*2.5. Phenolic Compounds*

Among the current interests of the scientific community is to find a sustainable source of bioactive moleculesin order to reduce the use of synthetic compounds. In this sense, macroalgae phenolic compounds have gained particular attention due to their specific bioactivities and health-promoting benefits, including antioxidant, antiproliferative, antimicrobial, antiallergic, antidiabetic, and neuroprotective properties [60–63]. Similar to terrestrial plants, these secondary metabolites are essential to the normal growth and development of macroalgae, supporting the natural defense system against various disturbing factors such as diseases, injuries, and environmental aggression [64]. Structurally, phenolic compounds from terrestrial plants are derived from gallic and ellagic acid, while the algal compounds are derived from polymerised phloroglucinol units (1,3,5-trihydroxybenzene) [17].

The phenolic compounds present in macroalgae vary from simple molecules, such as phenolic and cinnamic acids or flavonoids, to the more complex phlorotannin polymeric structures, their concentration being closely dependent on a number of intrinsic and extrinsic factors, such as species, seasonal variations, and environmental conditions [64]. Of all the seaweed phenolic metabolites, the main attention has been focused on phlorotannins (phloroglucinol, eckol, 7-phloroeckol, 6,6-bieckol, phlorofucofuroeckol A, fucodiphloroethol), identified in considerable quantities in brown *Ecklonia* species [40]. Other compounds such as hydroxybenzoic acid derivatives (gallic, p-hydroxybenzoic, vanillic, and syringic acids), hydroxycinnamic acids (ca ffeic, ferulic, sinapic, and p-coumaric acids), flavonoids (epicatechin, epigallocatechin, rutin, quercitrin, hesperidin, myricetin, and kaempferol), and bromophenols were identified in variable concentrations in all green, red, and brown species [17,40,64].

#### **3. Algae Applications in the Food Industry**

Algae species have been used as plain food since ancient times. In Asia and in the East, the tradition of eating algae is a long-standing one, while in the Western countries, the interest in consuming algae-based products is quite recent but gaining increasing terrain [65]. Algae have manifold uses in different industry fields as a result of their rich chemical composition and content of bioactive substances. Moreover, their gelling, thickening, and stabilizing properties have driven the isolation and development of products such as agar, alginate, and carrageenan [66,67]. Due to these properties, algae have a main use in the food industry as hydrocolloids or as functional ingredients in different fish and meat products (steaks, frankfurters, or sausages), milk-based and fermented products [68–70], or cereal-based products (flour, pasta, bread, and biscuits) [4,21]. Moreover, these algae-based hydrocolloids are of utmost importance for food industry innovative fields such as molecular gastronomy.

In the dairy industry, algae were added in order to improve the nutritional value of cheese and other milk-based products [68]. *Laminaria* was added to smoked cheese, yoghurt, and milk deserts, giving them not only improved nutritional properties but also good sensory characteristics. *Laminaria saccharina* algae from the North Sea can be also introduced into cottage cheese or fresh cheese composition in order to improve their iodine content [69]. In addition to their nutritional properties, algae have been shown to have the ability to increase product stability during shelf-life due to the presence of compounds with antibacterial and antioxidant potential. In this sense, it was demonstrated that when algae Wakame (*U. pinnatifida*) and Kombu (*Laminaria japonica*) were added into the cheese composition, the product quality was maintained for a longer storage period [70]. In the meat industry, algae were added in the diet of lambs and chickens in order to improve the content of DHA, EPA [71,72], and antioxidants [73,74].

Recent studies on the bioactivity of some common species, such as *H. elongata* and *U. pinnatifida*, recommend their use in the composition of functional foods, due to the high content of antioxidants and the potential to alleviate the metabolic syndrome [75]. A wide range of studies reported the high potential of using algae as a source of prebiotics [76]. Wang et al. [77] proved that rats which had 2.5% alginate supplementation exhibited an increase in *Bifidobacterium* and *Lactobacillus*. A dietary supplementation of 1% laminarin was proved to result in an increase in *Bifidobacterium* number in rats [78]. *U. pinnatifida* and *Porphyra ternera* extracts fed to rats resulted in lower bacterial enzyme activity in the cecum, and also, the enzymatic activities that were reduced are implicated in the conversion of procarcinogens to carcinogens, therefore implying a possible link between seaweed extract intake and the reduced risk of colon cancer [79].

The most use species of algae in the food industry are summarized in Table 1 below.



**Table 1.** *Cont.*


#### **4. Health E** ff**ects**

Seaweeds contain a large variety of bioactive compounds that may be involved in the prevention and treatment of many diseases. They have several mechanisms for disease prevention and/or treatment. In this regard, some epidemiological, clinical, and meta-analysis studies associate the lower incidence of di fferent chronic diseases, such as cancer, cardiovascular deficiency, diabetes, Parkinson disease, obesity related disorders, and metabolic syndrome, with a diet profile that includes seaweed consumption [49,90–93]. The main benefits on human health are presented in Figure 2.

