Elusive marine Verrucomicrobiota: Seasonally abundant members of the novel genera Seribacter and Chordibacter specialize in degrading sulfated glycans.

Members of the phylum Verrucomicrobiota play a significant role in various ecosystems, yet they are underrepresented in databases due to their comparatively lower abundance and isolation challenges. The use of cultivation-independent approaches has unveiled their hidden diversity and specialized metabolic capabilities, yet many of these populations remain uncharacterized. In this study, we focus on members of the family MB11C04 associated with North Sea spring blooms. Our analyses revealed recurrent MB11C04 populations with increased abundance in the late stages of spring blooms over ten-years. By examining their genomic content, we identified specialized genetic features for the degradation of complex polysaccharides, particularly sulfated and fucose-rich compounds, suggesting their role in utilizing organic matter during the collapse of the bloom. Furthermore, we describe two novel genera each with a novel species (Seribacter gen. Nov., Chordibacter gen. Nov.) in accordance with the SeqCode initiative based on high quality metagenome-assembled genomes. We also propose a new name for the family MB11C04, Seribacteraceae. Our findings shed light on the ecological significance and metabolic potential of Verrucomicrobiota populations in spring bloom events.