**2.4 Control of volume and osmolarity**

The Cryptophytes do not have a cell wall, so variations in osmolarity could induce turgidity causing it to burst or plasmolysis causing the cell to compress. However, they possess a remarkable organelle, a contractile vacuole (CV), which has a rhythmic activity with diastolic and systolic cycles allowing for filling and emptying of CV, respectively [39]. This rhythmic mechanism maintains the cell volume and osmolarity, the cycle last either10s in freshwater or 40s in marine water [40]. The CV is near where the flagella structure originates and functions usually by discharging excess water and ions to the vestibulum [41]. Cryptophytes and red algae also employ another mechanism for osmolarity and volume control, synthesis of floridoside (2-O-D-glycerol-α-D-galactoside); which is a low molecular weight carbohydrate that functions as an osmolyte [41–43]; a similar mechanism is used by red algae algae's what signals an endosymbiotic inheritance [41].

## **2.5 Plastid of the cryptophytes**

The plastids in alga and plants evolved from the endosymbiosis of a cyanobacterium, which means the incorporation of one cyanobacterium in a heterotrophic cell [44]; this primary endosymbiosis explains the plastid origin in chlorophytes, glaucophytes, and red algae. Other algae stramenopiles, haptophytes, and cryptophytes

*Cryptophyte: Biology, Culture, and Biotechnological Applications DOI: http://dx.doi.org/10.5772/intechopen.107009*

**Figure 2.**

*Secondary endosymbiosis of a red algae that evolved in a cryptomonad cell. The cryptomonads have four membranes plastids; the outer is the plastid endoplasmic reticulum which surrounds plastid and nucleus. The phycobilisomes disappeared, and only one pigment not organized in a structure that is in the thylakoid lumen. Nu—nucleus, No—nucleolus, Thy—thylakoid, Nm—nucleomorph, C—chloroplastid, Ri—ribosome, CER chloroplastid endoplasmic reticulum.*

are the product of a secondary endosymbiosis by an unknown eukaryotic host and a red algal symbiont (**Figure 2**). Many scientific works confirm this hypothesis [45, 46]. These organisms possess four membranes around the plastid; the outermost membrane is thought to be the phagocytotic vacuole membrane that endocytosed the red algae and evolved to become the chloroplastic endoplasmic reticulum (CER) [47]. The CER is contiguous to the exterior nuclear membrane [9, 19, 48] (**Figure 1**), it involves the two outer membranes of the plastid, and has ribosomes on its outer surface (**Figure 1**) [49]. However, in contrast to other algae, the plastid of cryptophytes is more complex [48, 49]; between its two outer and two inner membranes, there is a space that is thought to correspond to the remains of the endocytosed red algae cytoplasm, it is called periplastid compartment (PC). One of the significant adaptations for endocytosis was the loss of genetic information of the endocytosed cell (from the nucleus and chloroplast of the red algae). However, not all nuclear information disappeared in Cryptophytes and Chlorarachniophytes, as occurred in all other secondary endosymbioses, which were sent to the host nucleus by endosymbiotic gene transfer [50, 51]. The remanent of the nuclear information is harbored in the PC and constitutes consists of a small nucleus or nucleomorph (NM). The PC also harbors eukaryotic ribosomes, and numerous starch globules produced over the pyrenoid are visible in the PC (**Figure 1A**) [49]. The plastids of the cryptophytes require that most of their proteins be nucleus-encoded, and are synthesized as precursors in the cytosol, and subsequently imported through the four membranes surrounding the plastid [47, 52]. This is possible because there is a mechanism for importing proteins that allows crossing of 2–5 membranes when the information is sent to the PC, stroma, or the thylakoid lumen. There should also be a mechanism for the retrograde pathway, from plastid to other organelles [47]; the CER functions are associated with this pathway [53, 54]. Other CER functions are related to bidirectional lipid and metabolite transfer and division [55]. The proteins directed to the plastid are synthesized as preproteins with a bipartite N-terminal signal sequence, which is used for a co-translational translocation of them across the outermost membrane, and after passing this membrane, the signal sequence is cleaved off [53]. The mechanism for passing the second membrane is possibly like the other four

membrane plastids. The cryptophytes possess a nuclear-encoded symbiont-specific ERAD machinery (endoplasmic-reticulum-associated protein degradation) and also SELMA (symbiont-derived ERAD-like machinery); the origin of these mechanisms is unclear but is being studied [47]. To reach the stroma, Toc-Tic machinery (translocon of the outer and inner membrane of chloroplasts) similar to that of chlorophytes and diatoms may need to be present; this machinery existed in the common ancestor of all Archaeplastida, organisms with primary plastids [47, 51, 53].

All cryptophytes have one NM in each PC (**Figure 1**) with a double membrane with pores similar to those in the nucleus and three chromosomes that NM replicates in coordination with the nucleus [9, 44, 56]. The understanding of the presence of NM could resolve some fundamental questions such as the phylogeny of other algae with secondary plastids that also lack this vestigial structure [48]. The position of the NM is characteristic of the Cryptophyte species [9, 49].