**Figure 2.** Health e ffects of macroalgae on human health and wellbeing.

#### *4.1. Blood Pressure, Sugar, and Fat Reduction*

Seaweeds are known to be rich in linolenic acid and its derivatives. These compounds can reduce blood viscosity and also smooth the interaction between blood vessels and vasoconstrictor substances. It was shown that when linolenic acid concentration increased by 1%, the blood pressure can diminish by 5 mmHg [90,94–96]. In this sense, Ryan et al. [86] analyzed the possible reduction in blood pressure using DHA algae oil and found that blood pressure reduction and heart rate were significantly reduced.

Alginate was shown to reduce blood sugar level, with sodium alginate supplementation of patients with diabetes type II leading to a decrease in the blood peak glucose level [96–99]. Porphyran and peptides were also proved to reduce blood sugar and blood pressure in rats and rabbits (*Porphyra yezoensis* in 1.6 g/<sup>L</sup> and 0.47 mg/mL) [100,101]. Fucoidan can reduce blood fat and sugar levels by disrupting fat absorption. Fucoidan is known to improve endoplasmic reticulum stress-reduced

insulin sensitivity through adenosine monophosphate-activated protein kinase activation, and it can restore lipid homeostasis in mice with type II diabetes [102,103]. Linolenic acid aids the transformation of low-density lipoprotein (LDL) cholesterol to high-density lipoprotein (HDL) cholesterol and, therefore, can regulate fat metabolism [90,96,104].

#### *4.2. Anticoagulant and Antithrombotic Properties*

According to WHO, cardiovascular and cerebrovascular diseases have become the main cause of population mortality. Sulfated polysaccharides extracted from algae possess anticoagulant and antithrombotic properties [90]. In this regard, Ustyuzhanina et al. [105] showed that chemical transformation of branched xylofucans isolated from the brown algae *Punctaria plantaginea* into highly sulfated linear fucans effectively inhibited clot formation, having similar antithrombotic and anticoagulant effects to that of the heparinoid Clexane (enoxaparin) and the native fucoidan from *S. latissima*. *E. cava* was proved to be a grea<sup>t</sup> source of bioactive marine polyphenols, with antihyperglycaemic, antihyperlipidaemic, anti-inflammatory, and antioxidant effects, supported by evidence from in vitro studies as well as from those from human and animal trials already completed [60].

#### *4.3. Antiaging, Antidepression, and Antifatigue Properties*

A number of different physical and physiological factors are relevant when it comes to aging. The healthy function of the kidney and spleen plays an important role in human health. Seaweeds and seaweed-derived bioactive substances regulate the nervous system function, repairing DNA, promoting immunity, removing free radicals, regulating endocrine function, promoting healthy metabolism, and enhancing the kidney and spleen function [106]. *Fucus vesiculosus* aqueous extract increases the expression of integrin molecules. Topical application of the extract had a positive effect on the thickness and mechanical properties of human skin [90,96,106,107]. Polysaccharides have a large number of applications in the cosmetic industry. They act as rheology modifiers, suspending agents or wound-healing agents [108]. Carotenoids are powerful antioxidants possessing anti-inflammatory and antiaging properties. Several studies reported that astaxanthin, a xanthophyll carotenoid found also in macroalgae, can lower the oxidative stress protecting the mitochondria from the cumulative reactive oxygen species damage. Furthermore, astaxanthin was show to exhibit neuroprotective effects suggesting its possible use in the therapeutic treatment or prevention of neurodegenerative diseases such as Alzheimer's or Parkinson's disease [109,110].

Miyake et al. [111] performed a study on seaweed consumption and depressive symptoms during pregnancy concluding that a rich seaweed diet can be associated with a lower prevalence of depressive symptoms during pregnancy. Seaweed polysaccharides also possess antifatigue properties [112]. Higher hemoglobin, more oxyhemoglobin dissociation, and enhanced release of oxygen are responsible for the antifatigue property.

#### *4.4. Antimicrobial and Antioxidant Potential*

The antimicrobial compounds in algae are from several chemical classes, their level varying during algal growth and during seasons. In this sense, it was demonstrated that the *Polysiphonia* type produces antibiotic compounds constantly throughout the year, the *Laminaria* type has the maximum production during the winter, the *Dictyota* type during the summer, while *Codium* type has the best efficiency during the spring [113].

