Advances in ultrasound-enhanced recovery of marine algal polysaccharides: toward sustainable bioprocessing

Graphical abstract

Abstract

The growing emphasis on environmental sustainability has intensified interest in the extraction and valorization of polysaccharides from marine algal species due to their non-toxic nature and notable bioactivities. Although various conventional extraction techniques have been employed, these often involve high energy consumption, prolonged processing times, limited selectivity, and the potential degradation of polysaccharide bioactivity. To address these limitations, a range of sustainable and cost-effective extraction methods has been developed, including enzyme-assisted extraction (EAE), supercritical fluid extraction (SFE), subcritical water extraction (SWE), microwave-assisted extraction (MAE), and ultrasound-assisted extraction (UAE). Among these, the UAE, either as a standalone method or integrated within a biorefinery framework, has emerged as a promising and green technology. It offers higher extraction efficiency with reduced time, energy, and cost requirements. Polysaccharides extracted via ultrasound (US) have demonstrated enhanced anticancer, bioactive, and rheological properties. Furthermore, integrating the UAE with green solvents, such as natural deep eutectic solvents (NaDESs), or complementary techniques, including MAE, hot water extraction (HWE), and EAE, has been shown to further improve yields by 2–6 % and enhance functional quality. This review highlights the potential of the UAE, both individually and in combination with other methods for the efficient recovery of algal polysaccharides. It also evaluates the influence of the UAE on the biofunctional properties of the extracted compounds, underscoring their applicability across diverse industrial sectors. Nonetheless, further research is warranted to optimize process parameters, ensure safety, enhance antioxidant activity (AOA), and improve purification and characterization, as well as better understand the structure–activity relationships of these valuable biopolymers.

1. Introduction

Polysaccharides are high-molecular-weight carbohydrates consisting of repeated units of monosaccharides connected through the links of the glycosidic bond. These macromolecules are basic in the structure, energy reserves, and physiological processes of both terrestrial and aquatic beings [1,2]. This wide range of bioactivities, which encompasses anticoagulant, immunomodulatory, anti-inflammatory, antioxidant, antibacterial, and antiviral properties, is attributable to the fact that these have been increasingly used in pharmaceutical, nutraceutical, cosmetic, and food industries [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. These are also non-toxic, biodegradable, and inherently biocompatible, adding further to their usefulness as bio-actives that enable sustainability [13,14]. With the increasing concerns about global environmental sustainability, renewable biomaterials, algae have caught much interest as a high-yielding organism with polysaccharides. Both macroalgae (seaweeds) and microalgae present compelling advantages over traditional land-based biomass: rapid growth rates, non-reliance on arable land or freshwater, and the ability to sequester CO₂ and remediate wastewater [15,16]. These practical and ecological advantages are consistent with the ideas of a circular bioeconomy, and valorizing algal polysaccharides supports sustainable resource consumption and the development of environmentally benign products [17,18].

Macroalgae, including brown (Phaeophyceae), red (Rhodophyceae), and green (Chlorophyceae) seaweeds, are rich in structurally complex cell wall polysaccharides such as alginate, carrageenan, agar, fucoidan, and ulvan. These biopolymers serve critical roles in industrial applications as gelling, thickening, emulsifying, and stabilizing agents [[19], [20], [21]]. Microalgae, although less widely commercialized for polysaccharide production, possess significant untapped potential. Their extracellular polysaccharides (EPs) are characterized by intricate sugar compositions including rhamnose, galactose, xylose, and uronic acids with high degrees of sulfation, contributing to potent bioactivities [[22], [23], [24]]. Genera such as Porphyridium, Chlorella, and Arthrospira (Spirulina) polysaccharides have demonstrated antioxidant, antiviral, and wound-healing properties, highlighting their therapeutic promise. The cell walls of algal cells comprise a complex structure that impedes effective release of polysaccharides and are similar to a complex that includes minerals, hemicellulose, cellulose, and proteins [[25], [26], [27], [28]]. The biomass configuration is also highly variable as a result of species, cultivation conditions, season, and environmental conditions like salinity and nutrient quality, which brings issues with the optimization and standardization of yields [29].

Common extraction methods like solvent extraction, alcohol precipitation, acid/alkaline hydrolysis, and hot water extraction have many associated limitations, which include being energy-intensive, time-consuming, and having low selectivity. Such severe conditions could also degrade sensitive polysaccharide structures, which would harm their biological efficacy [[30], [31], [32], [33]]. Modern green extraction technologies have come up to counter these limitations [34,35]. EAE provides specificity in hydrolyzing cell wall components using cellulases or proteases, thereby improving both yield and selectivity [36]. More advanced solvent-free methods, such as SFE and SWE, offer cleaner, energy-efficient alternatives [37]. MAE reduces solvent usage, enhances cell disruption, and shortens extraction time while preserving bioactive integrity [38,39]. Pressurized liquid extraction (PLE) is an efficient, sustainable, and economical extraction approach. However, these extraction techniques are not cost-effective, require a large amount of algal biomass, and more time is required for extraction as well as heat-induced polysaccharide breakdown [40]. UAE is a potentially robust modern extraction method that can enhance polysaccharides’ yield, but its efficiency is dependent upon properties of the matrix used, and intended use [41]. US can also result in depolymerization, modify structure, and cavitation differ with equipment (probe or bath) as the probe-type UAE places the ultrasonic waves into the medium, where this produces cavitation in intense localized mass transfer. Whereas the bath-type UAE places the ultrasonic waves outside the medium, whereby the cavitation is less intense and uniform [42,43]. The advantages and disadvantages of the UAE have been illustrated in Fig. 1 (a) and Fig. 1 (b). The base of the UAE is that US waves applied through ultrasonic baths or probe-type systems encourage cavitation, heat, and mechanical effects, with cavitation being the main mechanism. When sound waves of 24–100 kHz frequency are applied to a liquid medium, cavitation (bubble formation and collapse) occurs. Cavitation results in cell rupture, fragmentation, restricted erosion, and penetration of components from the cell wall to the intercellular environment. In the case of a solid matrix, cavitation results in surface shedding, erosion, microfractures, and ultimately pore formation in the matrix, aiding solvent solubilization [[44], [45], [46], [47]]. Similar advances in integrated biorefinery strategies are opening up the possibility of extracting a broad range of polysaccharides from a single algal biomass. This not only increases commercial viability but also makes algal processing systems more sustainable [11,48,49]. For example, US-MAE and US-assisted enzymatic extraction are being used for more efficient extraction [50,51].

