Important biogeochemical processes take place in anoxic marine waters and sediments. Sulfur-cycling microorganisms such as sulfur-oxidizing and sulfate-reducing bacteria (SOB, SRB) play a key role. However, their microbial and metabolic diversity has not been mapped out. Furthermore, other sulfur-cycling microorganisms such as anaerobic sulfated polysaccharide degraders have remained completely out of sight. This thesis is an exploration of sulfur-cycling microorganisms in the Black Sea, a model ecosystem and the world’s largest anoxic marine basin. Genome-resolved metagenomics yielded an improved view on the diversity of putative SOB and SRB in the anoxic water column and hypotheses on their metabolic capacities. Further, a novel SRB and two novel anaerobic sulfated polysaccharide-degrading bacteria were isolated through anaerobic cultivation. A genome-based characterization revealed several interesting metabolic aspects, such as manganese reduction and exopolysaccharide formation. Finally, the mechanism of sulfated polysaccharide degradation was studied in depth by transcriptomics, indicating a novel L-fucose degradation pathway.