Sustainable Approach to Eradicate the Inhibitory Effect of Free-Cyanide on Simultaneous Nitrification and Aerobic Denitrification during Wastewater Treatment

Heterologous Expression of Cyanobacterial Cyanase Gene (CYN) in Microalga Chlamydomonas reinhardtii for Bioremediation of Cyanide Pollution

Simple Summary

Cyanide is a known toxic compound produced through natural and anthropogenic activities. Water can be polluted by cyanide ions through wastewater effluents. In high concentrations, cyanide is considered a strong metabolic inhibitor and can cause inhibition in mitochondrial complex IV (cytochrome c oxidase), and its assimilation can result in chronic poisoning and/or acute poisoning to humans and animals. Bioremediation systems involving the usage of transgenic algal systems have become preferable alternatives for the detoxification of cyanide contamination due to the accumulation and the biosorbent efficiency of transgenic Chlamydomonas in the removal of KCNO from fresh water.

Abstract

Recombinant DNA technology offered the creation of new combinations of DNA segments that are not found together in nature. The present study aimed to produce an ecofriendly bioremediation model to remediate cyanide pollution from a polluted marine system. Cyanide is a known toxic compound produced through natural and anthropogenic activities. An Agrobacterium-tumefaciens-mediated genetic transformation technique was used to generate transformed Chlamydomonas reinhardtii using plant expression vector pTRA-K-cTp carries isolated coding sequence of the cyanobacterial cyanase gene (CYN) isolated from Synechococcus elongatus (PCC6803). qRT-PCR analysis showed the overexpression of CYN in transgenic C. reinhardtii, as compared with the respective wild type. Growth parameters and biochemical analyses were performed under cyanide stress conditions using transgenic and wild C. reinhardtii for evaluating the effect of the presence of the cyanobacterial cyanase gene in algae. The transgenic C. reinhardtii strain (TC. reinhardtii-2) showed promising results for cyanide bioremediation in polluted water samples. Cyanide depletion assays and algal growth showed a significant resistance in the transgenic type against cyanide stress, as compared to the wild type. Genetically modified alga showed the ability to phytoremediate a high level of potassium cyanide (up to150 mg/L), as compared to the wild type. The presence of the CYN gene has induced a protection response in TC. Reinhardtii-2, which was shown in the results of growth parameter analyses. Therefore, the present study affirms that transgenic C. reinhardtii by the CYN coding gene is a potential effective ecofriendly bioremediator model for the remediation of cyanide pollutants in fresh water.

Heterotrophic nitrification and aerobic denitrification process: Promising but a long way to go in the wastewater treatment

The traditional biological nitrogen removal (BNR) follows the conventional scheme of sequential nitrification and denitrification. In recent years, novel processes such as anaerobic ammonia oxidation (anammox), complete oxidation of ammonia to nitrate in one organism (comammox), heterotrophic nitrification and aerobic denitrification (HN-AD), and dissimilatory nitrate reduction to ammonium (DNRA) are gaining tremendous attention after the discovery of metabolically versatile bacteria. Among them, HN-AD offers several advantages because individual bacteria could achieve one-stage nitrogen removal under aerobic conditions in the presence of organic carbon. In this review, besides classical BNR processes, we summarized the existing literature on HN-AD bacteria which have been isolated from diverse habitats. A particular focus was given on the diversity and physiology of HN-AD bacteria, influences of physiological and biochemical factors on their growth, nitrogen removal performances, as well as limitations and strategies in unraveling HN-AD metabolic pathways. We also presented case studies of HN-AD application in wastewater treatment facilities, pointed out forthcoming challenges of HN-AD in these systems, and presented modulation strategies for HN-AD application in engineering. This review may help improve the existing design of wastewater treatment plants by harnessing HN-AD bacteria for effective nitrogen removal.

Involvement of the cbb3-Type Terminal Oxidase in Growth Competition of Bacteria, Biofilm Formation, and in Switching between Denitrification and Aerobic Respiration

Paracoccus denitrificans has a branched electron transport chain with three terminal oxidases transferring electrons to molecular oxygen, namely aa₃-type and cbb₃-type cytochrome c oxidases and ba₃-type ubiquinol oxidase. In the present study, we focused on strains expressing only one of these enzymes. The competition experiments showed that possession of cbb₃-type oxidase confers significant fitness advantage during oxygen-limited growth and supports the biofilm lifestyle. The aa₃-type oxidase was shown to allow rapid aerobic growth at a high oxygen supply. Activity of the denitrification pathway that had been expressed in cells grown anaerobically with nitrate was fully inhibitable by oxygen only in wild-type and cbb₃ strains, while in strains aa₃ and ba₃ dinitrogen production from nitrate and oxygen consumption occurred simultaneously. Together, the results highlight the importance of the cbb₃-type oxidase for the denitrification phenotype and suggest a way of obtaining novel bacterial strains capable of aerobic denitrification.