Fig. 1.

Advantages and disadvantages of Ultrasound technique (a)Advantages; (b) Disadvantages.

Previous studies have extensively focused on the extraction, synthesis, and applications of algal polysaccharides by using both conventional and novel techniques. The majority of these studies have focused largely on freshwater algae or individual types of polysaccharides, including fucoidan found in brown algae. By comparison, systematic studies on the application of UAE to extract various polysaccharides from marine algal samples, especially red and green algae, are scarce. To the best of our knowledge, no comprehensive review has specifically discussed the US-assisted extraction of polysaccharides from marine algal sources. Therefore, this review aims to address this gap by presenting a focused resource that analyzes the specific impact of the UAE on the marine algal polysaccharides and their applications. The main objective of this review is to analyze the extraction yield of valuable marine algal polysaccharides in the US. It addresses the main algal polysaccharides and their chemical composition. This study also highlights the impact of the UAE on the bio-functionality of extracted polysaccharides and provides an overview of the industrial valorization of these polysaccharides.

2. Methodology

The methodology adopted for this narrative review began with the identification and definition of key themes linked to UAE of marine algal polysaccharides. Major scientific databases, Scopus https://www.scopus.com and Google Scholar https://scholar.google.com/ were used to identify relevant studies within the time frame of 2019 to 2025. The keywords “Ultrasound-assisted extraction,” “UAE,” “polysaccharides,” and “algae,” were searched in different combinations in these databases. Then, based on inclusion or exclusion criteria, relevant studies were chosen. The inclusion criteria were based on the publication related to the yield of algal polysaccharides by the UAE technique and a research paper. The publications were excluded on the basis of:

• It was irrelevant to the UAE of algal polysaccharides.

• Its full text was not available.

• It was not in the English language.

• It was not published between 2019 and 2025.

• It was a duplicate research paper, review paper, conference paper, book, or proceedings.

The final selection was based on articles that discussed the extraction, characterization, and valorization of polysaccharides. This strategy ensured a thorough but coherent synthesis of key literature.

3. Overview of main algal polysaccharides

Different species of micro- and macro-algae contain various polymers called polysaccharides. The classification of various algal polysaccharides along with their potent uses has been illustrated in Fig. 2. Fig. 2(a) presents the brown- algae derived sulfated polysaccharides, while Fig. 2(b) illustrates the red and green algae polysaccharides. Alginates, one of the anionic linear polysaccharides, are the salts of sodium, magnesium, or calcium mixed salts of alginic acid. These are present in the brown algae cell wall, and consist of 2 uronic acids, also called acidic sugars, (1,4)-linked β-D-mannuronic acid and α-L-guluronic acid [52,53]. These acids can combine either in homo or heteropolymeric blocks. Alginates provide strength and flexibility to the algae [54,55]. These are non-toxic, biodegradable, require low processing, have thickening characteristics, and can form stable gels, which is why these have broad applications in the food sector [52]. The main algal sources are Ascophyllum nodosum, Laminaria sp., Lessonia nigrescens, Ecklonia maxima, Macrocystis pyrifera, and Durvillaea Antarctica, as these have 40 % alginate in their tissues [55].

Fig. 2b.

Red and green algae-derived Polysaccharides and their potential uses.

Fig. 2a.

Brown algae-derived Sulfated Polysaccharides and their potential uses.

Glucans, primarily β-glucans or laminarin, are the storage polysaccharides present in the vacuoles of the brown algae (≈ 35 %). They are composed of β-1,3-linked d-glucose residues with variable levels of other monosaccharides, primarily mannito [56,57]. Agar, a polysaccharide present in the cell walls of red algae species, is composed of agarose and agaropectin. The agarose, gelling part, contains alternate β‑D‑galactopyranosyl and 3,6-anhydro‑α-L‑galactopyranosyl units. While agaropectin contains 5–10 % sulfate esters and methoxyl groups, as well as pyruvic acid, in addition to these units [58]. Another type of polysaccharides is Ulvans (aldobiuronans), hydrophilic sulfated anionic heteropolysaccharides, present in the cell walls of green algae, mainly Ulva sp. These are mainly composed of repeating units of α-L-rhamnose [59,60].

