Efficient bacterial isolate from roots of cactus degrading Reactive Black 5

Highlights

• A bacterial strain capable of degrading RB5 was isolated from nodules of Cactus.

• The strain efficiently degraded RB5 with metabolites detected by GC–MS.

• The strain identified as Proteus mirabilis and placed in phylogenetic position through 16S rRNA gene analysis.

• The effective degradation was achieved up to 100 ppm of dye.

Abstract

The current study aimed at azo dye degradation using a facultative anaerobe. A bacterial strain capable of decolorization and degradation of RB5 under static conditions was isolated from root nodules of Cactus. The strain efficiently degraded Reactive Black 5 (RB5) with metabolites detected by GC–MS. The strain was identified as Proteus mirabilis and placed in phylogenetic position through 16S rRNA gene analysis. The degradation increased with time to achieve up to 90 % after 72 h incubation at 37 ºC and pH value of 7. Variations of C source did not affect the process while yeast extract was effective. The effective degradation was achieved at 100 ppm of dye. The GC–MS detected metabolites included; acetaldoxime, oxirane trimethyl, acetic acid-2-propenyl ester, acetaldehyde semi carbazone, 2-pentanone, 4-hydroxy-4-methyl, hydrazine, 1-5-hexenyl-1-methyl and enanthamide Protox and phytotoxicity results showed fewer toxic ranges of these metabolites as compared to products of benzene and amines. The isolated strain can be useful in the bioremediation of azo dyes contaminated environments.

Introduction

Humans are naturally attracted to colored items like textiles, papers, leathers, and cosmetics. Initially, natural dyes were used to satisfy this esthetic sense but with the advancement of chemical technology, synthetic dyes were more appreciated by industrialists and consumers (Shah, 2014). Production of dyestuff is about 1 $\times$ 10⁶ tons per year globally and 70% of these are azo dyes (Sethi et al., 2012). Synthetic dyes have some properties that make them more demanding than natural dyes, such as a wide range of colors, low cost, rapidity, brightness, and easy production and application. Thus natural dyes have almost replaced synthetic dyes (Shipla and Shikah, 2015). Despite having so many advantages synthetic dyes have a low fixation rate thus effluents released from these industries are colored. During synthesis and dyeing processes, about 10%–15% is lost, of which about 50% are azo dyes (Sethi et al., 2012). The colored untreated effluent or treated by conventional methods is released into water bodies which eventually comes in contact with soils, plants, aquatic lives, animals, and humans (Shipla and Shikah, 2015).

Conventional treatment methods like photo-degradation, chemical degradation, and flocculation are of limited use and have high operational costs. Color is readily detected even in 10–20 mg L⁻¹ concentration and photosynthetic activity is reduced due to reduced sunlight penetration in water (Ayed et al., 2010). This group of dyes and their metabolites are considered as possible mutagens and carcinogens. Moreover, these are not readily degradable in the natural environment and are resistant to temperature (high or low), light, acidic, and alkaline conditions (Imran et al., 2014). Due to these hazardous effects on the environment and biological life legislations are made stringent (Sethi et al., 2012). There are at least 3000 types of azo dyes and these lie in the group of recalcitrant molecules as they have sulfonate group and azo bond (Sethi et al., 2012). RB5, a model azo organic pollutant was chosen to assess the performance of biodegradation (Elizalde-González et al., 2012).

Biological methods have progressively increased due to their limited amount of sludge production, environmental friendliness, and low cost (Chen et al., 1999). Acclimatized microbes decolorize and degrade azo dyes by breaking the azo bond in a reductive reaction to produce aromatic amines which are mutagenic and carcinogenic. Some potential microbes mineralize these metabolites of azo dyes under aerobic conditions (Sethi et al., 2012). Anaerobic bacteria, however, decolorize the dye nonspecifically and efficiently as compared to aerobic decolorization thus are more useful candidates (Cui et al., 2014). So, there is a great need to discover such cheap microbial biomass to reduce these pollutants from water.