# The Significance of Calcium in Photosynthesis
## 3 Ca\({}^{2+}\) Is Involved in the Processes of Photosynthetic Reaction  
Photosystem 2 (PS2) is composed of membrane-related redox enzymes, and Ca\({}^{2+}\) acts as a cofactor to participate in the formation of activation sites [21, 67]. Oxygen-evolving complex (OEC), a component of PS2, is involved in the decomposition of water molecules [68]. Extrinsic PsbQ, PsbP, and PsbO are OEC proteins [19, 22] that are closely related to CP47, \(\alpha\) subunit of cytochrome b559 and a small subunit in PS2 [69]. PsbQ and PsbP require Cl\({}^{-}\) and Ca\({}^{2+}\) as essential co-factors [19, 23]. PsbO-associated Ca\({}^{2+}\) is from the Mn\({}_{4}\)CaO\({}_{5}\) cluster [20, 21], and PsbO is closely related to the stability of the Mn\({}_{4}\)CaO\({}_{5}\) cluster [70]. Ca\({}^{2+}\) also participates in the s-state cycle, which is associated with water decomposition [71]. Many studies have suggested that the Mn\({}_{4}\)CaO\({}_{5}\) cluster is a necessary precondition for water oxidation [21, 72]. The mechanism of photosynthetic water oxidation based on calcium has been discussed in detail in other reviews [73, 74]. OEC splits water into oxygen molecules, protons, and electrons. Subsequently, electrons are transported to NADP\({}^{+}\), generating NADPH via linear electron flow (LEF); LEF probably generates ATP and NADPH for the Calvin cycle [75]. In this process, photosystem 1 (PS1) can transfer electrons of ferredoxin (FD) to NADP\({}^{+}\) and form NADPH through ferredoxin NADP\({}^{+}\) oxidoreductase (FNR) [25]. However, if FD does not transfer electrons to NADP\({}^{+}\) but passes through plastoquinone (PQ) to PS1 again, then this mode of electron transfer is called cyclic electron flow (CEF). PS1 participates in both LEF and CEF electron transfer, which plays an important role in the formation of CEF in electron transfer. PS1 is composed of multiple subunits (e.g., PsaA, -N, and -H) [76]. PsaN regulates photosynthetic electron flow through Ca\({}^{2+}\)-dependent phosphorylation [24], and it may also be related to electron transport from plastocyanin (PC) to PS1 [24]. PsaL and PsaA may also be associated with calcium [25]. FD is involved in the electron transport of PS1 [77] and has high affinity with Ca\({}^{2+}\) in its reduced state [27]. In addition, FD can interact with PsaD, PsaE, PsaC, and PsaH [26]. In microalgae and vascular plants, CEF containing two parts: proton gradient regulation 5 (PGR5)/PGR5-like photosynthetic phenotype1 (PGRL1)-dependent pathway and NADPH dehydrogenase (NDH)-related pathway [7]. [PERSON] et al. [78] confirmed that PGR5 protein is an essential part of _A. thaliana_ CEF components, and PGRL1 protein is found in _Rlnine chlamomonas_[79]. The CEF super complex is isolated from _R. chlamomonas_ containing PS1-light-harvesting (PS1-LHCI), cytochrome b\({}_{6}\)f complex (Cytbf), and FNR and PGRL1, and CAS is confirmed to be a part of the compounds [7]. CAS can interact with PGRL1 in vitro [7], and this interaction has a significant impact on CEF when CAS is downregulated in the Ca\({}^{2+}\)-dependent pathway [75]. Moreover, PGRL1 and CAS from _Chlamydomonas rheiensis_ can interact with homologous proteins from _A. thaliana_, indicating that the interaction mode between PGRL1 and CAS is conservative [80]. The NDH complex has been found in plants and cyanobacteria [81], and NDH can regulate the balance of the ATP/NADPH ratio and prevent over-reduction in electron flow [81]. [PERSON] et al. [82] reported that the external variable sequence of type 2 of NADPH dehydrogenase (NDH-2) in plants contains an EF-hand motif that binds to Ca\({}^{2+}\). NAD(P)H-dependent PQ reduction activity was found in the thylakoid membranes of potato and spinach [82]. In the chloroplast of _A. thaliana_, NADPH-dependent PQ reduction through the NDH-1 complex is strictly dependent on the presence of FD [82]. The NDH complex is activated by phosphorylating the NDH-F subunit and affects the dynamic levels of redox state of PQ [7].