Stimulatory and inhibitory effects of metals on 1,4-dioxane degradation by four different 1,4-dioxane-degrading bacteria

Highlights

• Effects of 8 metals on 1,4-dioxane degradation by 4 bacterial strains were studied.

• Cu(II) commonly had the most severe inhibitory effects on degradation.

• Cu(II) at low concentrations probably inhibited degradation enzymes.

• Mn(II) and Fe(III) stimulated degradation depending on the degrading strain.

• Stimulation by metals was likely associated with SDIMO types in the strains.

Abstract

This study evaluates the effects of various metals on 1,4-dioxane degradation by the following four bacteria: Pseudonocardia sp. D17; Pseudonocardia sp. N23; Mycobacterium sp. D6; and Rhodococcus aetherivorans JCM 14343. Eight transition metals [Co(II), Cu(II), Fe(II), Fe(III), Mn(II), Mo(VI), Ni(II), and Zn(II)] were used as the test metals. Results revealed, for the first time, that metals had not only inhibitory but also stimulatory effects on 1,4-dioxane biodegradation. Cu(II) had the most severe inhibitory effects on 1,4-dioxane degradation by all of the test strains, with significant inhibition at concentrations as low as 0.01–0.1 mg/L. This inhibition was probably caused by cellular toxicity at higher concentrations, and by inhibition of degradative enzymes at lower concentrations. In contrast, Fe(III) enhanced 1,4-dioxane degradation by Mycobacterium sp. D6 and R. aetherivorans JCM 14343 the most, while degradation by the two Pseudonocardia strains was stimulated most notably in the presence of Mn(II), even at concentrations as low as 0.001 mg/L. Enhanced degradation is likely caused by the stimulation of soluble di-iron monooxygenases (SDIMOs) involved in the initial oxidation of 1,4-dioxane. Differences in the stimulatory effects of the tested metals were likely associated with the particular SDIMO types in the test strains.

Introduction

The ether 1,4-dioxane has been used mainly as a solvent for extraction and as a detergent in chemicals manufacturing. It is also formed as a by-product in the production of ethylene oxide, ethylene glycol, and detergents. 1,4-Dioxane is a suspected human carcinogen, and can persist in aquatic environments due to its stable physicochemical properties (Zhang et al., 2017). Thus, 1,4-dioxane is a contaminant of emerging concern in aquatic environments.

During the last two decades, bacterial strains capable of degrading 1,4-dioxane as a sole carbon and energy source have been isolated from various environments (Inoue et al., 2016; Zhang et al., 2017; Li et al., 2018; Yamamoto et al., 2018c), and found to be distributed even in non-contaminated environments (Sei et al., 2010; He et al., 2018). As a result, biological treatment using 1,4-dioxane-degrading bacteria have been studied extensively as a promising strategy to treat and remediate 1,4-dioxane-contaminated waters (Zhang et al., 2017; Deng et al., 2018; Myers et al., 2018; Yamamoto et al., 2018b).

Biodegradation pathways of 1,4-dioxane have been proposed through the identification of degradation intermediates (Zhang et al., 2017). The common rate-limiting step in proposed biodegradation pathways is the initial oxidation of the carbon atom adjacent to the oxygen atom, namely 2-hydroxylation, which precedes the cleavage of the high-energy C–O bond of the cyclic ether structure (Li et al., 2017). This oxidation is catalyzed by soluble di-iron monooxygenases (SDIMOs) (Masuda et al., 2012; Inoue et al., 2016; He et al., 2017). 1,4-Dioxane-catalyzable SDIMOs are distributed over the six groups of SDIMOs currently recognized, although strains capable of metabolizing 1,4-dioxane are limited to group-5 and group-6 (He et al., 2017). Despite the increasing number of studies aimed at clarifying the diversity of 1,4-dioxane-catabolizing SDIMOs, their enzymatic characterization remains limited.

The enzymatic activity for 1,4-dioxane degradation can be affected by co-occurring compounds. Tetrahydrofuran (THF), a structural analog of 1,4-dioxane, is a well-known primary substrate for cometabolic 1,4-dioxane degradation (Zenker et al., 2000; Mahendra and Alvarez-Cohen, 2006; Sun et al., 2011; Sei et al., 2013b; Inoue et al., 2016). Competitive inhibition of 1,4-dioxane degradation by other analogous cyclic ethers has also been confirmed in previous studies (Inoue et al., 2018; Yamamoto et al., 2018a). Furthermore, chlorinated volatile organic compounds such as 1,1-dichloroethylene, cis-1,2-dichloroethene, and trichloroethene—which are major co-contaminants with 1,4-dioxane in groundwater—have been proven to inhibit 1,4-dioxane biodegradation via complex mechanisms including competition and impairment of degradative enzymes (i.e., SDIMO), the suppression of degradative gene expression, and via negative effects on other cellular activities in 1,4-dioxane-degrading bacteria (Mahendra et al., 2013; Zhang et al., 2016; Deng et al., 2018). In contrast, the effects of metals on 1,4-dioxane biodegradation and associated enzymes have been rarely studied. To our knowledge, only Pornwongthong et al. (2014) have examined the inhibitory effects of Cd(II), Cu(II), Ni(II), and Zn(II) on 1,4-dioxane degradation by Pseudonocardia dioxanivorans CB1190, in which the initial hydroxylation of 1,4-dioxane is catalyzed by THF monooxygenase (Thm) categorized under the group-5 SDIMOs. Due to the possible co-occurrence with 1,4-dioxane in industrial wastewaters and natural environments, the potential impacts of various metals on 1,4-dioxane biodegradation requires further research. In particular, given the diversity in the taxonomy of degrading bacteria (Inoue et al., 2016; Zhang et al., 2017; Li et al., 2018) and in 1,4-dioxane-catalyzable SDIMOs (He et al., 2017), further study of different 1,4-dioxane-degrading bacteria is urgently needed.

This study aimed to evaluate the effects of various metals on the initial oxidation of 1,4-dioxane by different 1,4-dioxane-degrading bacterial strains. Eight metal ions, including those reported to inhibit 1,4-dioxane degradation by P. dioxanivorans CB1190 (Pornwongthong et al., 2014), were used as the test metals. Four 1,4-dioxane-degrading bacterial strains belonging to Mycobacterium, Pseudonocardia, and Rhodococcus were used as the test strains, because most of known 1,4-dioxane-degrading bacteria capable of growing on 1,4-dioxane belong to these three genera (Inoue et al., 2016; Zhang et al., 2017; Li et al., 2018; Yamamoto et al., 2018c). The test strains had been confirmed to possess group-5 or group-6 SDIMOs depending on the strain. The study aimed to improve current understanding of the enzymatic aspects of 1,4-dioxane biodegradation to help identify the promising strains and design the overall system for effective biotreatment/bioremediation of 1,4-dioxane-contaminated waters.