Arsenic and sulfur mineralization is a natural phenomenon occurring in hydrothermal sys-tems where parameters like temperature and organic matter (OM) can influence the mo-bilization of the toxic metalloid in marine environments.In the present study we analyze the influence of temperature and OM (particularly sulfur-containing additives) on As and S precipitation based on the recent discovery of As-rich nanoparticles in the hydrothermal system near the coast of the Greek island Milos.To this end,we experimentally recreate the formation of amorphous colloidal particles rich in As and S via acidification (pH 3-4) of aque-ous precursors at various temperatures.At higher temperatures,we observe the formation of monodisperse particles within the first 24 h of the experiment,generating colloidal parti-cles with diameters close to 160 nm.The S:As ratio and particle size of the synthetized par-ticles closely correlates with values for AsxSy particles detected in the hydrothermal system off Milos.Furthermore,organic sulfur containing additives (cysteine and glutathione,GSH)are a key factor in the process of nucleation and growth of amorphous colloidal AsxSy par-ticles and,together with the temperature gradient present in shallow hydrothermal vents,dictate the stabilization of As-bearing nanomaterials in the environment.Based on these findings,we present a simple model that summarizes our new insights into the formation and mobility of colloidal As in aquatic ecosystems.In this context,amorphous AsxSy parti-cles can present harmful effects to micro-and macro-biota not foreseen in bulk As material.