Czerny, Jan (2008) Influence of future CO2 concentrations on growth and nitrogen fixation in the bloom-forming cyanobacterium Nodularia spumigena. (Diploma thesis), Christian-Albrechts-Universität, Kiel, Germany, 81 pp.

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Abstract

Besides the well-known atmospheric climate change, anthropogenic CO2 emissions are resulting in elevated CO2 concentrations and acidification in the surface ocean. Phytoplankton physiological responses to these chemical alterations are influencing ocean primary production and thus the fixation of CO2 and its transport into deep waters. Diazotrophic cyanobacteria play a special role in this process since they are able to provide the ecosystem with nitrogen; the nutrient limiting primary production in most of the ocean regions. Future changes in carbon and nitrogen fixation can influence the ocean in its function as the most important sink for anthropogenic CO2. Stimulating effects of elevated [CO2] on carbon fixation have been observed in eukaryotic and prokaryotic phytoplankton species as well as in natural phytoplankton assemblages. For the non-heterocystic oceanic cyanobacterium Trichodesmium it was shown that not only carbon fixation but also nitrogen fixation rates rise in response to high [CO2]. Whether this is a general pattern in diazotrophic cyanobacteria is unknown. This study shows that Nodularia spumigena, an important heterocystic bloom forming cyanobacterium of the Baltic Sea, reacts to elevated [CO2] and corresponding acidification as projected for the year 2100 with strongly impeded growth and reduced nitrogen fixation. These effects were accompanied by storage of nutrients and significant changes in the elemental composition of the cells. Carbon and phosphorus cellular content increased about twice as much under high [CO2] than cellular nitrogen content. Findings of impeded growth of N. spumigena under lowered ambient pH are contrasting the findings in marine phytoplankton but show similarities to several studies on heterocystic bloom forming cyanobacteria in freshwater. Aggregation and the formation of surface scums are typical for blooms of N. spumigena and its freshwater relatives. Regarding the preference for high pH N. spumigena is hypothesized to be adapted to a microclimate caused by photosynthetic activity under high growth densities. However, as the cyanobacterial filaments are initially suspended in the water column, first aggregation and bloom development of N. spumigena will probably be delayed under future [CO2] and pH.

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