This lecture covers microbial sulfur oxidation processes, including chemolithotrophic and phototrophic pathways, the role of different bacteria and archaea, and the oxidation of thiosulfate as a key intermediate. It also delves into the Sox pathway, tetrathionate intermediate pathway, and branched thiosulfate oxidation pathway, explaining the mechanisms involved in each. Additionally, it discusses the environmental significance of sulfur oxidation, particularly in the context of acid mine drainage and bioleaching.