3.1. Chemical composition of main algal polysaccharides

Chemical composition determination plays a vital role in the extracted polysaccharides. The biological and antioxidant potential of polysaccharides is greatly influenced by their monosaccharide composition, mainly in fucose-rich sulfated polysaccharides [61]. These biological activities include antioxidant, cytotoxic, antidiabetic, and prebiotic potential [[62], [63], [64]]. For example, one study has shown that fucoidan extracted from brown algae exhibited strong antiviral activity against HSV-2 infection [65]. Another study also exhibited cytotoxic properties against breast carcinoma cells and human lung carcinoma [66]. Fucose sulfated polysaccharides extracted from a heterotrophic marine alga were recognized as natural antioxidants [45]. Uronic acids, including glucuronic and galacturonic acids, can contain maximum AOA, but at higher amounts can minimize the anticoagulant and anti-complement properties [67,68]. That is why it is crucial to determine the composition of saccharides by using analytical techniques such as High-Performance Liquid Chromatography (HPLC) to gain a deeper understanding of polysaccharides [69]. Although HPLC is a fast and effective technique, it has certain limitations, such as derivatization artifacts and poor separation of sugar isomers [70,71]. Advanced analytical techniques, including Liquid Chromatography- Mass Spectrometry (LC-MS) and Ultra-high performance liquid chromatography (UHPLC) coupled with MS, offer better sensitivity and resolution, resulting in more accurate characterization [72,73]. Table 1 presents the chemical composition of algal polysaccharides extracted by UAE.

Table 1.

Composition of ultrasound-assisted extracted algal polysaccharides.

Polysaccharide Type	Algal source	Monosaccharide Composition (%)	Chemical Composition (%)	Sulfate Content (%)	Uronic Acid content (%)	References
Fucoidan	Seaweed	−	Moisture: 11.87Carbohydrates: 58.65Protein: 8.53	22.97	1.08	[75]
Polysaccharides	Brown algae	Galacturonic acid: 2.03 Glucuronic acid: 1.31 Rhamnose: 3.02 Fucose: 40.41 Mannose: 4.19 Xylose: 2.53Galactose: 27.13Glucose: 19.30	Carbohydrates: 73.83 Proteins: 4.74Fat: 0.99Ash: 5.57	−	−	[69]
Fucoidans	Arctic brown seaweed (F. vesiculosus)	Fucose: 74.4 Xylose: 7.9 Mannose: 3.6Galactose: 8.3Glucose: 5.8	−	24.7	86.4	[67]
	F. distichus	Fucose: 65.4 Xylose: 5.8 Mannose: 5.8Galactose: 18.2Glucose: 4.8		22.4	36.2
	F. serratus	Fucose: 61 Xylose: 5.4 Mannose: 2.5Galactose: 21.8Glucose: 9.3		18.9	44.3
	A. nodosum	Fucose:73.6 Xylose: 14.7 Mannose: 4.7Galactose: 3.9Glucose: 3.1		18.8	110.2
Polysaccharides	Red algae	−	RIP-1 Total Sugars: 25.22 Proteins: 23.29 RIP-2Total sugars: 27.37Proteins: 19.75	RIP-1: 0.29 RIP-2: 10.01	−	[94]
Endopolysaccharides	Microalgae	−	440.6 mg/g	−	103.7 mg/g	[92]
Exopolysaccharides			802 mg/g		96.8 mg/g
Polysaccharides	Brown seaweed	Glucuronic acid: 0.28 Fucose: 1 Galactose: 1.03Mannose: 0.74Xylose: 0.12	Carbohydrates: 46.60Protein: 0.37	12.47	11	[68]
Polysaccharides	Brown algae	Mannose: 21.88Galacturonic acid: 78.12	Carbohydrates: 91.63	2.520	32.378	[98]
Agar	Red seaweed	Galactose: 566 mg/g	Carbohydrates: 591 mg/g Protein: 250 mg BSA/g	3.1	−	[58]
Polysaccharides	Seaweed	−	Lipids: 25.60Ash: 11.01	−	−	[60]
Fucoidans	Seaweed	D-mannose: < 10 L-rhamnose: < 10 D-glucose: 48.9 L-galactose:20.9D-xylose: < 10L-fucose: 19.3	Sugar: 27.97	11.45	14.47	[74]
Carrageenan	Red seaweed	3,6 anhydragalactose: 41.12	Protein: 2.47	23.07	−	[50]
Fucoidans	Brown seaweed	Glucose: 2.88Rhamnose: 0.84Fucose: 8.79	−	37.57	−	[112]
Polysaccharides	Brown seaweed	−	Protein: 10.68–13.13 Fat: 1–2.84 Ash: 28.15–34.35 Fiber: 44.36–54.99 Moisture: 2.3–4.78	−	−	[2]
Agar	Seaweed	−	−	2.95	−	[90]
Polysaccharides	Marine algae	Glucose:16.39 Mannose:14.75Arabinose: 1Galactose: 693.03	−	−	−	[45]
Fucoidan	Marine seaweed	Fucose: 35.65 Mannose: 28.94 Galactose: 29.95Xylose: 7.74Glucose: 3.19	Carbohydrate: 58.65Protein: 10.17	27.16	1.1	[65]
Polysaccharides	Marine macroalgae	Glucose: 31.47Fucose: 36. 30Galacturonic Acid: 33	−	45.09	−	[100]
Polysaccharide	Irish seaweed	Galacturonic Acid: 3.77 Glucose: 33.27 Galactose: 3.91Rhamnose: 7.18Fucose: 2.60	Protein: 9.93 mg bovine serum albumin equivalents/100 mg Sugars: ≈ 32 GE /100 mg	−	−	[113]
Polysaccharides	Brown algae	Mannose: 5 Rhamnose: 33 Glucuronic acid: 5.6 Glucose: 13 Galactose: 31.5Xylose: 11.8Fucose: 33.3	Carbohydrate: 28.09 Protein: 25.09 mg/g	226.46 mg/g	64.28 mg/g	[114]
Carrageenan	Red seaweed	Galactose: 6.45 Glucose: 2.20 Fucose: 0.44Arabinose: 0.08Xylose: 0.23	Protein: 0.04	−	10.06	[86]

4. Ultrasound-assisted extraction of specific polysaccharides

UAE has gained considerable interest due to enhanced yield and bio-functionality and has been commonly used for the extraction of algal polysaccharides. Fig. 3 depicts the general steps involved in the extraction of marine algal polysaccharides, emphasizing the importance of cavitation in their extraction.

