Free calcium (Ca²⁺) ions can be highly toxic as they can precipitate phosphate ions, which are important for various metabolic pathways. Therefore, cyanobacteria regulate their intracellular Ca²⁺-concentration [Ca²⁺]_i via pumps, channels and calcium binding proteins.^1,2 In multicellular cyanobacteria, Ca²⁺ signaling also plays an important role in heterocyst differentiation, a cellular compartment required for atmospheric N₂ fixation.³ Shortly after nitrogen depletion, a condition that triggers heterocyst differentiation, a transient increase of [Ca_²⁺]_i ⁴ in pro-heterocysts have been reported.³ In the filamentous cyanobacterium Nostoc sp. PCC 7120 two calcium binding proteins have been identified: CcbP (cyanobacterial calcium binding protein)³ and CSE (Ca²⁺ Sensor EF-hand).⁵ CcbP is mainly known for its buffer property in capturing free Ca²⁺ ions. CSE binds Ca²⁺ via two characteristic Ca²⁺ sensor EF hand domains^.5 CSE appears unfolded in the absence of calcium and undergoes a strong conformation change upon Ca²⁺ binding. CSE is strongly downregulated during nitrogen depletion.^5,6 Although there is a strong connection between Ca²⁺ signaling and heterocyst differentiation, the specific functions of both calcium binding proteins, CcbP and CSE, remain elusive. Our aim is to further investigate the role of calcium signaling and calcium homeostasis in multicellular cyanobacteria with respect to heterocyst formation and photosynthesis. Therefore, we investigate the characteristics of knockout or overexpressing mutants of those proteins in Nostoc sp. with respect to cell-cell-communication, growth under nitrogen limitation and heterocyst formation.

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