Extracts obtained with different solvents from a wide range of algae species, including *Ulva fasciata, Bryopsis plumosa, Chaetomorpha antennina, Acrosiphonia orientalis, Sargassum wightii, Grateloupia filicina, Hypnea pannosa, Gracilaria corticate Portieria hornemannii, Cheilosporum spectabile, Centroceras clavulatum, Chnoospora bicanaliculata, and Padina tetrastromatica*, were tested for their antimicrobial activity against *E. coli*, *S. aureus*, and *S. pyogenes*. From the tested solvents, the mixture between methanol and toluene (3:1 v/v) had the highest efficiency in extracting the compounds with antimicrobial potential from

fresh biomass [114,115]. In another study, Tuney et al. [116] used methanol, acetone, diethyl ether, and ethanol to extract the bioactive compounds from 11 seaweed species. Diethyl ether extracts of fresh *C. mediterranea*, *E. linza*, *U. rigida*, *G. gracilis*, and *E. siliculosus* exerted high antimicrobial effects (10–15-mm halo) against several organisms (including *Enterococcus faecalis, Staphylococcus aureus, Pseudomonas aeruginosa*, and *Escherichia coli*). Instead, Bhuyar et al. [117] tested the ethanolic extract of the red alga *Kappaphycus alvarezii* against *Bacillus cereus*, the results indicating an inhibition zone with less than 10 mm of diameter.

The *Phylum Rhodophyta* (red algae) is recognized as one of the oldest groups of algae, characterized by the presence of phycoerythrin (a red protein-pigment complex), carrageenan (a sulfated polysaccharide), and phlorotannins. All of these compounds having strong antimicrobial activity. Another red alga extracts, *Symphyocladia latiuscula*, were proved to exhibit antimicrobial activity against a broad spectrum of microorganisms, the strongest antimicrobial effect being observed against *Vibrio mimicus* (50 μg/mL) and *Vibrio vulnificus* (50 μg/mL) [113,118].

Species such as *Laminaria saccharina*, *Laminaria digitata*, *Himanthalia elongata*, *Palmaria palmata*, and *Enteromorpha spirulina* are recognized as edible algae. Among these, *H. elongata* contains considerable amount of phenolics, tannins, and flavonoids. These antioxidant compounds that have a significant DPPH scavenging activity (50% inhibition (EC50) level at 0.125 μg/mL extract) can promote *H. elongata* as a natural alternative for food preservation. Moreover, the *H. elongata* methanolic extract at a concentration of 6% inhibited the growth of food spoilage (*Pseudomonas aeruginosa* and *Enterococcus faecalis*) and food pathogenic microorganisms (*Listeria monocytogenes* and *Salmonella abony*). Lower concentrations of the same brown seaweed extract (3%) extended the lag phase and decreased the exponential growth rate and final population densities of microorganisms in the culture [61,119].

The antimicrobial activity of other bioactive compounds extracted from marine algae was assessed against various microorganisms such as *Staphylococcus aureus*, *Salmonella choleraesuis*, *Mycobacterium smegmatis*, *Candida albicans*, and *Escherichia coli*. From the isolated compounds, three of them (namely cycloeudesmol (10–50 μg/mL), laurinterol (1–5 μg/mL), and debromolaurinterol (10–50 μg/mL)) exhibited antimicrobial activity at concentrations close to that of streptomycin (complete inhibition after 48 h) [120,121].

Studies of Al-Saif et al. [122] revealed the high antimicrobial potential of several algae strains (*Ulva reticulate*, *Caulerpa occidentalis*, *Cladophora socialis*, *Dictyota ciliolate*, and *Gracilaria dendroides*) against *Escherichia coli* (ATCC 25322), *Enterococcus faecalis* (ATCC 29212), *Pseudomonas aeruginosa* (ATCC 27853), and *Staphylococcus aureus* (ATCC 29213). The chloroform extract of *Gracilaria dendroides* had the highest antimicrobial activity against *E. coli* (32.6 mm inhibition zone). Wahidi et al. [123] tested the antimicrobial activity of extracts of macroalgae from Marrocan Atlantic coast against Gram-positive (*Bacillus subtilis* and *Staphylococcus aureus*) and Gram-negative (*Escherichia coli* and *Pseudomonas aeruginosa*) bacteria. Their results showed that the ethanolic extract of *Cystoseira brachycarpa* (500 μg/disc) had the highest inhibition diameter (>20 mm) for all tested bacteria, similar to that of control (rifampicine 30 μg).