Fig. 3.

An overview of the general UAE process of polysaccharides.

4.1. Brown algae-derived sulfated polysaccharides

4.1.1. Fucoidan

The UAE was optimized and compared with heat-assisted extraction (HAE) for extracting fucoidan from four frozen thalli of Arctic brown algae. Fucales, phaeophyceae, and extracted polysaccharides were characterized. UAE under optimum conditions by fractional factorial design (FFD) resulted in 15.5 to 21.6 % yield, significantly higher than HAE (11.9 to 10.4 %) due to more diffusion of fucoidan in the solvent [67]. Hot-water-assisted US extraction was used to extract fucoidan from the brown algae Sargassum henslowianum C. Agardh, and their purification, physicochemical characteristics, and in vitro antithrombotic activity were determined. This combined method contributed to a 6.25 % yield, and after purification, it exceeded 65.2 % [74]. Alboofetileh, Rezaei, Tabarsa, & You. [75] have optimized the UAE process to extract fucoidan of seaweed Nizamuddinia zanardinii by Single-factor experiment and Box-Behnken design (BBD) and characterized these polysaccharides. UAE yielded 3.51 % fucoidan with improved AOA, significantly dependent upon US power, time, and temperature, and liquid-to-solid (L/S) ratio. The positive impact of US power and temperature assisted in cell wall disruption of seaweed; consequently, cells diffused into the extraction solution. These parameters further enhanced the extraction by decreasing the solvent viscosity, vapor pressure, and cavitation phenomenon. At a higher US duration, a lower yield was due to fucoidan’s heat sensitivity and breakdown. Higher yield by high L/S ratio was due to lower fucoidan in water, so lower viscosity of solvent assisted in solubility and diffusion of fucoidan into the solvent. By increasing the L/S ratio, the viscosity increased, so the yield decreased. Fucoidan also showed 62.36 and 56.83 % cytotoxic effect against HeLa and HepG2 cancer cell lines, respectively (see Table 2).

Table 2.

Ultrasound-assisted extraction (UAE) of marine algal polysaccharides.

Algal Source	Species	Polysaccharide	Method	Sonicator Type	Key conditions					Yield (%)	References
					Frequency (kHz)	Power (W)	Time (min)	Temperature (°C)	LSR
Brown seaweed	Sargassummuticum	Alginate	US-assisted aqueous extraction	Bath	0.04	150	5–30	25	20:1	15	[55]
Red seaweed	K. alvarezii	Carrageenan	UAE	Bath	−	120	60	70	−	76.70	[86]
Brown algae	F. virsoides	Fucoidan	UAE + CE	−	−	−	UAE: 5CE: 180	80	−	13.27	[100]
	C. barbata		UAE		26	200	30	25		7.57
Irish brown seaweed	F. vesiculosus	Fucoidan	CE + US-assisted depolymerization	Probe	US: 20	−	CE: 60US: 30	CE: 80	−	22.95	[104]
Brown Seaweed	S. horneri	Crude polysaccharides	DES-assisted Ultrasonic	Probe	−	−	30	50–90	1:1–1:50	11.31	[99]
Marine seaweed	N. zanardinii		UAE		20	200	40	55	−	3.6	[65]
			EUAE		−	−	1440	50		7.87
			UMAE			700	40	90		5.53
Macroalgae	Laminaria hyperborea	Fucose	HAE + US + thermal treatment	Bath	US: 0.05–0.06	80	HAE: 30–90 Thermal: 0–30	HAE: 80–120Thermal:100	−	460.6 mg/100 g	[56]
	L. digitata									151.05 mg/100 g
	A. nodosum									2971.7 mg/100 g
	L. hyperborea	Glucans								908 mg/100 g
	L. digitata									134.8 mg/100 g
	A. nodosum									494.2 mg/100 g
Marine algae	S. limacinum Meal	polysaccharides	EAUE	−	40	40–80	10–50	30–70	−	11.86	[45]
Seaweed	H. pannosa	Agar	UAE	Probe	−	750	15–45	−	−	32.99	[90]
			UMAE			Microwave: 200–600	5–15			26.9
Brown seaweeds	S. wightii	Polysaccharides	UAE	Probe	50–60	−	30	−	−	≈ 41	[2]
	S. asperum									≈ 39
	C. sinuosa									46.91
	Padina tetrastromatica									35
Brown Seaweed	L. digitata L.	Alginate	US- assisted acid treatment	Bath	−	−	10–30	40–70	−	30.9	[83]
Brown algae	S. siliquosum	Fucoidan	UAE	Horn	−	50–200	10–20	−	5–25 mL/g	4.78	[76]
Red macroalgae	Palmaria palmata	Sulphated polysaccharides	UAE	−	40	−	50	25	−	54.3	[96]
Brown seaweed	E. maxima	Fucoidan	US- assisted enzymatic extraction	Horn	−	−	−	40–65	−	7.9	[51]
Red seaweed	K. alvarezii	Carrageenan	UMAE	−	−	UAE: 200–600	UAE: 5–20	UAE: 35MAE: 93	40:1	61.25	[50]
Green seaweed	U. rigida	Sulfated polysaccharides	UAE	Bath	−	−	50–130	40–80	15–75 mL/g	13.22	[59]
Microalgae	Dic- tyosphaerium sp.	Water-soluble polysaccharides	UAE	Probe	−	100–900	10–60	30–80	−	ER = 7.83	[91]
Brown algae	Sargassum cymosum C. Agardh	Alginate	US- assisted citric acid extraction	Probe	20	120–180	5–15	−	−	54.20	[81]
Seaweed	Sargassum henslowianum C. agardh	Fucoidans	Hot-water-assisted US	−	−	350	210	60 and 80	−	65.2	[74]
Green seaweed	Ulva spp.	Hybrid protein-polysaccharide	US- assisted pH shifting	Probe	−	550	1–30	−	−	47	[60]
Red seaweed	G. sesquipedale	Agar	WH-US	Probe	24	400	30	−	−	10.1	[58]
			NaOH + WH-US					90		2.3
Seaweed	E. kurome	Polysaccharides	UAE	−	−	400	10	−	1:80	5.81	[98]
Brown macroalgae	A. nodosum	Alginate	US- assisted depolymerization	Probe	20	−	32	−	−	22.6	[85]
	F. vesiculosus									7.92
Brownseaweed	Laminaria japonica	Polysaccharides	UAE	Probe	195 w/mL	−	30	−	−	9.73	[68]
Microalgae	V. punctata	Bound polysaccharide	US	−	−	1–60	0.5–4	−	−	65.1 mg/g DW	[92]
Red algae	R. intricata	Polysaccharide	US- assisted water extraction	−	−	−	20–60	60–100	50–150 g/ml	37.78	[94]
Arctic brown algae	F. vesiculosus	Fucoidan	UAE	Probe	−	−	20–60	25	10–30 ml/g	21.6	[67]
	Fucus distichus									17.9
	Fucus serratus									15.5
	A. nodosum									16.1
Brown seaweed	F. vesiculosus by product	Alginate	UAE	Probe	20	−	6–34	−	−	6.19	[52]
Microalgae	T. obliquus	Starch	US-assisted enzymatic extraction	Horn	−	−	CT: 5 BP and BC: 5–60	−	−	75	[93]
	Chlorella sp.									79.8
Brown seaweed	Silvetia compressa	Polysaccharide	UAE	−	−	1.2–3.8 W cL−1 power density	−	50–65	10–30 ml/g	23	[97]
Brown algae	S. angustifolium	Polysaccharide	UAE	Bath	−	400–1200	−	4–12	20–50 mL/g	7.14	[69]
Brown Macroalgae	A. nodosum	FSPs	UAE	−	20	500	2 and 5	−	−	195.36 mg fucose/100 g	[79]
			UMAE		−	Micrwave: 250–1000				3533.7 mg fucose/100 g
Seaweed	N. zanardinii	Fucoidan	UAE	Probe	−	100–200	40–60	70–90	60–80 mL/g	3.51	[75]
Red algae	Pterocladia capillacea	Carbohydrates	UAE	Bath	−	−	30–240	−	−	59.3	[115]
Brown seaweed	S. wightii	Fucoidan	UAE	−	−	−	30	−	−	14.61	[77]
Brown algae	S. angustifolium	Alginate	UAE	Probe	−	320–640	5–25	25–65	−	46	[53]
Brown seaweed	S. mcclurei	Fucoidan	UAE	−	−	360	49	54	−	7.1203 mg/g	[66]
Brown seaweed	Sargassum fusiforme	Sulfated polysaccharides	US	−	−	−	30–90	50 and 70	−	15.30	[78]

