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Photosynthesis and nitrogen fixation are ones of the most vital processes on earth. Almost all living organisms depend on a source of combined nitrogen. Often nutrients are short in supply and in natural waters the biological activity is usually limited by the availability of N2. However, this is not the case when the N2-fixating cyanobacteria are the predominant primary producers.

To investigate this nitrogenase activity the acetylene reduction method is most widely used. The method is based on the property of nitrogenase to reduce compounds with a triple bond. In addition to reduction of N2 to NH4+, nitrogenase reduces acetylene to ethylene.

For the first time, a high-resolution light action spectra were recorded for nitrogenase activity in cyanobacteria. Response to changes in acetylene and oxygen concentrations and in light intensity is investigated with a time resolution of 20 seconds, revealing many aspects of the dynamics of the nitrogen fixation process.