The protein content in algae can be as high as 47% of the dry weight [35] , but these levels vary according to the season and the species. The protein content of brown algae is generally low (5-15% of the dry weight), whereas higher protein contents are recorded for green and red algae (10-30% of the dry weight). Except for brown algae U. pinnatifida which has a protein level between 11 and 24% (dry weight) [35]. Higher protein level were recorded for red algae, such as Porphyra tenera

(33-47% of dry mass) [35] or Palmariapalmata (8-35 of dry mass) [121]. These levels are comparable to those found in soybean.

There are studies about the variation of protein content of marine algae as a func­tion of the seasonal period [1, 39]. Higher protein levels were observed during the end of the winter period and spring whereas lower amounts were recorded during summer.

The in vivo digestibility of algal protein is not well documented, and available studies about their assimilation by humans have not provided conclusive results. However, several researchers have described a high rate of alga protein degradation in vitro by proteolytic enzymes. For instance, the relative digestibility of alkali — soluble proteins from P. tenera is higher than 70% [38] . On the other hand, some compounds limiting the digestibility of alga proteins, such as phenolic compounds or polysaccharides, have been described. Studies performed on brown algae show the strong inhibitory action of soluble fiber on in vitro pepsin activity and their negative effects on protein digestibility [59] .

Typical amino acid composition of different species of algae is outlined in Table 2 according to Dawczynski et al. [23] . The quality of food protein depends on its essential amino acids. These algae present high concentration of arginine, valine, leucine, lysine, threonine, isoleucine, glycine, and alanine, although the predomi­nant amino acids are glutamine and asparagine. Glutamine and asparagine exhibit interesting properties in flavor development, and glutamine is the main responsible in the taste sensation of “Umami.”

The concentration of essential amino acids, such as, threonine, valine, isoleucine, leucine, phenyl alanine, lysine, and methionine, are higher in U. pinnatifida than in Laminaria sp. U. pinnatifida has higher concentrations of Lysine that has Hizikia fusiforme and Laminaria sp. has higher concentrations of Cysteine than has

U. pinnatifida. Interestingly, taurine is not a typical component of traditional European food and taurine content represents a nutrient feature which is character­istic of red algae, such as Phorphyra sp. Taurine is detected at low concentrations in brown algae varieties.

In general, algae possess proteins that have a high nutritional value since they contained all the essential amino acids in significant amounts (see Table 2).

The organoleptic characteristic of algae are principally due to their free amino acid profile [126], which in turn depends on environmental factors in its culture grounds [44]. Generally, the free amino acid fraction of algae is mainly composed of alanine, aminobutyric acid, taurine, ornithine, citrulline, and hydroxyproline [89].

Other proteins present only in red and blue-green algae are phycobiliproteins (phycocyanin in blue-green algae, phycoerythrin in red algae), a group of protein involved in photosynthesis. Purified phycobiliproteins can have several uses, such as cosmetics, colorants in food, and fluorescent labels, in different analytical tech­niques [33, 138], These proteins are characterized by having a tetrapyrrolic pig­ment, called phycobilin, covalently attached to their structure. Important medical and pharmacological properties, such as hepatoprotective, anti-inflammatory, and antioxidant properties [9, 10, 156], have been described and are thought to be basi­cally related to the presence of phycobilin. Besides, phycobiliproteins might have an important role in different photodynamic therapies of various cancerous tumors and leukemia treatment [157] , Different works have been aimed to the selective extraction and analysis of the phycobiliproteins from algae, such as Herrero et al. [53] and Simo et al. [174], that identified the two subunits of each protein, namely allophycocyanin-a, allophycocyanin-b, c-phycocyanin-a, and c-phycocyanin-b, from S. platensis. In the red microalga Porphyridium spp., the red-colored pigment phycoerythrin [62, 195] has been described.