Microbes are the fundamental drivers of Earth’s biogeochemical cycles. Their enormous taxonomic, functional and metabolic diversity are of essential importance for ecosystem functioning − from cellular to community scales, over time and space. Among the multitude of microbial metabolisms in the oceans, polysaccharide degradation is one key process, featuring distinct substrate niches and bacteria-algae interactions. Nonetheless, many aspects regarding the distribution of polysaccharide-degrading taxa and their genetic regulation remain open. One central question is how hydrolytic abilities, and the diversity of metabolic functions in general, affect microbiome assembly over time. Studying temporal variability is especially important in vulnerable and changing systems such as the polar oceans, where the climate crisis exerts substantial pressure on biological communities. This Habilitation summarizes my research on bacterial polysaccharide degradation, intraspecific diversity, and microbiome seasonality in the Arctic Ocean. The enclosed studies present interdisciplinary insights into the biogeography, identity, genetic repertoire, and regulatory dynamics of polysaccharide degraders on cellular, microhabitat and ocean-wide scales. Experimental incubations revealed distinct “hydrolytic community fingerprints” in the Atlantic, Pacific, and Southern Oceans. Furthermore, studying the genetic machinery and cellular regulation under both simple and complex substrate conditions, including dissolved and particulate polysaccharides, illuminated the microscale underpinnings of larger community dynamics. This integrated molecular and culture-based evidence contributes to a conceptual perspective on polysaccharide utilization in contrasting marine systems. The establishment of model organisms was essential for studying genetic regulation and intraspecific diversity; addressing hydrolytic capacities and other traits including siderophore production, aromatics degradation, and metabolite secretion – mediators of central element cycles and chemical ecology. Connecting genotypes to niches furthermore contributed to broader eco-evolutionary concepts on species delineation and population assembly. Finally, my research characterized microbial communities over seasonal and environmental gradients in the Arctic Ocean. Time-series observations via autonomous devices revealed community dynamics over polar day and night, and across different sea-ice and polar water regimes. This microbial inventory in the environmental context establishes a baseline of Arctic microbial ecology, and allows benchmarking future ecosystem shifts. Overall, the presented research contributes important conclusions for the understanding of microbial diversity and biogeochemical functions in cellular, spatial and temporal dimensions, underlining the relevance of microbes for ecosystem functioning in the current and future ocean.