Particle-attached bacteria play a key role in the decomposition of complex phytoplankton polysaccharides. During a spring phytoplankton bloom at Helgoland Roads, marine microalgae (phytoplankton) dominated global carbon cycling, with significant proportions of their biomass consisting of polysaccharides. These polysaccharides were rapidly remineralized by heterotrophic bacteria, particularly particle-attached bacteria, which were more diverse and exhibited dynamic adaptive shifts over time. Metagenome-assembled genomes (MAGs) of 305 species-level bacteria were obtained, including 152 particle-attached bacteria, 100 of which were novel for the sampling site. Particle-attached bacteria had larger genomes with higher proportions of polysaccharide utilization loci (PULs), which targeted a broader range of polysaccharide substrates, including both soluble and complex polysaccharides. These bacteria were more prevalent in targeting poorly soluble or complex polysaccharides. Despite representing only 1% of all bloom-associated bacteria, particle-attached bacteria played a pivotal gatekeeping role in the solubilization and subsequent degradation of phytoplankton-derived polysaccharides. The study highlights the importance of particle-attached bacteria in the breakdown of algal polysaccharides, which are a significant component of marine microbial communities. The findings suggest that the diversity of polysaccharide niches among the most active particle-attached clades is a determining factor for the proportion of algal polysaccharides that can be rapidly remineralized during short-lived phytoplankton blooms.Particle-attached bacteria play a key role in the decomposition of complex phytoplankton polysaccharides. During a spring phytoplankton bloom at Helgoland Roads, marine microalgae (phytoplankton) dominated global carbon cycling, with significant proportions of their biomass consisting of polysaccharides. These polysaccharides were rapidly remineralized by heterotrophic bacteria, particularly particle-attached bacteria, which were more diverse and exhibited dynamic adaptive shifts over time. Metagenome-assembled genomes (MAGs) of 305 species-level bacteria were obtained, including 152 particle-attached bacteria, 100 of which were novel for the sampling site. Particle-attached bacteria had larger genomes with higher proportions of polysaccharide utilization loci (PULs), which targeted a broader range of polysaccharide substrates, including both soluble and complex polysaccharides. These bacteria were more prevalent in targeting poorly soluble or complex polysaccharides. Despite representing only 1% of all bloom-associated bacteria, particle-attached bacteria played a pivotal gatekeeping role in the solubilization and subsequent degradation of phytoplankton-derived polysaccharides. The study highlights the importance of particle-attached bacteria in the breakdown of algal polysaccharides, which are a significant component of marine microbial communities. The findings suggest that the diversity of polysaccharide niches among the most active particle-attached clades is a determining factor for the proportion of algal polysaccharides that can be rapidly remineralized during short-lived phytoplankton blooms.