During the last decade recurring dense blooms of the toxic dinoflagellate Alexandrium ostenfeldii have increasingly been reported in shallow coastal waters of the Baltic Proper. The blooms typically occur when inorganic nutrient concentrations (e.g. nitrate, NO3) are low but dissolved organic nitrogen (DON) is available in high concentrations and in waters overgrown by vegetation.The pathway of DON uptake during bloom development and peak is unknown, as are the interaction between the dinoflagellate and its co-occurring bacterial community. What bacterial community co-exist with the highly toxic A. ostenfeldii and does an algal-bacteria interaction facilitate DON uptake? This project investigates the interactions between bacteria and A. ostenfeldii during the initial part and peak of the bloom using molecular tools. Metagenomics will identify and characterize the bacterial community during bloom development and metatranscriptomics connect gene expression and gene functions of N utilization and metabolism to community succession and toxicity over time.