"Large, filamentous nitrogen-fixing cyanobacteria (Trichodesmium spp., Nodularia spumigena and Aphanizomenon spp.) play an important role for primary production, input of nitrogen, and the microbial food webs in the ocean, in brackish waters (e.g. the Baltic Sea) and in lakes. Gas vesicles within filaments make these cyanobacteria positively buoyant to reach high light intensities in surface waters where they cover the energy needed for nitrogen fixation through photosynthesis. These large cyanobacteria create their own microenvironment with steep gradients of gases and nutrients at the sea surface. Hence, their growth conditions are significantly different to those of the bulk water. Direct measurements of small-scale physical-chemical characteristics and biogeochemical processes associated with large, filamentous nitrogen-fixing cyanobacteria are hence crucial to understand the role that these cyanobacteria have on a large scale in respect to their overall input of carbon and nitrogen from the atmosphere into aquatic systems. The present project aims at a detailed understanding of the carbon and nitrogen fluxes in aggregates of large, nitrogen-fixing cyanobacteria and their associated heterotrophic community."