(US) ultrasound; (LSR) liquid: solid ratio; (SFE) Supercritical fluid Extraction; (NE) native extraction; (CE) conventional extraction; (UAE) ultrasound-assisted extraction; (DESs) Deep Eutectic Solvents; (EUAE) alcalase-US; (UMAE) US-microwave; (HAE) hydrothermal-assisted extraction; (EAUE) Enzyme-assisted ultrasonic extraction; (L/S) liquid to solid; yield, (ER) extraction rate, % of dry biomass; (WH-US) US- assisted hot water; (CT) Cyclic treatment; (BP) Batch treatment with pulsed ultrasonication; (BC)Batch treatment with continuous ultrasonication; (SLR) Solid–liquid ratio; (FSPs) Fucose-sulphated polysaccharides.

UAE and MAE were implemented on the brown macroalgae Sargassum siliquosum to extract fucoidan, and further, these polysaccharides were purified. The UAE achieved a lower yield of 4.78 % than conventional HWE and MAE, due to polysaccharide degradation resulting in damage to low-molecular-weight fucoidan. HWE and MAE contributed to 5.08 and 6.94 % fucoidan yield, respectively. The yield was directly dependent on the US temperature and S/L ratio up to a certain level [76]. Alboofetileh, Rezaei, Tabarsa, Rittà, et al. [65] extracted marine seaweeds N. zanardinii fucoidan by different conventional and novel extraction methods and characterized them. SWE exhibited the highest extraction of 13.15 % because high pressure and temperature cause changes in the characteristics of water to promote the disruption of cells and solvent penetration to increase the extraction yield. The extraction yield was followed by US-assisted enzymatic extraction (7.87 %) > MAE (6.17 %) > acalase extraction (5.58 %) > US-assisted microwave extraction (5.53 %) > HWE (5.2 %) > conventional extraction (CE = 4.8 %) > flavourzyme extraction (4.36 %) > viscozyme extraction (4.28 %), and the lowest by UAE (3.6 %). However, US-assisted microwave and enzymatic extraction contributed to antibacterial activity against P. aeruginosa (4 and 4.5, respectively) and potent antiviral activity with no cytotoxic effects. UAE was optimized to extract Vietnamese brown seaweed Sargassum mcclurei fucoidan, and the resulting extract's bioactivity was investigated. The fucoidan yield reached 7.1203 mg/g, which was higher than the HWE (6.0239 mg/g). The AOA and cytotoxic impact on breast and human lung carcinoma half-maximal inhibitory concentration (IC₅₀) values were 0.4261 mg/mL, 95.80, and 129 μg/mL, correspondingly [66]. HWE and UAE were compared to isolate brown seaweed Sargassum wightii fucoidan. UAE contributed to a higher yield of 14.61 %, compared to HWE (10.59 %). Metal chelating activity by UAE (38.79 %) was also higher than HWE (31.72 %). But UAE exhibited lower radical scavenging activity of 61.93 % than HWE (54.27 %) by 2,2-Diphenyl-1-picrylhydrazyl (DPPH) assay [77].

