**1. Introduction**

An ecosystem is qualified as extreme when it's physicochemical parameters are most often hostile to life Grégoire, Fardeau, Guasco, Bouanane, Michotey and Bonin [1]. Indeed, any biotope characterized by a very low or very high value of the main parameters that influence their life cycle can be characterized as an extreme environment [2]. These parameters are essentially temperature, salinity, pH, pressure, radiation, desiccation, and oxygenation. Organisms with the ability to live in extreme environments are called "extremophiles". And as a result, several groups have been described taking into account the extreme conditions they can tolerate [1]. These are essentially prokaryotic microorganisms, mostly belonging to the Archaea

group. Eukaryotes can also be recorded in extreme environments. They are essentially unicellular algal or fungal organisms [3].

Among these environments, hypersaline ecosystems are very widespread and they can be classified into natural and artificial biotopes. While the natural environments are essentially represented by salt lakes, lagoon and, Sabkhas, the artificial hypersaline environments are represented by saltworks. These latter are transitional ecosystems between the marine and the continental domain [4], consisting of shallow ponds used for the production of halite (NaCl) from seawater which is pumped to the first series of ponds. After an evaporation cause a sufficient increase in salinity, the water is transferred to the next series of ponds, and so on, until brine saturated with NaCl is obtained, from which the halite precipitates in the last series of ponds recognized the crystallization ponds. The salinity in each of the ponds is thus maintained more or less constant over time [5]. This process leads to the selection of the variety of microbial heterotrophs and autotrophs and ciliated protozoa [4]. Species were adapted to different salinity variations [6]. The Sfax solar saltern (Tunisia) is an artificial paralic ecosystem characterized by its floristic and faunal richness [7], as well as by its microalgae richness [8, 9]. This biotope has been the site of several studies since 1998: (i) microalgae [10], (ii) ciliates [11] and (iii) zooplankton [12–14] especially the branchiopod crustacean *Artemia salina* [15, 16]. The cultures of microalgae sampled from the Sfax solar saltern have been the subject of several studies [17–20].

Microalgae are very rarely grouped according to their energy metabolism or even according to their ability to synthesize the necessary metabolites, but rather according to their morphological properties [21]. There are therefore different taxonomic classes of microalgae, the main ones being Rhodophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae, Dinophyceae and Cyanobacteria. Microalgae occupy a very important place in nature since they are at the base of a long food chain and contain impressive nutritional proerties [22]. Moreover, they have various fields of exploitation, due to the value-added molecules. The biochemical composition of microalgae proves that they contain high value natural fatty acids (omega-3), which can produce a high value dietary supplement [23]. Furthermore, microalgae contain a high amount of proteins reaching up to 70% of the dry matter for *Spirulina* and also producing mineral elements such as calcium and magnesium [24]. Also, extremophile microalgae have many substances recognized by their bioactive properties such as antiviral, antiproliferative and anticancer properties [25]. These biomolecule possess a powerful antioxidant effect as determined by several authors [26, 27]. Finally, microalgae are largely used in wastewater treatment [28].

In this chapter we will present the biodiversity of the halophilic microalgae of the Sfax solar saltern and the different techniques used for the isolation and valorization of culture or metabolites extracted from three microalgal species.
