# Investigations of the Energy Transfer
## 1 Introduction  
_Arthrospira (Spirulina) platensis_ is planktonic, photoautotrophic, filamentous cyanobacteria, which is morphologically characterized by multicellular spiral trichomes [1]. From an economic perspective, _A. platensis_ is considered to be one of the most valuable cyanobacteria due to its large-scale cultivation and biotechnological potential. The rich content of micro- and macro-nutrients, along with a high content of valuable compounds such as proteins, lipids, and pigments, made _A. platensis_ an interesting microorganism for industrial sectors such as cosmetics, pharmaceuticals, biomedical, and food. With few exceptions, cyanobacterial phycobiliproteins are found in vivo assembled in huge macromolecular complexes, which are known as phycobilisomes (PBSs). The PBSs are the main light harvesting antenna systems found in cyanobacteria and red algae, which are unique complexes among all photosynthetic organisms that are located on the stromal face of the thylakoid membrane [2]. The PBS is a macromolecular complex formed by soluble, colored proteins, known as phycobiliproteins (PBPs) and linker polypeptides [3; 4]. PBPs are the only light-harvesting proteins that covalently bind the chromophores, and based on their spectral properties, they are classified in four types: phycocyanin (PC; \(\lambda\) max absorption = 610-625 nm) [5], phycoerythrin (PE; \(\lambda\) max = 540-570 nm), phycoerythrocyanin (PEC; \(\lambda\) max = 560-600 nm), and allophycocyanin (APC; \(\lambda\) max = 650-660 nm) [3; 6; 7]. The PBS can be found in several shapes, from hemidiscoidal and hemielipsoidal to bundle shape. Regardless of their shape, the basic structure of the PBS can be divided into two structural different compartments, consisting of a cylindrical core from which several rods of stacked disks radiate out [7]. The core is always formed by APC and can contain three to five cylinders [6]. The number of rods varies greatlyfrom six, usually in cyanobacteria [4], to 14 in some red algae [8]. Rods are typically assembled from PC and in some species, PE and PEC are added [2]. Phycobiliproteins are organized in a trimeric discoidal structure, which is constructed from \(\alpha\)/\(\beta\) polypeptide heterodimers [6]. Typically, two trimeric disks are stacked together to form a hexamer, which is the building block of the PBS [9]. Linker proteins are colorless polypeptides and are vital for the normal functioning of the PBS complex. Their main roles are to direct the proper assembly of the complex and to modulate energy flow between chromophores [3; 10; 11]. The unique assembly mode of PBSs allows only the unidirectional energy transfer from higher energy chromophores to lower ones. As a result of this, the energy flows from the rods of PE/PC to the APC core and finally to the photosystem through the L\({}_{\text{cm}}\) final emitter [3; 7; 12]. The PBPs have a broad absorption spectrum, narrowing the absorption gap of chlorophyll, and as such, increasing the total energy that can be used in photosynthesis [4]. Absorption spectra of different PBPs coincide with the excitation spectra, so pigments that absorb at lower wavelengths transfer and concentrate their energy to the pigments with higher wavelengths and low energy. This is possible only if the pigments are at a favorable distance and have the right orientation of the dipole moments [13]. In _A. platensis_, the PBS has a hemidiscoidal shape with a tricylindrical core and six radiating rods. The core is formed by stacked discs of APC, while the rods are formed by PC hexamers [14]. The number of hexamers on each rod has a high variability, from one to three hexamers, being strongly influenced by the cultivation conditions. In recent years, the PBPs from _A. platensis_, especially PC, captured the attention of the energy sector thanks to its use as a photosensitizer in solar cells [15]. In addition to PC, [PERSON] et al. used the entire PBS as solar concentrator for a photo voltaic device [16]. To extend the applicability of the PBPs from _A.