**1. Introduction**

Cyanophycin, abbreviated CGP (cyanophycin granule peptide), is next to poly-γ-glutamic acid and poly-ε-lysine, the third polyamino acid known to occur in nature [1]. It serves as a nitrogen/carbon reserve polymer in many cyanobacterial strains as well as in a few heterotrophic bacteria. CGP consists of the two amino acids, aspartate and arginine, forming a poly-l-aspartic acid backbone with arginine side chains. The arginine residues are linked to the β-carboxyl group of every aspartyl moiety via isopeptide bond [2].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CGP was discovered in 1887 by the botanist Antonio Borzi during microscopic studies of filamentous cyanobacteria [3]. He observed opaque and light scattering inclusions by using light microscopy and created the name *cianoficina*. Early electron microscopic studies showed a strong structure variation of the CGP granules, depending on the fixatives and poststains used during electron microscopic examinations [4, 5]. This led to a controversy about the ultrastructure of these inclusions until the 1970s. Later, electron microscopic studies described CGP granules as membrane less, electron dense and highly structured cytoplasmic inclusions [6, 7].

With a C/N ratio of 2:1, CGP is extremely rich in nitrogen and consequently an excellent nitrogen storage compound. During the degradation of CGP and subsequent degradation of arginine, a function as energy source was also proposed [8].
