Latin America, like other areas in the world, is faced with the problem of high arsenic (As) background in surface and groundwater, with impacts on human health. We studied As biogeochemical cycling by periphyton in Lake titicaca and the mine-impacted Lake Uru Uru. As concentration was measured in water, sediment, totora plants (Schoenoplectus californicus) and periphyton growing on stems, and As speciation was determined by X-ray absorption spectroscopy in bulk and eDtA-extracted periphyton. Dissolved arsenic was between 5.0 and 15 μg L −1 in Lake Titicaca and reached 78.5 μg L −1 in Lake Uru Uru. As accumulation in periphyton was highly variable. We report the highest As bioaccumulation factors ever measured (BAFs periphyton up to 245,000) in one zone of Lake Titicaca, with As present as As(V) and monomethyl-As (MMA(V)). Non-accumulating periphyton found in the other sites presented BAFs periphyton between 1281 and 11,962, with As present as As(III), As(V) and arsenosugars. DNA analysis evidenced several taxa possibly related to this phenomenon. Further screening of bacterial and algal isolates would be necessary to identify the organism(s) responsible for As hyperaccumulation. Impacts on the ecosystem and human health appear limited, but such organisms or consortia would be of great interest for the treatment of As contaminated water. The arsenic (As) geogenic background of surface and groundwater is naturally high in South America, predominantly originating from young volcanic rocks and their weathering products in arid oxidizing conditions 1-4. As a result, about 4.5 million people in South America are chronically exposed to high levels of As (>50 µg L −1) 5 , and certain Andean populations have developed a unique capacity to adapt to As toxicity 6,7. Concerning Andean lakes, extreme As concentrations are observed in hypersaline lakes colonized by extremophile bacterial communities 8 , and lower but still significant concentrations are observed in other, less saline lakes, which are major freshwater resources 5. In many areas of the Altiplano, mining and smelting activities add to natural rock weathering processes in the As budget 9. The biogeochemical cycling of As has been studied in freshwater and marine ecosystems, and in hypersaline environments, but its trophic transfer and speciation in living organisms mainly concerns the marine environment and As contaminated freshwater systems 10. In the high altitude lakes of the Andean Altiplano (above 3500 m asl), shallow zones (<2 m) are colonized by totoras (Schoenoplectus californicus, syn Scirpus californicus). These macrophytes were used for construction purposes in traditional Andean culture. Nowadays, they are mainly used as cattle fodder and have been tested successfully in constructed wetlands in North America for the removal of metals (Zn, Cu, Cd, Pb) and nutrients from wastewater 11-14. The filtration potential of wetland plants does not rely on absorption by the plant, but on physico-chemical and biologically driven processes taking place on submerged stems and in the rhizosphere 15,16. In particular, the periphyton, an assemblage of algae and bacteria forming a