| Summary: | The surface ocean absorbs large quantities of the CO 2 emitted to the atmosphere from human activities. As this CO 2 dissolves in seawater, it reacts to form carbonic acid. While this phenomenon, called ocean acidification, has been found to adversely affect many calcifying organisms, some photosynthetic organisms appear to benefit from increasing [CO 2 ]. Among these is the cyanobacterium Trichodesmium , a predominant diazotroph (nitrogen-fixing) in large parts of the oligotrophic oceans, which responded with increased carbon and nitrogen fixation at elevated p CO 2 . With the mechanism underlying this CO 2 stimulation still unknown, the question arises whether this is a common response of diazotrophic cyanobacteria. In this study we therefore investigate the physiological response of Nodularia spumigena , a heterocystous bloom-forming diazotroph of the Baltic Sea, to CO 2 -induced changes in seawater carbonate chemistry. N. spumigena reacted to seawater acidification/carbonation with reduced cell division rates and nitrogen fixation rates, accompanied by significant changes in carbon and phosphorus quota and elemental composition of the formed biomass. Possible explanations for the contrasting physiological responses of Nodularia compared to Trichodesmium may be found in the different ecological strategies of non-heterocystous ( Trichodesmium ) and heterocystous ( Nodularia ) cyanobacteria.
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