Colony-forming and single-cell picocyanobacteria nitrogen acquisition strategies and carbon fixation in the brackish Baltic Sea

Picocyanobacteria are widespread and globally significant primary producers. In brackish waters, picocyanobacterial populations are composed of diverse species with both single-cell and colony-forming lifestyles. Compared to their marine counterparts, brackish picocyanobacteria are less well characterized and the focus of research has been weighted toward single-cell picocyanobacteria. Here, we investigate the uptake dynamics of single and colony-forming picocyanobacteria using incubations with dual carbon-13 and inorganic (ammonium and nitrate) or organic (urea and amino acids) nitrogen-15 sources during August and September 2020 in the central Baltic Sea. Phytoplankton community and group-specific uptake rates were obtained using an elemental analyzer isotope ratio mass spectrometer (EA-IRMS) and nano secondary-ion mass spectrometry (NanoSIMS). Picocyanobacteria contributed greater than one third of the ammonium, urea, amino acids, and inorganic carbon community uptake/fixation in September but < 10% in August when phytoplankton biomass was higher. Overall, single-cell ammonium and urea uptake rates were significantly higher for single-celled compared to colonial picocyanobacteria. In a 6-yr offshore central Baltic Sea time series (2015–2020), summer abundances of colonial picocyanobacteria reached up to 10⁵ cells mL⁻¹ and represented > 5% of the average phytoplankton biomass, suggesting that they are periodically important for the ecosystem. Colonial strain identification was not distinguishable using 16S rRNA gene amplicon data, highlighting a need for refined tools for identification of colonial forms. This study shows the significance of single-celled brackish picocyanobacteria to nutrient cycling and the importance of considering uptake and lifestyle strategies when assessing the role of picocyanobacteria in aquatic ecosystems.