Abstract
Extensive use of chromium (Cr) and arsenic (As) based preservatives from the leather tanning industry in Pakistan has had a deleterious effect on the soils surrounding production facilities. Bacteria have been shown to be an active component in the geochemical cycling of both Cr and As, but it is unknown how these compounds affect microbial community composition or the prevalence and form of metal resistance. Therefore, we sought to understand the effects that long-term exposure to As and Cr had on the diversity and structure of soil microbial communities. Soils from three spatially isolated tanning facilities in the Punjab province of Pakistan were analyzed. The structure, diversity and abundance of microbial 16S rRNA genes were highly influenced by the concentration and presence of hexavalent chromium (Cr (VI)) and arsenic. When compared to control soils, contaminated soils were dominated by Proteobacteria while Actinobacteria and Acidobacteria (which are generally abundant in pristine soils) were minor components of the bacterial community. Shifts in community composition were significant and revealed that Cr (VI)-containing soils were more similar to each other than to As contaminated soils lacking Cr (VI). Diversity of the arsenic resistance genes, arsB and ACR3 were also determined. Results showed that ACR3 becomes less diverse as arsenic concentrations increase with a single OTU dominating at the highest concentration. Chronic exposure to either Cr or As not only alters the composition of the soil bacterial community in general, but affects the arsenic resistant individuals in different ways.
Highlights
Anthropogenic metal contamination is a problem frequently encountered near long-term industrialized areas [1]
We found that in the presence of metals novel Proteobacteria were abundant community members and that arsenic concentration dramatically influenced the diversity of arsenic resistance genes
Principal Components Analysis revealed that much of the variance in soil properties and chemistry between the contaminated sites was associated with pH, arsenic, total chromium, and Cr (VI) levels, while control soils were correlated with di and monovalent cations, bicarbonate, and phosphate concentrations (Figure 1)
Summary
Anthropogenic metal contamination is a problem frequently encountered near long-term industrialized areas [1]. In Pakistan, the leather industry has left soils surrounding production facilities contaminated with chromium (VI & III) and arsenic. Accumulation of Cr (VI) and arsenic in soils is of concern due to their mutagenic and carcinogenic properties in humans [2,3]. Chromium is found as chromite [Cr(III)] or chromate [Cr(VI)] [8]. Chromite is less toxic and poorly mobile due to its low water solubility at neutral pH and its sorption characteristics [8]. Chromate is acutely toxic, mutagenic, teratogenic and carcinogenic (USEPA, 1998b). It is soluble in water and mobile in soils and sediments [8]. Cr (VI) resistance is thought to involve the use of a chromium efflux pump [9,10] while detoxification (i.e. reduction to Cr (III)) is less understood but several genes have been proposed [11]
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