Abstract
Arsenic (As), a highly toxic metalloid, naturally present in Camarones River (Atacama Desert, Chile) is a great health concern for the local population and authorities. In this study, the taxonomic and functional characterization of bacterial communities associated to metal-rich sediments from three sites of the river (sites M1, M2 and M3), showing different arsenic concentrations, were evaluated using a combination of approaches. Diversity of bacterial communities was evaluated by Illumina sequencing. Strains resistant to arsenic concentrations varying from 0.5 to 100 mM arsenite or arsenate were isolated and the presence of genes coding for enzymes involved in arsenic oxidation (aio) or reduction (arsC) investigated. Bacterial communities showed a moderate diversity which increased as arsenic concentrations decreased along the river. Sequences of the dominant taxonomic groups (abundances ≥1%) present in all three sites were affiliated to Proteobacteria (range 40.3–47.2%), Firmicutes (8.4–24.8%), Acidobacteria (10.4–17.1%), Actinobacteria (5.4–8.1%), Chloroflexi (3.9–7.5%), Planctomycetes (1.2–5.3%), Gemmatimonadetes (1.2–1.5%), and Nitrospirae (1.1–1.2%). Bacterial communities from sites M2 and M3 showed no significant differences in diversity between each other (p = 0.9753) but they were significantly more diverse than M1 (p<0.001 and p<0.001, respectively). Sequences affiliated with Proteobacteria, Firmicutes, Acidobacteria, Chloroflexi and Actinobacteria at M1 accounted for more than 89% of the total classified bacterial sequences present but these phyla were present in lesser proportions in M2 and M3 sites. Strains isolated from the sediment of sample M1, having the greatest arsenic concentration (498 mg kg-1), showed the largest percentages of arsenic oxidation and reduction. Genes aio were more frequently detected in isolates from M1 (54%), whereas arsC genes were present in almost all isolates from all three sediments, suggesting that bacterial communities play an important role in the arsenic biogeochemical cycle and detoxification of arsenical compounds. Overall, results provide further knowledge on the microbial diversity of arsenic contaminated fresh-water sediments.
Highlights
Arsenic (As) is one of the most prevalent toxic metalloids on Earth, occurring primarily as the inorganic species oxyanion arsenate (H3AsO4) [As(V)] and arsenite (H3AsO3) [As(III)]
Some bacteria, representing different phylogenetic groups involved in Astransformation use processes such as reduction, oxidation and methylation mechanism to tolerate arsenic [8,9,10,11]
Minerals of metallic sulphides containing arsenic are dissolved in the Andes Mountains, affecting superficial and ground waters that cross the Atacama Desert which are used as drinking water sources
Summary
Arsenic (As) is one of the most prevalent toxic metalloids on Earth, occurring primarily as the inorganic species oxyanion arsenate (H3AsO4) [As(V)] and arsenite (H3AsO3) [As(III)]. Arsenic present in the environment originates from both natural and anthropogenic sources. Microorganisms in As-rich environments have evolved mechanisms to utilize arsenic for metabolic processes or to detoxify the cell. They influence the biochemical cycle of As, bio-transforming As-species varying in solubility, mobility, bioavailability and toxicity [5,6,7]. In response to toxic concentrations of heavy metals, many aquatic organisms, including microorganisms, can develop tolerance. Sometimes called dissimilatory arsenate respiration, a respiratory arsenate reductase consisting of two subunits (ArrA and ArrB) is responsible for the reduction of arsenate. The reductase is encoded by the arr operon, which always includes the arrA and arrB genes, with some strains containing an additional membrane subunit ArrC [13]
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