Interactions between bacteria and phytoplankton can influence primary production, community composition, and algal bloom development. However, these interactions are poorly described for many consortia, particularly for freshwater bloom-forming cyanobacteria. Here, we assessed the gene content and expression of two uncultivated from Lake Erie blooms. These organisms were targeted because they were previously identified as important catalase producers in blooms, suggesting that they protect from HO. Metatranscriptomics revealed that both transcribed genes for uptake of organic compounds that are known cyanobacterial products and exudates, including lactate, glycolate, amino acids, peptides, and cobalamins. Expressed genes for amino acid metabolism and peptide transport and degradation suggest that use of amino acids and peptides by may regenerate nitrogen for cyanobacteria and other organisms. The genomes lacked genes for biosynthesis of cobalamins but expressed genes for its transport and remodeling. This indicates that the obtained cobalamins externally, potentially from , which has a complete gene repertoire for pseudocobalamin biosynthesis; expressed them in field samples; and produced pseudocobalamin in axenic culture. Both were detected in blooms worldwide. Together, the data support the hypotheses that uncultured and previously unidentified taxa exchange metabolites with phytoplankton during harmful cyanobacterial blooms and influence nitrogen available to phytoplankton. Thus, novel may play a role in cyanobacterial physiology and bloom development. Interactions between heterotrophic bacteria and phytoplankton influence competition and successions between phytoplankton taxa, thereby influencing ecosystem-wide processes such as carbon cycling and algal bloom development. The cyanobacterium forms harmful blooms in freshwaters worldwide and grows in buoyant colonies that harbor other bacteria in their phycospheres. Bacteria in the phycosphere and in the surrounding community likely influence physiology and ecology and thus the development of freshwater harmful cyanobacterial blooms. However, the impacts and mechanisms of interaction between bacteria and are not fully understood. This study explores the mechanisms of interaction between and uncultured members of its phycosphere with population genome resolution to investigate the cooccurrence of and freshwater in blooms worldwide.