Characterization of arsenate reduction by arsenic tolerant lvllcrobacterjum foljoruj'vf strain szllsolated from gold ores

Abstract

Arsenic is a metalloid of global concern that primarily exists in two inorganic forms of severe toxicity, As (III) and As (V). The reduction of As (V) to As (III) increases toxicity, mobility and bioavailability of arsenic. Understanding how microorganisms reduce As (V) is important to elucidate As (V) reduction mechanism and inevitably, discover approaches to minimise its toxic impact on the environment. This study was aimed at investigating the capability of arsenic tolerant Microbacterium foliorum strain SZ1 isolated from gold ores to undergo As (V) reduction to As (III). This strain demonstrated complete reduction of 1 mM As (V) achieved within 120 hours under aerobic condition indicating a possible mechanism of detoxification through regulation of ars operon. Further optimization of factors enhancing As (V) reduction capacity of strain SZ1 resulted in complete reduction of 1 mM As (V) achieved within 36 hours in Tris minimal medium supplemented with 10 mM sucrose and 0.1 % (w/v) tryptone at pH 7. The effect of cell adaptation or acclimation towards As (V) reduction was investigated. Well-adapted strain SZ1 recorded complete reduction of 0.5 mM As (V) to 3 mM As (V) within 18 hours to 42 hours incubation. Exopolysaccharides (EPS) was observed to be secreted during reduction of As (V) and subjected to further characterization through chemical analysis of neutral carbohydrate and protein contents and Fourier transform infra-red (FT-IR) analysis. As As (V) concentration increased, so did the protein and carbohydrates concentration of EPS, indicating that EPS played an important role in enabling strain SZ1 to resist and reduce arsenic. Haldane inhibition model was used to fit the reduction rate at different initial As (V) concentrations and the parameters µmax, Ks and Ki were determined to be 0.14 h-1, 0.39 mM and 35.3 mM, respectively. In addition, presence of As (III) as the final product was further confirmed by detection through high performance liquid chromatography (HPLC) analysis. Field emission scanning electron microscopy analysis (FESEM) showed that cells grown in the presence of As (V) exhibited distinct changes in cell morphology and presence of EPS. Exploration of the draft genome of M. foliorum SZ1 identified the presence of ars operon (arsC-arsC-ACR3-arsT-arsC-arsR-arsC) and another two stand-alone genes, arsC and arsB which further confirmed SZ1’s tolerance towards high concentration of arsenic. From the screening of plant growth promoting (PGP) traits, strain SZ1 was able to produce siderophores and indole acetic acid which highlighted its potential use in microbe-assisted arsenic phytoremediation. This is the first study that elucidates the characterization of As (V) reduction by M. foliorum SZ1.