Particle-attached bacteria play a crucial role in the decomposition of complex polysaccharides from phytoplankton during spring blooms. The study analyzed bacterial communities in different size fractions (free-living, 0.2–3 μm; particle-attached, 3–10 μm and >10 μm) during a 2018 spring phytoplankton bloom at Helgoland Roads, southern North Sea. Free-living bacteria included clades like *Aurantiivirga*, “Formosa”, *Cd*, *Prostillococcus*, and SAR11, while particle-attached bacteria dominated clades such as BD1-7, *Stappiaceae*, *Nitrinicolaaceae*, and *Polaribacter*. Particle-attached bacteria were more diverse and showed dynamic shifts in taxonomic composition and polysaccharide-targeting enzyme repertoires. A total of 305 metagenome-assembled genomes were obtained, including 152 particle-attached bacteria, 100 of which were novel. These bacteria had larger genomes with higher proportions of polysaccharide utilization loci, suggesting they targeted a broader range of polysaccharides, including complex ones like xyans, cellulose, and pectins. Particle-attached bacteria, despite being only 1% of all bloom-associated bacteria, played a pivotal gatekeeping role in solubilizing and degrading phytoplankton-derived polysaccharides. The study highlights the importance of particle-attached bacteria in breaking down complex polysaccharides, which are crucial for carbon cycling in marine ecosystems. The findings suggest that these bacteria are essential for the efficient decomposition of algal polysaccharides, contributing significantly to the global carbon cycle.Particle-attached bacteria play a crucial role in the decomposition of complex polysaccharides from phytoplankton during spring blooms. The study analyzed bacterial communities in different size fractions (free-living, 0.2–3 μm; particle-attached, 3–10 μm and >10 μm) during a 2018 spring phytoplankton bloom at Helgoland Roads, southern North Sea. Free-living bacteria included clades like *Aurantiivirga*, “Formosa”, *Cd*, *Prostillococcus*, and SAR11, while particle-attached bacteria dominated clades such as BD1-7, *Stappiaceae*, *Nitrinicolaaceae*, and *Polaribacter*. Particle-attached bacteria were more diverse and showed dynamic shifts in taxonomic composition and polysaccharide-targeting enzyme repertoires. A total of 305 metagenome-assembled genomes were obtained, including 152 particle-attached bacteria, 100 of which were novel. These bacteria had larger genomes with higher proportions of polysaccharide utilization loci, suggesting they targeted a broader range of polysaccharides, including complex ones like xyans, cellulose, and pectins. Particle-attached bacteria, despite being only 1% of all bloom-associated bacteria, played a pivotal gatekeeping role in solubilizing and degrading phytoplankton-derived polysaccharides. The study highlights the importance of particle-attached bacteria in breaking down complex polysaccharides, which are crucial for carbon cycling in marine ecosystems. The findings suggest that these bacteria are essential for the efficient decomposition of algal polysaccharides, contributing significantly to the global carbon cycle.