The arsenic is a toxic element for the human and other organisms´ health, it may affect the exploitation of water, the agriculture development and the sustainable rational use of soil, the result is reflected within the socioeconomic growth lag in the affected area. Arsenic is mobilized in the environment by a combination of several natural processes, such as weathering reactions, biological activities and volcanic emissions, as well as by a group of anthropogenic activities. Microorganisms

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... ract with the arsenic through a different mechanism such as adsorption, redox, precipitation, etc. On the other hand, the rate of arsenic releasing in geochemical environments is controlled mainly by microorganisms. In the environment, the major arsenic transformations include microbial oxidation, reduction, methylation and demethylation, those reactions have an enormous impact in the arsenic behavior that depends on its redox state that exhibits differences in its mobility and toxicity. Normally, the arsenic resistance comes through the production of genes within operon ars, their location is usually plasmid or chromosomic. In this study, it was achieved the isolation of 11 strains from which 2 have been reported as an arsenic resistant in this work. The samples were collected from water, sediment and biofilm of Xichu River, which has been impacted by mining activity, exhibiting a high arsenic concentration (98 µg/l). All the strains were tested by several methods as arsenic resistance, colorimetric test for arsenate reduction, pDNA extraction as well as ars and aox genes amplification, which are the genes involved in arsenic transformation. It was obtained a great microbial diversity with different skills in arsenic transformation. One of the isolated strains Rodoccoccus gordoniae, did not show the presence of any gen but it was the most likely microorganisms to grow in the presence of arsenic. Almost all the microorganisms showed arsenic resistant until 20 mM of arsenite and arsenate. The main contribution of this study [...]