Seaweeds are known to be high in mineral content. More than 30% of the dry weight of marine algae is ash which contains various kinds of minerals, as they are bathed in the rich seawater. Some of the minerals are necessary for our health while some are toxic in varying degrees. Most of the macroalgae have high Ca, Mg, P, K, Na, and Fe contents [116], as can be seen in Table 4.

In comparison with higher plants, their outstanding feature is their high iodine content [89]. Seaweeds are the best natural sources of biomolecular dietary I. Some seaweeds contain 1,000 times as much iodine as found in a marine fish like cod. Seaweeds provide di-iodotyrosin (I2T) which is precursor to essential thyroid hor­mones thyroxine (T4) and triiodothyronine (T3) [14] .

The mineral content in general is highly dependant on the environmental growing conditions (season, temperature, physiological state, geographic varia­tions…). For example, in a recent study of Porphyra and Laminaria from France, Spain, Korea, and Japan [152] it was found, by using ICP-MS, that seaweeds from Korea and Japan tended to display the highest concentrations of Pb and Cd. In contrast, Spanish and French samples showed the highest levels of some microelements essential to human nutrition. Moreover, Porphyra presented

higher concentrations of most elements (Cd, Co, Cr, Mo, Ni, Pb, Sb, Se, and V), except for As, than Laminaria.

However, the linkage of certain minerals with anionic polysaccharides (alginate, agar, or carrageenan) might limit the absorption and extraction of these minerals. In such cases, mineral availability is a function of the type of linkage between the polysaccharide and the mineral. For instance, the weakness of the linkages between polysaccharides and iodine allows rapid release of this element. In contrast, the strong affinity of divalent cations (particularly Ca2+) for carboxylic polysaccharides (alginates) probably limits the availability of associated minerals. From a nutri­tional standpoint, this high affinity might be compensated by the high mineral con­tents of seaweeds [95].

Other compounds with proven bioactivity are those related with photosynthesis, mainly pigments such as chlorophylls, carotenoids or proteins like opsins. Among them chlorophylls are the most wide spread compounds. Chlorophylls and their intermediate metabolites have proved its contribution to antioxidant and antimicro­bial activities. For example, in supercritical CO2 extracts of S. platensis, chloro — phyll-a, pheophytin-a, pheophytin-a O-allomer, and pyropheophytin-a were detected by LC-MS/MS among the contributors to antioxidant activity measured by DPPH radical scavenging method [107]. On the other hand, phytol was detected by GC-MS among the bactericidal compounds present in D. salina extracts [111], as can be observed in Fig. 2, being all of them secondary metabolites of chlorophylls.

Certain alkaloids have been isolated from seaweeds. Among the many chemical classes present in plant species, alkaloids stand out as one of major importance in the development of new drugs, because they possess a wide variety of chemical structures and have been identified as responsible for many of the pharmacological properties of medicinal plants. Caulerpin, a bisindole alkaloid, was isolated from the green alga Caulerpa racemosa in 2009 [24]. This alkaloid showed low toxicity and a variety of important biological activities already described in the literature, among which it is important to mention the antitumor, growth regulator and the plant root growth stimulant properties. De Souza et al. isolated caulerpin from lipoid extract of C. racemosa and its structure was identified by spectroscopic methods, including IR and NMR techniques and demonstrated in vivo and in vitro its anti­nociceptive and anti-inflammatory activities [24].

Microalgae have been also studied in the search of alkaloids, in this sense most of this research has been conducted to identify toxins [78]. The non-sulfated alka­loid toxins of freshwater cyanobacteria (anatoxins and saxitoxin) are all neurotox­ins. The sulfated polysaccharides, C-toxins and gonyautoxins are also neurotoxins, but the sulfated alkaloid cylindrospermopsin blocks protein synthesis with a major impact on liver cells. Some marine cyanobacteria also contain alkaloids (lyngbya — toxins, aplysiatoxins) which are dermatoxins (skin irritants), but have also been associated with gastroenteritis and more general symptoms such as fever [ 78] . Several freshwater bloom forming cyanobacterial genera, including Anabaena, Aphanizomenon, Oscillatoria, and Cylindrospermum, produce the neurotoxin, ana — toxin-a, an alkaloid with a high toxicity to animals [117].