The chemical structures of the polysaccharides obtained from seaweeds are different depending on the taxonomic group to which they belong, their species, the season when these seaweeds were harvested, and the respective extraction method. The colloids authorized in the food industry, and widely used worldwide, are alginate (extracted from brown algae), agar, and carrageenan (extracted from red algae) ^[1][2][3][4][15,16,17,18].

Alginate (Figure 1) is a polysaccharide naturally found in brown seaweed in the form of alginic acid. This anionic polymer is based on monomers of β-d-mannuronic acid (M) and 1,4 α-l-guluronic acid (G) ^[5][19]. Depending on the position of the monomeric units in the chain, the molecular weight of the polymer, and the nature of its associated counter ions, the properties of this polysaccharide can differ ^[6][7][1,20].

At the level of food certification, the molecular weight of alginate is not considered. However, it is contemplated for good practices in the extractive industry associated with the food industry ^[9][21].

The industrial extraction of this polysaccharide involves several steps: washing the seaweed to remove impurities and pre-treatment with heated acid (usually hydrochloric acid for 24 h) to remove pigments, proteins, and lipids ^[10][22]. Next, the solid–liquid extraction takes place, where the solid residue is subjected to an alkaline treatment (sodium carbonate) followed by a centrifugation or filtration process. After this process, hydrochloric acid is added to the liquid extract to precipitate the alginate dissolved in the solution in the form of sodium alginate. After precipitation, the solution with the precipitate is centrifuged/filtered to obtain the precipitated alginate. Afterwards, the alginate is dried and milled for later application ^[11][23].

Alginate is classified as a non-organic compound and is approved by the Food and Drug Administration (FDA, USA) and the European Food Safety Authority (EFSA, EU) as a food ingredient ^[12][24]. In this context, the application and labeling of food products containing alginate are regulated according to the European Union Commission Regulation (1333/2008) as E400 (alginic acid), E401 (sodium alginate), E402 (potassium alginate), E403 (ammonium alginate), E404 (calcium alginate), and E405 (propylene glycol alginate) ^[9][21].

The main characteristics of alginate are its high degree of viscosity and absorption, which make it possible to thicken food products, such as jellies, marmalades, sauces (e.g., mayonnaise), syrups, and ice cream ^[13][14][25,26].

The FDA has approved alginate for human consumption after toxicological testing. However, the FDA requires evidence of good practices in alginate extraction and the use of alginate at threshold concentrations, which vary according to the type of food product ^[12][24].

Agar (Figure 2) is a very important polysaccharide industrially extracted from the red seaweeds genus Gracilaria and Gelidium, from the phylum Rhodophyta ^[15][16][17][27,28,29]. Generally, the industrial extraction method is based on a thermal treatment of the seaweed biomass in an aqueous solution (between 2–4 h at 105–110 °C) for immediate filtration while the extract is hot (as the agar gels very quickly at 50 °C). After the filtration process, the extract either gels or is maintained in a viscous solution due to the amount of agar present in the solution. However, the gel itself is normally yellowish or brown in color because some of its constituents are degraded (proteins, monosaccharides). Therefore, the freezing/thawing technique is used to obtain a concentrated agar with a clear color, as this technique allows the agar to be washed with water, avoiding pre-treatment to reduce impurities during extraction. Finally, the agar obtained is dried in an oven with air circulation and then milled for later application in industry ^[11][18][19][23,30,31].

It is a gel-forming polysaccharide consisting of 70% agarose and 30% agaropectin molecules, composed of residues of (1–4)-3, 6-anhydro-l-galactose and β 9 (1–3)-d-galactose ^[15][27]. To date, there is no evidence that its molecular weight has any significance for food safety, and therefore, it is considered safe regardless of its molecular weight ^[16][28]. However, the quality of the agar differs greatly between species belonging to these two orders. For example, agar extracted from Gelidium corneum (Gelidiales) is considered more suitable for pharmaceutical applications, ^[17][29]. On the other hand, agar extracted from Gracilaria gracilis (Gracilariales) is normally used, almost exclusively, in the food industry. However, normally, this agar has one more step in the industrial extraction system, which consists of an alkaline pre-treatment with sodium hydroxide, to increase the quality of the rheological properties of the agar obtained ^[18][30].

Agar is considered safe for human consumption by regulatory authorities in the United States of America (FDA) and the European Union (EFSA). Despite the inclusion of agar (E406) in the list of approved food additives, its application in food products is regulated and limited. It is estimated that approximately 90% of commercialized agar is destined for the food industry ^[18][30].