A biorefinery method by applying water and UAE to extract fucoidan from seaweeds, Sargassum fusiforme, was used. It contributed to a 15.30 % yield with a potent AOA with superoxide anion scavenging and ferric ion reduction ability of 80.15 and 63.31 %, respectively [78]. Different extraction techniques were employed on the macroalgae A. nodosum to extract bioactive compounds and antioxidants. US-assisted microwave extraction resulted in the highest fucoidan extraction (3533.75 mg fucose per 100 g dm), followed by MAE and UAE (1699.80 and 195.36 mg fucose/100 g dm, respectively). There was no significant change in AOA of the extracted polysaccharides by all extraction methods [79]. The Hydrothermal-assisted extraction resulted in 6978.4, 943, and 2782.3 mg/100 g dm fucose as well as 640, 219.5, and 2344.1 mg/100 g dm glucans in A. nodosum, L. digitata, and L. hyperborean, respectively. It also improved AOA, values reaching 59.64 µM trolox/mg fde and 45.34 % by ferric reducing antioxidant power (FRAP) and DPPH in A. nodosum [56]. In a previous study, when Garcia-Vaquero et al. [57] applied high-intensity UAE on the macroalgae Laminaria digitata and extracted fucose and glycan, they obtained a higher yield of fucose (2268.9 mg fucose/100 g dm). Polysaccharides and antioxidants extraction of three different species of brown algae was enhanced by hydrothermal-assisted extraction, preceded by US and thermal techniques, and further optimized by BBD. It contributed to an additional 460.6, 151.05, and 2971.7 mg/100 g dm fucose in L. hyperborean, L. digitata, and A. nodosum, respectively. The achieved values of glucans were 700.7, 134.8, and 494.2 mg/100 g dm, respectively.

4.1.2. Alginate

UAE was used to extract alginate from the brown seaweed Sargassum muticum, and the extracted polysaccharides were characterized. UAE, with less energy and time consumption, yielded 15 % alginate. The extracted alginate also provided stable thermo-rheological properties and non-cytotoxic impact against human cell lines, with 25.6 % inhibition against caucasian human glioblastoma [55]. UAE was used to extract alginic acid under optimum conditions by response surface methodology (RSM), from the brown algae Sargassum angustifolium of Persian Gulf shores. The extracted polysaccharides were also characterized. UAE resulted in a maximum of 46 % yield, significantly dependent upon the temperature at some points. The yield was independent of process conditions at all points due to temperature neutralization. It also exhibited 20.3 % cytotoxic impact against the breast cancer cell line. The previous study exhibited a lower yield, which can be attributed to the tropical climate of Brazil [53]. Similarly, the UAE contributed to a 24 % alginate in the brown macroalgae S. angustifolium [80].

UAE, optimized by RSM, was used to extract seaweed Fucus vesiculosus by-product alginate, and the extracted polysaccharides were characterized. UAE yielded 6.94 % alginate, similar to CE (8.23 %), with non-significant quality degradation, due to cavitation bubbles collapsing at solid–liquid crossing points, resulting in liquid jets. These jets increased the mass transfer by penetration [52]. Chica et al. [81] optimized the US-assisted citric acid extraction method by BBD to improve the yield and functional characteristics of the alginate from the brown seaweed Sargassum cymosum C. agardh. This combined approach yielded 54.20 % alginate with a viscosity and AOA of 10.43 mPa•s, and 80.81 µM Trolox g⁻¹. The mechanism behind this is that the US produced mechanical impacts due to acoustic cavitation, resulting in the disruption of the rigid cell wall by strong shear forces. This physical phenomenon helps citric acid in better penetration to biomass, which causes an increase in alginic acid release and extraction. When acid-alkaline extraction was used to extract sodium alginate from Iranian brown seaweed Nizimuddinia zanardini, it resulted in a lower yield of 24 % [54]. Similarly, citric treatment also exhibited a lower alginate yield of 28.81 % from brown macroalgae Sargassum latifolium [82]. US-assisted acid treatment was carried out to enhance brown seaweed L. digitata L. alginate extraction from its biomass. Ethanol, a biomass byproduct, was also further utilized. It resulted in a 30.9 % yield within a ≈ 4–24 fold less duration than the CE methods [83]. When Trica et al. [84] extracted brown seaweed Cystoseira barbata alginate by acid treatment, it contributed to only 19 % yield. A green biorefinery approach was used to extract brown seaweed A. nodosum and F. vesiculosus’s fucoidan, laminarin, mannitol, alginate, and protein. US resulted in a higher yield of 7.92 % of alginate in F. vesiculosus than in A. nodosum (7.68 %) [85].

