ-Proteobacteria. The broad diversity of bromate-reducing bacteria suggests the widespread capability for microbial bromate reduction. While the cometabolism of bromate via nitrate reductase and (per)chlorate reductase has been postulated, five of our bromate-reducing isolates were unable to reduce nitrate or perchlorate. This suggests that a bromate-specific reduction pathway might exist in some microorganisms. Bioaugmentation of activated carbon filters with eight of the bromate-reducing isolates did not significantly decrease start-up time or increase bromate removal as compared to control filters. To optimize bromate reduction in a biological drinking water treatment process, the predominant mechanism of bromate reduction (i.e., cometabolic or respiratory) needs to be assessed so that appropriate measures can be taken to improve bromate removal.