Abstract
Soil contamination by PAH and heavy metals is a growing problem. Here, we showed that a new isolate, Pseudomonas brassicacearum strain LZ-4, can simultaneously degrade 98% of 6 mM naphthalene and reduce 92.4% of 500 μM hexavalent chromium [Cr (VI)] within 68 h. A draft genome sequence of strain LZ-4 (6,219,082 bp) revealed all the genes in the naphthalene catabolic pathway and some known Cr (VI) reductases. Interestingly, genes encoding naphthalene pathway components were upregulated in the presence of Cr (VI), and Cr (VI) reduction was elevated in the presence of naphthalene. We cloned and expressed these naphthalene catabolic genes and tested for Cr (VI) reduction, and found that NahG reduced 79% of 100 μM Cr (VI) in 5 minutes. Additionally, an nahG deletion mutant lost 52% of its Cr (VI) reduction ability compared to that of the wild-type strain. As nahG encodes a salicylate hydroxylase with flavin adenine dinucleotide (FAD) as a cofactor for electron transfer, Cr (VI) could obtain electrons from NADH through NahG-associated FAD. To the best of our knowledge, this is the first report of a protein involved in a PAH-degradation pathway that can reduce heavy metals, which provides new insights into heavy metal-PAH contamination remediation.
Highlights
Environmental pollution caused by heavy metals and polycyclic aromatic hydrocarbons (PAHs) is an enormous environmental problem[1]
To isolate naphthalene-degrading bacterial strains, the final enrichment culture was diluted in BH medium and plated on BH agar plates sprayed with naphthalene
A previous study showed that some PAH-degrading bacteria can survive in the presence of other pollutants[26], and that Pseudomonas gessardii LZ-E has been reported as a potential candidate for the remediation of combined pollution, because it can simultaneously degrade naphthalene and reduce Cr (VI) without inhibition[23]
Summary
Environmental pollution caused by heavy metals and polycyclic aromatic hydrocarbons (PAHs) is an enormous environmental problem[1]. Chromium is an important industrial material, and hexavalent chromium is a serious environmental threat due its toxic effects on humans and biodiversity[5] These chemical compounds are deposited in soil and sediment, and when they reach high concentrations, they become harmful to the environment and human health[6]. Decontamination of PAHs and heavy metals is a complex problem, as conventional remediation methods, such as chemical degradation of these organic pollutants and precipitation of chromium by transforming Cr (VI) into poorly soluble trivalent chromium [Cr(III)], may cause further environmental damage[7, 8]. Numerous studies have been conducted to elucidate the enzymes involved in chromium reduction and naphthalene degradation and the genes encoding them in both gram-negative and gram-positive bacteria that can remediate naphthalene and chromium[15,16,17]. We elucidated the mechanism through which the naphthalene catabolic protein NahG elevated Cr (VI) reduction in strain LZ-4 by determining the chemical and enzymatic reactions
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