In "Journey to the Microcosmos: Life Without Oxygen? Challenge Accepted," viewers explore how microorganisms survive in mud and sediment where oxygen disappears. The video explains the oxic-anoxic interface, oxygen and sulfur gradients, and how microbial metabolism shifts in anoxic environments. Featured organisms include spirochetes, the ciliate Caenomorpha, purple sulfur bacteria such as Thiospirillum, and large sulfur bacteria including Achromatium oxaliferum and Macromonas. Key concepts include electron acceptors, photosynthesis, sulfide diffusion, calcite and calcium oxalate storage, and microbial adaptation to changing chemical conditions. This biology and microbiology content supports curriculum topics in cells, ecosystems, biochemistry, and the cycling of matter in aquatic and sediment environments. Part of the "Journey to the Microcosmos" series.