During the expedition M200, a team of German and US scientists will conduct an examination of the link between manganese (Mn) cycling and other elemental redox cycles in the Baltic Sea’s oxygen-depleted basins. This work is linked to a cruise in summer 2020 on the R/VBorgese that first characterized the distribution and specia-tion of Mn throughout various basins within the Baltic Sea. On M200 we will investigate the biogeochemical processes driving turn-over of Mn at a subset of these stations, and will establish links between Mn redox cy-cling and other redox couples across the redoxcline. As the primary reactive Mn phases are predicted to exist within the sub-oxic zone, the focus will be on the links be-tween nitrogen (N), reactive oxygen species (ROS), and iodine (I). We will also explore links between Mn and the sulfur and carbon cycle across these zones. We will target microbial communities involved via cultur-ing approaches, and aim to understand the physical and geochemical controls regulat-ing these interactions. Starting from Rostock we will sample sta-tions in the Gotland Basin and the Landsort or Fårö Deeps. This cruise will focus on two sites, where we will conduct high resolution profiling of physical and geochemical redox structure and chemical speciation of select-ed elements, including water sampling for laboratory analyses. We will perform extensive shipboard incu-bations targeting rates of transformations, and enrich and culture microorganisms, e.g. phototrophic Mn oxidizing organisms, and microbes capable of coupling Mn redox changes to N and methane cycling. In paral-lel, water mass mixing on the measured ge-ochemical distributions will be studied. The overall goal is to achieve a mechanistic understanding of complex redox interac-tions in the Baltic redox transition zones, and comparable marine systems.