Arsenic in the Antofagasta Region of Chile is worldwide recognized as highly As contaminated area. Due to its abundance, mainly as As (V) and As (III), certain microorganisms such as several types of bacteria have evolved and developed the necessary genetic components which confer resistance mechanisms. These mechanisms include reactions of reduction, oxidation and methylation. The aim of this work was to study the most relevant arsenic resistant bacteria that exist in highly arsenic contaminated sediments in El Tatio geyser field. This place is a suitable habitat to study the adaptation of endemic species subjected to extreme environmental conditions. All bacterial strains isolated were grown with increasing concentration of arsenate, exhibiting high levels of arsenate resistance ranging from to 5 to 30 mM. Inductively coupled plasma mass spectrometry (ICP-MS) was employed to determine the concentration of As within intact cells of each bacterial strain. Results showed a great accumulation of this element. The separation of bacterial cells into cytoplasmic and membrane fractions were carried out by differential centrifugation in order to know the distribution of arsenic in the different cellular compartments. Most of arsenic was mainly located in the cytoplasmic fraction. Indeed, the optimization of chromatographic methods coupled to ICP-MS allowed us to separate and quantify the different arsenic species as a result of bacterial transformations. The results demonstrated that in half of the isolated strains, between 20-70% of arsenate was reduced to arsenite. Only in one case it was observed the presence of methylated species of arsenic such as DMA and MMA.
