**2.6 Pigments and light harvesting by red algae endosymbiont and cryptophytes**

Like all phytoplankton, cryptophytes have chlorophyll *a* as the primary lightharvesting pigment [57], and other accessory pigments ∝-carotene, alloxanthin, chlorophyll c2, and the PBPs for the capture of low light intensity in wavelengths not well absorbed by chlorophyll *a* (500–650 nm) [17, 58, 59].

The red algae and cyanobacteria have a color that depends on the predominance of PBPs, the orange-red PER or the blue PCY, which are in several hundred and are highly organized in supramolecular complexes, the phycobilisomes (PBS) (**Figure 2**). The PBS are their main light-harvesting antennae and cover the stromal surface of thylakoids [60]; the PBSs have mobility that lets them distribute the absorbed energy between photosystems (PSI and PSII) [61]. PBPs are composed of two kinds of α and β protein subunits and are more stable in trimer (3α + 3β) or hexamer (6α + 6β). The protein part of apoprotein is covalently bonded to a chromophore or phycobilin [32, 60, 62], these chomoproteins are united to colorless linker polypeptides [63], and constitute the light-harvesting antenna for transferring energy to chlorophyll *a* to PSII and possibly to PSI [32, 60, 62].

Like red algae, cryptophytes have PBPs pigments, but they do not have PBSs, and, they only produce one kind of PBP pigment per cell (Cr-PE or Cr-PC), packed into the thylakoid lumen [17, 64] without any arrangement [32, 65, 66], it gives the cell a red or blue color [9, 18, 19], but cells possess other accessory pigments, allowing them to display a great diversity of colors [67]. The ratio of Chlorophyll a: PBPs of cryptophytes can be several times higher than that of non-PBPs pigments [18]. The endosymbiosis provided the cryptophytes with new machinery that allowed diversification of light capture [65]; the PBPs are an auxiliary or second light-harvesting system, allowing them to occupy light spectra niches for more efficient light capture [65].

The PBPs of the cryptophytes are composed of two α and two β protein subunits and four linear tetrapyrrole chromophores or phycobilins covalently bonded by one or two thioether bonds to specific cysteine residues on the protein [68]; these PBPs provide unique spectral properties of absorption and emission fluorescence (**Table 1**).

The number and location of phycobilins within the protein are the primary factors that determine the visible absorption, the fluorescence spectrum, and the energy transfer pathway for any given PBP [66]. The complex of chromophores and protein subunits is a complete light-capturing unit; the α subunits of PBPs are encoded in the nuclear genome (derived from the ancestral host), whereas the β subunits are encoded in the plastid as in red algae, so the PBPs are unique chromoprotein


*Adapted from table VI [67, 68]. The PBPs are named, including the wavelengths of maximum absorption [9, 17].N.R. = no reported.*

#### **Table 1.**

*Different classes of phycobiliprotein (PBPs) in cryptophytes, spectral ranges of the main absorption and fluorescence emission maxima at visible wavelengths (nm) (Cr, cryptophytes; PE, phycoerythrin; PC, phycocyanin); and the actual Cryptomonad genera.*

complexes that originated from secondary endosymbiosis [17, 65]. Another function of PBPs in cryptophytes is to help them with photoacclimation; this process involves changes in PBPs concentrations and shifts in the PBPs absorbance peaks when they are grown under red, blue, or green light [58]. The anterior means the photosynthetic system of cryptophytes is very different from other algae [75, 76], the location of PBPs in thylakoid lumen, the presence of dimeric PBPs forms, and the principal connection to PSII, but the precise mechanism remains to be discovered.

Another structure in the chloroplast is the pyrenoid, where the enzyme RUBISCO responsibly for CO2 fixing is located, there is one pyrenoid per plastid, and its position is an identifying characteristic of each species (**Figure 1**) [31].