In general, the microbial species on which the algae extracts have the strongest inhibitory activity are *Staphylococcus aureus* [9], *Escherichia coli* [124], *Salmonella* spp [62,125,126], *Bacillus cereus* [127,128], and *Listeria monocytogenes* [129,130]. For example, 100% ethanolic extracts of *Pithophora oedogonium* and *Botrydiopsis arhiza*, at concentrations of 2, 4, 6, and 8 mg/mL, were investigated for their antimicrobial activity against *Salmonella* and *Staphylococcus* sp. While *B. arhiza* extracts showed no inhibition capacity, the *P. oedogonium* extract (4 mg/mL) inhibits the growth of the above-mentioned strains [126]. Jang and Lee [128] evaluated the antibacterial potential of 51 Korean domestic algae methanolic extracts against foodborne pathogens, such as *B. cereus*, *S. aureus*, and *L. monocytogenes*. From the tested extracts, microorganisms were specifically sensitive to *Laurencia okamurae* Yamada and *Dictyopteris undulata* Holmes extracts which exerted antibacterial potential comparable with that of streptomycin [128]. *C. linum* methanolic extract at a concentration of 500 μg/mL was most effective against *B. cereus*, with a 27 mm inhibition zone, comparable with that of the standard antibiotic (chloramphenicol, 100 μg/mL). The high antimicrobial activity of the *C. linum* methanolic extract may be associated with its significant phenolic content (672.3 mg/g gallic acid equivalent), and high scavenging activity (IC50 9.8 μg/mL) [62].

In the e ffort of finding new natural antimicrobials, the algae represent a rich source of bioactive compounds with manifold activities. In this direction, several studies were conducted assessing di fferent fractions of methanolic or ethanolic seaweed extracts. For example, the ethyl acetate soluble fraction of *E. cava* methanolic extract exhibits high antibacterial activity against *L. monocytogenes* having an minimum inhibitory concentration (MIC) value of 256 μg/mL and an minimum bactericidal concentration (MBC) value of 512 μg/mL. Instead, the chloroform fraction of the ethanolic extract of *Myagropsis myagroides* was even more e fficient in inhibiting the *L. monocytogenes* growth, with an MIC value of 63 μg/mL [129,130].

One worthy nutritional property of algae is linked to their high content of polyphenols, flavonoids, and carotenoids [131]. The major phenolic compounds isolated from the marine algae included anthraquinones, coumarins, and flavonoids, with rutin, quercetin, and kaempferol flavonoids being identified in all the algal species. According to Al-Saif et al. [122], the highest concentration of these three flavonoids was found in alga *Gracilaria dendroides* (rutin, 10.5 mg/kg; quercetin 7.5 mg/kg; kaempferol 15.2 mg/kg). These compounds were proved to be the most e ffective flavonoids in inhibiting bacterial growth (*E. coli*, *P. aeruginosa, S. aureus*, *E. faecalis*). The eckol (phlorotannin compound) isolated from the ethyl acetate extracts of *E. cava* species showed potential antimicrobial activity against methicillin resistant *S. aureus,* the MIC values ranging from 125 to 250 μg/mL [132]. In the case of phlorotanins isolated from *E. bicyclis*, namely eckol, dieckol, dioxinodehydroeckol, fucofuroeckol-A, 7-phloroeckol, and phlorofucofuroeckol-A, the MIC values for the inhibition of *S. aureus* and methicillin-resistant *S. aureus* ranged between 32 and 64 μg/mL [133]. The antioxidant activity of three representative Black Sea macroalgae, *Ulva lactuca* (green algae), *Cystoseira barbata* (brown algae), and *Ceramium rubrum* (red algae), was assessed according to the antioxidative capacity in lipid soluble substances procedure (ACL method). Of these, *C. barbata* showed the highest antioxidant activity (141.5 Trolox equivalent units, nmols/g dry weight) [134].

## *4.5. Antiallergic E*ff*ect*

The worldwide trend is to use natural substances to cure allergies, and this has led to an increased interest in algal bioactive compounds, particularly in seaweed phenols. From the phenolic compounds, curcumin, epigallocatechin gallate, flavonoids, and quercetin were proved to have a significant antiallergic activity [20,21]. Additionally, fucoidan extracted from *U. pinnatifida* was proved to reduce the chemical and immunological responses in an animal model [21,135,136].

The porphyran, a sulfate polysaccharide isolated from *Porphyra tenera* and *Porphyra yezoensis*, is also known to possess antiallergic properties. The oral administration of porphyran (obtained from dried nori, 2% in drinking water) to mice with ear edema suppressed the evolution of the disease [137]. Aside from the antiallergic potential, the porphyran was also found to exert anti-inflammatory activity, their reactive oxygen species scavenging potential being considered the main mechanism responsible for this action [21]. Phlorotannins from *E. arborea* have been used since ancient times as folk medicine due to their antiallergic properties as reported by literature data [138]. Phlorotannins, carotenoids, polysaccharides, PUFAs, and phycocyanins were all found to exhibit antiallergic properties [139].
