Introduction

A. Seaweeds as an underexploited bioresource

Seaweeds have been used as a food in Asian countries, especially in China, Japan, and Korea, since ancient times (Chapman and Chapman, 1980; Indegaard and Minsaas, 1991; Nisizawa et al., 1987). In Western European countries, seaweeds are mainly used in the pharmaceutical, food, and cosmetics industry as a source of hydrocolloids (Indegaard and Ostgaard, 1991; Juanes and Borja, 1991). Around 16 million tons of seaweeds (fresh weight basis) and other marine plants are annually produced or collected with an estimated value of 5575 million euros (FAO, 2007) worldwide; at the same time, seaweeds are currently considered as an underexploited natural resource (Cardozo et al., 2007; Khan et al. , 2009). Moreover, seaweeds are a potential source of new biologically active substances and essential nutrients for human nutrition (MacArtain et al., 2007; Smit, 2004). Therefore, systematic studies on nutrition and health protection of specific marine algae consumed in Europe (Denis et al., 2010) and other countries are currently developed to provide the consumer with nutritional recommendations on a scientific base. These studies will also contribute to the economic exploitation of seaweeds.

B. Nutritional assessment of seaweeds

Brown and red seaweeds possess a good nutritional value and can be an alternative source of proteins, minerals, and vitamins (Jimenez-Escrig and Cambrodon, 1999; Plaza et al., 2008; Ruperez and Saura-Calixto, 2001). Oil content is generally low but contains a great amount of essential fatty acids (Gomez-Ordonez et al., 2010; Ruperez and Saura-Calixto, 2001; Sanchez-Machado et al., 2004).

Biochemical and nutritional aspects of seaweed proteins have been reported. Enzymatic degradation of algal fibers could be attempted to improve protein digestibility (Fleurence, 1999) and also to prepare bioactive peptides. A great deal of interest has been developed nowadays to isolate antihypertensive bioactive peptides, which act as angiotensin-con-verting enzyme (ACE) inhibitors because of their numerous health beneficial effects (Wijesekara and Kim, 2010).

Minerals are attributed to different ions associated with the charged polysaccharides of seaweeds. Seaweeds contain sulfate, representing different percentages of the ashes (Gómez-Ordónez et al., 2010; Rupeerez and Saura-Calixto, 2001). Sulfate anion is derived from homo- or heteropoly-saccharides in brown algae or from galactans in red ones. Sulfate seems to be a typical component of marine algal polysaccharides, related to high salt concentration in the environment and with specific functions in ionic regulation. Such sulfated mucilages are not found in land plants. Mineral bioavailability depends on the linkage type between polysaccharide and mineral and also on polysaccharide digestibility (Gomez-Ordonez et al., 2010). Typically, there is a strong positive correlation between sulfate content and biological activity of polysaccharides from seaweeds (Jiao et al., 2011).

Besides, seaweeds are considered an excellent source of dietary fiber with a high proportion of soluble to total dietary fiber (Gomez-Ordonez et al., 2010; Jimenez-Escrig and Sanchez-Muniz, 2000; Ruperez and Saura-Calixto, 2001). Dietary fiber in seaweeds is mainly composed of indigestible sulfated polysaccharides (Gomez-Ordonez et al., 2010; Ruperez et al., 2002), which are resistant to human digestive enzymes (Ruperez and Toledano, 2003). Several storage and structural polysaccharides commonly found in brown and red seaweeds are laminaran, alginate, fucan, carrageenan, and agar (Gomez-Ordonez et al., 2010; Ruperez et al., 2002). Alginates from brown seaweeds are traditionally used as hydrocolloids, while fucans are most interesting because of their biological activity (Rioux et al., 2007). Fucans from brown seaweeds are by-products in the preparation of alginates for the food and cosmetic industries (Boisson-Vidal et al., 1995). Different biological activities and potential health benefits of sulfated polysaccharides derived from marine algae have been reviewed recently ( Jiao et al., 2011; Wijesekara et al., 2011).

Seaweeds have to survive in a highly competitive environment subjected to light fluctuation, oxygen exposure, dehydration process, etc.; therefore, they develop defense strategies in different metabolic pathways. Thus marine organisms are rich sources of structurally diverse bioactive minor compounds such as carotenoids, polyphenols, minerals, vitamins, and fatty acids (Cardozo et al., 2007). Besides, they possess other major compounds such as complex carbohydrates and protein, from which bioactive sulfated polysaccharides and peptides can be isolated.

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