Have you ever dreamed of going to an anoxic world? This thesis takes you on a fascinating journey along the steep oxygen gradient at the deepest point of the Baltic Sea, the Landsort Deep. On your trip from the oxygenated surface water into the anoxic deep water and sediment, you will meet many microorganisms whose importance for sustaining ecosystem functions cannot be underestimated. Meet, for example, the organic matter degraders, fermenters, sulphate reducers, methane producers and methane oxidisers. Find out if they are more susceptible to share genetic material compared to other microorganisms. And finally, are the Cyanobacteria in the dark and anoxic sediment dead or alive? Petter Thureborn carries out research in the field of microbial ecology with a special focus on the Baltic Sea. This is his PhD thesis.