Fundamentals and Application Potential of Arsenic-Resistant Bacteria for Bioremediation in Rhizosphere: A Review

Abstract

As a hazardous environmental metalloid toxicant, arsenic (As)-at elevated levels in water and soil-has created a major public health concern through its entry into the food chain by accumulation in crops. Among the various methods reported thus far for reclamation of As-contaminated crop fields, bioremediation using bacteria with plantgrowth-promoting traits has been found to be a most promising solution. There is every possibility that bacterial isolates with the ability to remove or immobilize As could be used for successful bioremediation. However, bioremediation needs to define its boundaries between promise and field application, as most studies have been restricted to laboratory results only. Rhizosphere interactions play a critical role in monitoring As bioavailability to crop plants, thus a better understanding of it might improve rhizoremediation technologies. The challenges rely on the application of these novel approaches under field conditions. Despite some limitations, the prospect for successful stimulation and exploitation of microbial metabolism for As rhizoremediation appears to be very promising.

Molecular Insight Into Key Eco-Physiological Process in Bioremediating and Plant-Growth-Promoting Bacteria

“…Among all other bacterial isolates, arsenite oxidation system (another prime mechanism for arsenic resistance) is well-characterized in Alcaligenes faecalis where the first "arsenic gene island" was reported (Silver and Phung, 2005b;Phung et al, 2012). Arsenite oxidase is a heterodimeric protein that consists of two subunits: one large molybdopterin-containing subunit with an iron (3Fe-4S) center encoded by asoA/aoxA and another small rieske active subunit (Anderson et al, 1992) encoded asoB/aoxB genes (Silver and Phung, 2005b;Phung et al, 2012;Mallick et al, 2015). Upstream of the aoxA, there is a Twin Arginine Translocation (TAT) leader sequence which is involved in assemblage of arsenite oxidase and its translocation from cytoplasm to periplasm (Mallick et al, 2015).…”

“…Arsenite oxidase is a heterodimeric protein that consists of two subunits: one large molybdopterin-containing subunit with an iron (3Fe-4S) center encoded by asoA/aoxA and another small rieske active subunit (Anderson et al, 1992) encoded asoB/aoxB genes (Silver and Phung, 2005b;Phung et al, 2012;Mallick et al, 2015). Upstream of the aoxA, there is a Twin Arginine Translocation (TAT) leader sequence which is involved in assemblage of arsenite oxidase and its translocation from cytoplasm to periplasm (Mallick et al, 2015). On the other hand, an anaerobic form of arsenate reductase (Arr) is a heterodimer (Mallick et al, 2015) and is encoded by arrA and arrB genes that were primarily characterized in Shewanella spp.…”

“…Upstream of the aoxA, there is a Twin Arginine Translocation (TAT) leader sequence which is involved in assemblage of arsenite oxidase and its translocation from cytoplasm to periplasm (Mallick et al, 2015). On the other hand, an anaerobic form of arsenate reductase (Arr) is a heterodimer (Mallick et al, 2015) and is encoded by arrA and arrB genes that were primarily characterized in Shewanella spp. and in Bacillus selenitireducens (see Figure 2) (Afkar et al, 2003).…”

“…For example, Babu et al (2015) reported the presence of gene bmtA (encoding metallothionein) in Pseudompnas koreensis, along with other arsenic resistance genes. On the other hand, methylation of arsenic and its subsequent volatilization is a multistep complex process and information is significantly lacking in this regard (Mallick et al, 2015). Another gene, namely arsM, was found to be responsible for arsenic methylation (Bertin et al, 2011).…”