Carrageenan is extracted from red seaweed of the order Gigartinales. The first historical use of carrageenan was for food purposes and occurred in Ireland ^[20][32]. Carrageenan is a polysaccharide consisting of alternately linked galactose and 3,6-anhydrogalactose units, by alternating α-1,3 and β-1,4 glycosidic bonds, and whose molecular weight (greater than 100 kDa) is required for safe use in food terms ^[21][22][33,34]. In this case, there are three types of carrageenan (Figure 3) normally marketed: kappa-carrageenan (κ) (Figure 3a), which forms rigid gels with syneresis; iota-carrageenan (ι) (Figure 3b), which is characterized by producing elastic and smooth gels; and finally, lambda-carrageenan (λ) (Figure 3c), which originates viscous solutions without ever gelling ^[18][30].

In the carrageenan extraction industry, the pre-treatment of seaweed through a depigmentation step is necessary (with sodium hypochlorite or organic solvent) to obtain a clear color in the final product ^[20][23][32,35]. The carrageenan extraction step must be carried out in an alkaline (e.g., sodium hydroxide) or aqueous solution. Subsequently, carrageenan can be recovered by alcoholic precipitation, drum drying, or precipitation in aqueous potassium chloride and subsequent freezing (as in the case of κ-carrageenan). However, only methanol, ethanol, and isopropanol can be used for precipitation and purification of carrageenan ^[24][25][26][36,37,38]. To ensure food quality and safety, carrageenan is hot-dried at a temperature above 40 °C in a drying oven with forced ventilation before use ^[23][35].

Unlike phycocolloids in the food area, degraded/hydrolyzed seaweed polysaccharides were the aim of several studies (Table 1 and Table 2) with promising results as possible inducers of resistance and as biostimulants. With the emerging need to reduce synthetic-compound use in agriculture in the European Union, polysaccharides and their oligosaccharides are gaining new scientific interest to serve as alternatives. Seaweed extracts have already demonstrated the potential to promote seed germination and plant vigor and improve cultivars ^{[6][27][28][29][30][31][32][33]}[1,3,39,40,41,42,43,44]. As an advantage, seaweeds do not compete for land space, which allows the exploration of polysaccharides in a sustainable and circular economy way, possibly alleviating the effects of climate change (Figure 4).

Alginates are biodegradable and non-toxic compounds traditionally used as natural fertilizers due to their superabsorbent or water-retaining properties. The carboxylic acid groups present on the alginic acid chain, combined with the metallic ions in the soil, form high-molecular-weight polymers that can absorb moisture and retain large amounts of water. Generally, water retention is a problem in sandy soils. These soils, when watered, dry up easily and drain away valuable nutrients beyond the plant roots. The use of alginates can improve this problem, stimulating plant root system development and increasing soil microbial activity ^[6][7][1,20].

Seaweed alginates and oligo-alginates, produced by the enzymatic degradation of alginic acid, were reported to activate defense responses against pathogens in wheat plants ^[34][45], date palm roots ^[35][46], tomato plants ^[36][47], olive trees ^[37][48], and against TMV ^[38][12] by regulating defense-responsive signaling pathways. To induce resistance against viral infections, including TMV, alginates activate different defense enzymes such as phenylalanine ammonia-lyase (PAL), peroxidase (POD), and ascorbate peroxidase (AP), which elicit their metabolic pathways and the synthesis of secondary metabolites, such as phenolic compounds with antiviral activity ^[34][35][38][12,45,46]. In tomato plants, the alginate confers resistance against a fungal infection by inducing antioxidant defense and antifungal pathogenesis-related (PR) protein expression by signaling pathways mediated by salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) ^[36][47]. In an experiment with olive trees, the alginate induced resistance against a Verticillium dahliae fungal infection by restricting the pathogen’s growth and strengthening the host defense metabolism ^[37][48].

In addition, seaweed alginates and oligo-alginates can stimulate growth, seed germination, and shoot elongation in different plant species ^[8][38][12,14] by enhancing nitrogen assimilation and basal metabolism ^[39][49].

Therefore, alginates constitute an important source of potential elicitors in plants and a particular interest in agriculture. The chemical characterization of alginates or oligo-alginates and their mechanism to boost plant growth remains unclear.

Agar is a polysaccharide extracted from the red seaweed genus Gracilaria and Gelidium. Due to the huge availability of biomass from these seaweeds, it represents an excellent choice for commercial cultivation and to study their bioactive potential. Many species of these genus have been evaluated for their antibacterial, antioxidant, antifungal, antiprotozoal, anti-inflammatory, antiviral, cytotoxic, antihypertensive, spermicidal, and embryotoxic activities ^[40][50]. The type and number of substituents on the agar structure are crucial elements for the bioactivities’ efficacy. The sulfur content generally correlates with activity, which is why active agars are typically sulfated ^[41][51].

