Diazotrophic filamentous cyanobacteria dominate the phytoplankton community in the Baltic Sea in summer. They shape the nutrient cycles in this ecosystem and affect the usability of the Baltic Sea for tourism and fisheries. Phosphorus (P) is essential for cyanobacteria growth, bloom formation, carbon- and nitrogen fixation and regulates these functions. However, it is still largely unknown how this regulation works and what mechanisms are involved. The current knowledge is based on their C:N:P stoichiometry or parameters, such as alkaline phosphatase activity. These are indicators for the P-based nutrition state; but are unable to provide information how the regulation functions and what structures are involved. The aim of the proposed study is to investigate  how P-containing cellular structures of the main Baltic Sea cyanobacteria (Nodularia spumigena, Aphanizomenon sp.) are involved in the regulation of growth, carbon and nitrogen fixation and how it is related to  dissolved inorganic and organic P concentrations in the surrounding water and to the cellular C:N:P stoichiometry. We focus on polyphosphates, phospholipids and ATP which are cellular components with high phosphorus content and can influence the cell stoichiometry significantly. Moreover, these cellular constituents are fundamental to cell growth and metabolism, therefore, disturbance of their normal levels can lead to important functional and structural alterations.

The studies will be done in batch experiments under naturally occurring P concentrations. Gradients of nutrients, biomass, and the C:N:P stoichiometry will be produced by the growth of cyanobacteria itself, similar to runs in the ecosystem.  We expect from these studies, a higher and more specific knowledge gained through the P regulation of cyanobacteria and their possible reaction to changing conditions in the future. The study shall help to forecast the possible reaction of the cyanobacteria under future conditions. Our findings will be crucial for improving the biogeochemical model simulations used to predict cyanobacteria bloom events under projected climate conditions, since the anticipated increases in CO₂ levels and temperature are expected to favor cyanobacteria growth. The proposed study is included in the Research Program (Research Focus I) of the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) and in the Phosphorus Campus, a joint project of the IOW and the University of Rostock under the umbrella of the Leibniz Association.

The profound scientific experience of the participating researchers in studying numerous aspects of the P cycle in the Baltic Sea and elsewhere combined with the innovative methodological approaches proposed herein together with the close cooperation within the IOW will ensure the success of the planned investigations.