Abstract
Simple SummarySynthetic azo dyes are used in various industries like apparel, food, paper, etc. The improper discharge of these recalcitrant dyes poses a serious threat to the environment. However, in nature, certain microorganisms including cyanobacteria can degrade these recalcitrant compounds by producing various reactive oxygen species (ROS), particularly by hydrogen peroxide (H2O2). This study aimed at developing a genetically modified cyanobacterium for better H2O2 accumulation that results due to superoxide radicals dismutation by enzyme superoxide dismutase (SOD). The modified strain (MS-sodC+) was created by integrating Cu/Zn SOD gene (sodC) from another cyanobacterium, and its expression was driven by a strong constitutive psbA gene promoter. The modified strain (MS-sodC+) resulted in over-accumulation of H2O2 during azo dye treatment with a higher rate of dye removal than the wild-type strain (WS-sodC). Therefore, to encourage cyanobacteria for biodegradation of toxic azo dyes faster than the natural rate enhanced H2O2 accumulation through SODs, overexpression may serve as a valuable bioremediation tool.Discharge of recalcitrant azo dyes to the environment poses a serious threat to environmental health. However certain microorganisms in nature have developed their survival strategies by degrading these toxic dyes. Cyanobacteria are one such prokaryotic, photosynthetic group of microorganisms that degrade various xenobiotic compounds, due to their capability to produce various reactive oxygen species (ROS), and particularly the hydrogen peroxide (H2O2) when released in their milieu. The accumulation of H2O2 is the result of the dismutation of superoxide radicals by the enzyme superoxide dismutase (SOD). In this study, we have genetically modified the cyanobacterium Synechococcus elongatus PCC 7942 by integrating Cu/Zn SOD gene (sodC) from Synechococcus sp. PCC 9311 to its neutral site through homologous recombination. The overexpression of sodC in the derivative strain was driven using a strong constitutive promoter of the psbA gene. The derivative strain resulted in constitutive production of sodC, which was induced further during dye-treated growth. The genetically engineered Synechococcus elongatus PCC 7942 (MS-sodC+) over-accumulated H2O2 during azo dye treatment with a higher dye removal rate than the wild-type strain (WS-sodC−). Therefore, enhanced H2O2 accumulation through SODs overexpression in cyanobacteria may serve as a valuable bioremediation tool.
Highlights
Synthetic dyes including azo dyes are recalcitrant compounds and their usage in various industries like apparel, food, paper, etc., have increased significantly, which pose a serious threat to the environment
All experiments related to dye decolorization and degradation were carried out with three individuals cyanobacterial transformants, and the results presented are expressed as averages ± standard error (SE) of triplicate samples
Improper discharge of synthetic azo dyes poses a serious threat to the environment because of its recalcitrant nature due to the presence of highly reactive amine (N=N) and sulfonic (–SO3 − ) groups [25,26,27]
Summary
Synthetic dyes including azo dyes are recalcitrant compounds and their usage in various industries like apparel, food, paper, etc., have increased significantly, which pose a serious threat to the environment. These dyes are chemically and photolytically stable, are highly persistent in natural environments, and their presence in water and soil is undesirable due to their appearance and toxicity [1]. Cyanobacteria, being oxygenic photosynthetic organisms, inevitably generate various reactive oxygen species (ROS) by their photosynthetic electron transport [5] These ROS include free radicals such as superoxide anion (O2− ), hydroxyl radical ( OH), as well as non-radical molecules such as hydrogen peroxide (H2 O2 ) and singlet oxygen (1 O2 , delta state), and their induced production are an unavoidable consequence of stress conditions [6]. Production of H2 O2 by cyanobacteria is not surprising, as cyanobacteria photo produce O2 at a higher rate
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