**1. Introduction**

Cryptophytes or cryptomonads are eukaryote algae that are biflagellate and unicellular, with sizes between 3 and 50 μm, most are photosynthetic and motile, and a few are palmelloid and form colonies surrounded by mucilaginous sheaths [1]. They are classified into the kingdom Chromista, phylum Cryptophyta, class Cryptophyceae, and order Cryptomonadales. They live in environments from fresh to marine water usually of good quality [2, 3]; also they can be found at varying light conditions and at different temperatures, including those that are extreme such as the Antarctic [4], blooms of cryptophytes have been reported in fresh [5] and marine waters [6]. Cryptophyte's genus *Goniomonad* lacks plastids; they are heterotrophic and feed on bacteria and small organic particles. Due to their small size and biochemical composition, Cryptophytes are essential contributors to the food chains of a diversity of organisms [7–10]. Some cryptophytes are plastid donors for dinoflagellates like *Dinophysis acuminata* [11] and ciliates like *Mesodinium rubrum* and *M. major* [12]. Their importance has been underestimated mainly due to their delicate structure, which can be easily altered or broken by common fixatives such as Lugol

and formalin. However, cryptophyte inter-species morphology is not that different allowing for species-level taxonomy by light microscopy [13, 14].

The abundance of cryptophytes is increasing in places like the Antarctic Peninsula [6] and Chesapeake Bay [12], where a notable change has been observed in species composition and size distribution, significantly influencing local ecosystems.

There is general agreement that cryptophytes evolved from a secondary endosymbiosis, which occurred by the engulfment of a red alga by an unknown eukaryote [15, 16]; this event resulted in a cell with two nuclei, two cytoplasms, one of each is in the chloroplast, which is covered with four membranes, and with unique content and distribution among algae of harvesting-light pigments, they have chlorophylls *a* and *c*2, phycocyanin (PCY) or phycoerythrin (PER) [17, 18]. The cryptophytes are complex cells with specific movements and unique structures that allow easy recognition. The main morphological characteristics are slightly ovoid asymmetric cells, two asymmetric flagella with mastigonemes bipartite, an internal and an external periplast plate that surrounds the cell membrane, the structure of the furrow/gullet (groove/throat) [19], ejecti- or ejectosomes that allows them to suddenly alter its swimming direction in the opposite direction [1, 3, 8, 20].

The composition of the cryptophytes, especially in fatty acid, phycobiliproteins (PBPs), and carbohydrates, has attracted the attention of aquaculture and diverse business sectors, such as pharmaceutical, nutraceutical, chemical, and cosmetic industries [21, 22]. The culture of these cells and the production of these substances have some difficulties to overcome, as these delicate cells have a low growth rate compared to other cells in the market.

This chapter will address the Cryptophytes, mainly focusing on those that are photosynthetic, observing their biology, biochemical composition, culture systems, and some of their products with antioxidant potential, as well as fluorescent pigments which are of increasing interest as a marker in biotechnology applications.
