Cyanobacteria fix atmospheric CO2 into sugars using water and sunlight in a process 
called photosynthesis, generating O2 as a by-product. Cyanobacteria not only 
contribute substantially to global primary production, but also to the global nitrogen 
cycle. Nitrogen fixation is a particular feature of a wide range of bacteria and archaea. 
Filamentous cyanobacterial strains spatially separate oxygenic photosynthesis and 
anoxic nitrogen fixation in two different cell types - the vegetative cells and 
specialised cells called heterocysts, respectively. As a model organism for 
cyanobacterial photosynthesis and nitrogen fixation, Anabaena (Nostoc) sp. PCC 
7120 was used in this thesis to study the effects of Ca2+ on regulation mechanisms of 
the metabolic equilibrium between the two most abundant elements - carbon and 
nitrogen - whose ratio is an indicator of the metabolic performance. 
Ca2+ is a ubiquitous second messenger, enzyme co-factor and component of 
membranes in eukaryotic cells and organisms, such as mammals and plants. 
However, the importance of Ca2+ in cellular processes in prokaryotes, particularly in 
cyanobacteria, is not as well understood. High amounts of Ca2+ are known to be toxic, 
therefore, the concentration of free intracellular Ca2+ is tightly regulated by Ca2+ 
channels and pumps, and Ca2+-binding proteins, respectively. Results in this thesis 
have demonstrated that Ca2+ is a key regulator of the intracellular carbon and 
nitrogen balance, through regulation of gene expression of the respective carbon and 
nitrogen uptake transporters, which is vital for metabolic homeostasis of the cells. A 
novel Ca2+-binding protein (CSE) in Anabaena sp. PCC 7120 was described and 
connected to the maintenance of the metabolic balance between photosynthetic 
light reactions and nitrogen fixation, based on the characterisation of the cse gene 
and protein. The CSE protein shows many features of a Ca2+ sensor and regulator 
protein and thus is likely to interact with a protein partner upon sensing changes of 
intracellular Ca2+ levels. Overexpression of CSE impaired photosynthetic electron 
transport across the thylakoid membrane, possibly through defects in the proper 
assembly of the light-harvesting complexes, called phycobilisomes in cyanobacteria, 
and damage of photosystem II dimers. Deletion of CSE, in contrast, severely 
disturbed heterocyst development and filament integrity, possibly through the lack 
of functional Ca2+ signalling during early heterocyst differentiation or the release of 
Ca2+ ions due to the knockout of this Ca2+-binding protein. These results indicate a 
possible dual function of CSE as a Ca2+ buffer as well as a Ca2+ sensor protein.