4.2. Red algae-derived polysaccharides

4.2.1. Carrageenan

Novel extraction techniques were employed to extract kappa (κ)-carrageenan from the seaweed Kappaphycus alvarezii, and their physicochemical characteristics were determined. UAE resulted in a 76.70 % yield, slightly lower than CE (77.33 %). However, it was higher than the native extraction (45.47 %) and SFE (53.40 %). The UAE also improved physicochemical properties with a maximum viscosity of 658.7 cP [86]. That yield was comparatively higher when carrageenan was extracted by native extraction from red algae species K. alvarezii, K. striatum, K. malesianu, and Eucheuma denticulatum, resulting in 40–50 % yield [87]. The consecutive US-microwave extraction was optimized using RSM to extract red seaweed K. alvarezii carrageenan. It was also compared with aqueous and alkaline extraction. This consecutive approach resulted in 61.25 % κ-carrageenan, comparatively higher than MAE (56.05 %), aqueous (44.53 %), and alkaline extraction (39.58 %). The mechanism behind the higher yield by US-microwave extraction occurred because US breaks the algae matrix, resulting in enhanced solvent contact and mass transfer. The microwave technique, then, by implementing thermal energy, causes the rupture and extraction of carrageenan. This sequential process also exhibited shear-thinning behavior and improved apparent viscosity with a gel strength and AOA of 588.03 g and 32.35 by 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay, respectively [50]. Compared to this study, 60 and ∼ 20 % hybrid carrageenan from red seedweed Mastocarpus stellatus was obtained by MAE and CE [88]. Alkaline extraction of seaweed Hypnea bryoides carrageenan also exhibited a lower yield (26.74 %) [89]. Mild water extraction of κ-Carrageenan from different red algae species (K. alvarezii, K. striatum, and K. malesianusalso) and iota-carrageenan from Eucheuma denticulatumalso also contributed to a lower yield of 32–42 % [87].

4.2.2. Agar

Martínez-Sanz et al. [58] produced the seaweed Gelidium sesquipedale agar-based extracts by synergistic HWE and US technique, as well as characterized them. The combined process resulted in a higher yield of 10 to 12 % compared to the combined approach using HWE + US and alkali, which led to only 2–3 % agar polysaccharides. This combined approach also exhibited high AOA (8 mg TE/g extract by ABTS assay) and gelling properties. Three different green extraction techniques were optimized by RSM and compared to extract Agar from red seaweed Hypnea pannosa. UAE exhibited the highest yield (32.99 %) of superior quality agar among all, preceded by MAE (27.04 %), US-assisted microwave extraction (26.19 %), and CE (12 %). UAE also resulted in the highest gel-forming ability and viscosity, achieving values of 485.50 g cm⁻² and 1.32 %, respectively. The reduction of agar extraction at higher power was due to monosaccharides’ formation as a result of agar breakdown, or saturation impact occurrence by excessive bubble generation near the US probe tip, which consequently led to low energy transfer to seaweed [90].

4.3. Microalgae-derived polysaccharides

Chen et al. [91] optimized the UAE to extract microalgae Dictyosphaerium sp. water-soluble polysaccharides as well as refined and characterized them, along with evaluating their AOA. UAE contributed to a yield of 7.12 % with a notably strong AOA of 93.41 and 73. 68 % by DPPH and ABTS assay. The obtained yield was significantly dependent upon UAE temperature, power, extraction solvent, and S/L ratio. Babich et al. [92] determined the nutrient media composition on the growth of microalgae Vischeria punctata and polysaccharides production, extracted by UAE. UAE resulted in a lower yield of bound polysaccharides (65.1 mg/g) than chemical treatment with heat (132.2 mg/g). A higher treatment time, the polysaccharides' structure disrupted by UAE, contributed to a lower yield. Di Caprio et al. [93] used pulsed and cyclic UAE to extract starch from microalgae Tetradesmus obliquus and Chlorella sp. Pulsed treatment exhibited significantly higher carbohydrate yield of 74 and 87 % with 6 times less energy consumption than the continuous one (≈ 30 and 25 %). The lower yield by continuous treatment is due to a higher decline in the applied power.

4.4. Green algae- derived polysaccharides and other bioactive polysaccharides

UAE was optimized by RSM to extract green macroalgae Ulva rigida sulfated polysaccharides, and their AOA was also determined. UAE contributed to a maximum of 13.12 % yield with an improved AOA. Extracted polysaccharides exhibited 1.57 and 1.50 mg TE/mL AOA by DPPH and ABTS assay [59]. The hybrid protein-polysaccharide extracts of green seaweed Ulva sp. were prepared by the pH shift method in combination with the US pretreatment. This extraction technique, by rupturing the cell wall, resulted in a higher extraction, achieving a level of 51 % polysaccharides from the extracts, with the ulvan main component. It also improved the AOA with 12.4 μg/mL IC₅₀ [60].

A US-assisted water extraction process was carried out to extract red algae Rhodymenia intricata's polysaccharides, and their AOA, before and after the purification was also investigated. This process yielded 37.78 % polysaccharides. High yield by large mesh size was because there was more crushing, so more cell wall disruption, but decreased at higher mesh sizes, as polysaccharides agglomeration and settlement in the solution can occur due to fine particles. A markedly high S/L ratio can minimize the sonication effect and lower the extraction process, consequently resulting in lower yield. UAE also resulted in higher AOA, achieving a maximum value of 42.80 % by DPPH assay before and 98.30 mg TE/g by ABTS assay after purification in samples, respectively [94]. The protein-polysaccharide extracts from brown algae (A. nodosum and S. latissima) were extracted through a sequential extraction by US, pH optimization, and an enzymatic method, as well as these were further characterized. The sequential treatment of US with acid/alkali extraction contributed to a higher yield with maximum emulsifying attributes and an improved AOA of ≈ 99 % and 70 Trolox equivalents/g of extract. They achieved a maximum value of 19.6 % in A. nodosum with alginate and fucoidan, key polysaccharides. This yield was higher than the sequential enzymatic method and the control sample, which exhibited 6.5 and 5.4 % yield, respectively. A. nodosum also exhibited 62.9 % anti-hypertensive properties [95]. On the other hand, a biorefinery method in which the UAE, combined with different NaDESs, was used to extract red macroalgae Palmaria palmate polysaccharides. UAE-NADEs contributed to a maximum of 54.3 % yield. However, that yield was lower than pulsed light (PL)-water extraction, which resulted in 61.7 % polysaccharides. It also resulted in a lower AOA than PLE-NADEs, achieving a value of 14.51 µmol trolox/g and 116 IC₅₀ µg/mL seaweed by Trolox Equivalent Antioxidant Capacity (TEAC) and Oxygen Radical Absorbance Capacity (ORAC) assay [96].