In a study about seaweeds’ carbohydrate polymers as plant growth promoters, agar extracted from two red seaweeds, Gracilaria gracilis and Asparagopsis armata, showed positive results in the growth and seed germination of kale plants ^[8][14].

Although there are many studies regarding the bioactivities of agarophytes ^[40][50], there is still a lack of studies regarding the bioactivities of agar, especially its effect on plants.

Carrageenans and their oligomers, extracted from various red seaweeds, present a significant source of bioactive substances that activate plants’ defense mechanisms and offer resistance against abiotic and biotic stresses. This can be achieved by modulating various physiological and biochemical processes ^[42][52]. Additionally, carrageenans control several metabolic activities in plants, including cell division, purine and pyrimidine synthesis, assimilation of nitrogen and sulfur, and photosynthesis ^[8][43][14,53].

Lemonnier-Le Penhuizic et al. ^[44][54] demonstrated that oligosaccharides (of varying molecular weights, but less than 500 Da) of λ-carrageenan act as inducers of embryogenesis. It should be noted that, both alginate and agar oligosaccharides were also tested, but with less significant results than those obtained by carrageenans. In general, the oligosaccharides obtained from carrageenan promote an increased plant height, greater leaf biomass, and better carbon fixation, as well as superior nitrogen assimilation and greater overall plant growth ^[43][53], in addition to promoting plant defenses as elicitors and activating their defense mechanisms against pathogens ^[45][55].

Tobacco plants ^[46][56] and eucalyptus trees ^[47][57] treated with commercially available κ-, ι-, and λ-carrageenans showed positive results in their growth. The oligo-carrageenans enhanced photosynthesis, basal metabolism, and the synthesis of secondary metabolites such as essential oils and polyphenolic compounds. In addition, κ-, ι-, and λ-carrageenans were reported to induce long-term protection against viral, bacterial, and fungal infections at a systemic level by activating the phenylalanine ammonia-lyase (PAL) enzyme and enhancing the accumulation of phenylpropanoids with potential antimicrobial activities ^[48][49][58,59].

As said previously, the level of sulfation of the polymer is suggested to influence their bioactivity and, therefore, their targeted applications for plant defenses ^[50][13]. The sulfate group content differs depending on the type of carrageenan: κ-carrageenan has 20–30% of sulfate group content, ι-carrageenan has 28–35%, and λ-carrageenan has 32–39% ^[49][51][59,60]. Among the three carrageenans, λ-carrageenan was considered the most potent elicitor due to its high sulfur content, inducing systemic resistance in plants. Plants treated with λ-carrageenan, either through leaf infiltration or foliar spray, showed resistance against several pathogens by inducing SA-, JA-, and ET-dependent defense pathways ^{[49][52][53][54][55]}[59,61,62,63,64].

Ι-carrageenan, sprayed on leaves, was reported to stimulate the growth of tobacco plants by enhancing photosynthesis, basal metabolism, and cell cycle, as well as ascorbate (ASC) levels and ascorbate peroxidase (AP) enzyme activity ^[56][65]. This oligo-carrageenan can elicit resistance against the moth Trichoplusia ni in Arabidopsis thaliana by inducing various defense mechanisms, including JA- and SA-dependent pathways, proteinase inhibitors, and an alteration of the products of glycosylate hydrolysis ^[57][66].

Κ-carrageenan, used in leaf spray treatment, was reported to stimulate the growth of chickpea plants, maize plants ^[58][67], and pine trees ^[59][68] by enhancing the basal metabolism and the production of secondary metabolites. Additionally, κ-carrageenan, applied through leaf infiltration, showed resistance against several pathogens by inducing SA-, JA-, and ET-dependent defense pathways ^{[60][61][62][63]}[69,70,71,72].

When compared to λ-carrageenan, κ-and ι-carrageenan showed better results in the growth of the roots and leaves in kale by inducing the production of indole-3-acetic acid (IAA), responsible for the plant’s development ^[8][14].

In sum, carrageenans and oligo-carrageenans can be employed as naturally occurring growth-enhancing, anti-fungal, and anti-viral agents.

In these cases, the mechanisms by which these poly- and oligosaccharides operate are yet unknown, nor is the characterization completed to have a direct correlation between polysaccharides, molecular weight, and the respective bioactivity. Thus, it will be essential to clarify from the biochemical point of view the potential danger of the polysaccharide degradation through the digestive system, or not, and what is the potential of low molecular weight oligosaccharides in plant health.