Sargassum algae accumulate a high concentration of arsenic, posing a serious threat to pollute coastal ecosystems. The process of As accumulation, distribution and metabolization in algae cells needs to be understood in order to assess the toxicity and the environmental effects. Preliminary results indicate heterogeneous distribution of the arsenic in the cell, with high arsenic accumulation on cell walls. However, the form in which arsenic binds to the wall is not inferred from only bulk speciation. Thus, we aim to characterize the As speciation in the cell compartments of Sargassum algae, focusing on the differences in species between the cell walls and the cell internal organelles, to elucidate the mechanisms of As binding and storage. Moreover, the As speciation during the seaweed degradation will be studied by comparing samples collected from the open sea and from the stockpiled in the beach, which will make possible to assess the As potential environmental effects.