The UAE process was optimized using BBD to extract brown seaweed S. compressa polysaccharides and phlorotannins, and the impact of the UAE parameters was also determined. UAE resulted in 23 % yield of polysaccharides, exhibiting a negative relation with ethanol amount due to polarity alteration in the solvent [97]. Karthika Parvathy et al. [2] extracted polysaccharides from freeze-dried brown seaweeds, including S. wightii, Spatoglossum asperum, Colpomenia sinuosa, and Padina tetrastromatica, using different extraction methods and characterized them. The UAE contributed to a 30–50 % yield, with a maximum yield (46.91 %) and AOA (36.24 %) by DPPH in C. sinousa, which is lower than EAE (50–60 %) and higher than MAE (20–40 %) and CE (15–45 %). UAE and MAE exhibited 5–25 % more extraction than CE. A study was carried out to extract brown algae L. japonica polysaccharides by using different techniques, and their structural and antioxidant characteristics were investigated. UAE and EAE significantly contributed to a higher yield of 9.73 % and 9.12 % than HWE (5.48 %) and acid-assisted extraction (6.54 %). They also resulted in improved AOA, UAE achieving a value of 42.75 % by DPPH due to hydroxyl groups in these extracted substances. The EAE contributed to 56.26 % AOA [68].

On the contrary, when the effect of different extraction processes on the structural and cytotoxic properties of the marine sp. Ecklonia kurome pyran polysaccharides were compared and investigated. EAE exhibited a higher yield of 14.29 % than HWE and UAE, which contributed 5.97 and 5.81 % polysaccharides, respectively. The UAE also led to the highest structure destruction of polysaccharides among others. After purification of the obtained polysaccharides, all processes showed anti-tumor properties in polysaccharides against human breast cancer cells (MCF-7) and caused apoptosis [98]. Enzyme-assisted US extraction was applied to extract marine algae Schizochytrium limacinum meal, and their AOA was evaluated. It resulted in 11.86 % polysaccharides with a strong AOA of 0.157 mg/mL, because US generates a cavitation phenomenon, which produces micro-jets, a turbulent process, and breaks down small-sized droplets [45].

DES-based US extraction was used to extract brown seaweeds Sargassum horneri polysaccharides, and these were characterized. It resulted in a lower yield of 11.31 % with 71.28 % ABTS radical scavenging activity, than HWE (13.52 %), but with a lower time consumption. The higher yield by increasing the solvents' molar ratio was because of enhanced viscosity and surface tension, and a lower yield by increasing ratios further might reduce the contact between solvents. The reduced yield at higher S/L ratios was because the excess solvent makes the extraction process more complex after the system attains equilibrium. At higher water content, the mixture becomes more polar, so there is a reduction in contact among molecules [99]. Novel techniques, UAE, and cold plasma alone and as a pretreatment of CE were applied to extract polysaccharides from brown algae Fucus virsoides and C. barbata. UAE resulted in a lower yield of 6.72 %, three times lower than cold plasma (7.29 %) and CE (11.74 %) in the case of F. virsoides. When the UAE was combined with CE, it showed little similarity in yield, achieving a value of 11.41 %. The reason can be low temperature or less extraction time of the UAE. US extracted polysaccharides also exhibited a lower AOA (19.97 µmol TE g⁻¹ in F. virsoides) by ORAC assay, and the DPPH value was also lower (≈ 34 % in C. barbata) [100]. Similarly, when Okolie et al. [61] applied UAE on seaweed A. nodosum polysaccharides, it contributed to a lower yield of 4.56 % than CE (11.9 %). UAE was applied to the brown algae S. angustifolium to extract crude polysaccharides and to determine its effect on the structural characterization, as well as their AOA and antitumor activity. It resulted in a 7.14 % yield with anti-tumor activity upto 40.56 %. It also exhibited an AOA IC₅₀ value of 0.24 mg/mL by ABTS and 0.32 mg/mL by DPPH assay. Further, when these were fragmented, 38.12 and 61.07 % polysaccharides were obtained with the maximum AOA (0.17 mg/mL) and anti-tumor activity (70.81 %) in the 2nd fraction [69].

5. Bio- functional properties and industrial use of extracted polysaccharides

The Bio-functional properties include cytotoxic and functional properties. The functional properties are bioactive and rheological properties, including viscosity, AOA, gelling ability, and antimicrobial. The AOA of extracted polysaccharides is dependent upon their structural properties and is a key property to be used in functional food products and medicine development [45,69,101]. Viscosity of polysaccharides, mainly agar, is the image of the purity and quality of a product, exhibiting various applications [90]. The emulsifying characteristics affect the texture and sensory properties of the foods, so they have a critical role in food product development [95]. The polysaccharides, including alginate, agar, and carageenans from seaweeds, extracted by UAE alone or combined with heat in the case of agar, have potential applications as food additives, gelling agents, stabilizers, and coatings [58,85]. The extracted fucoidan has significant biological potential, such as antioxidant, anticancer, and macrophage-stimulating ability, specifying its potential applications in food (processed and functional), pharmaceutical, and chemical sectors [75,100]. The obtained polysaccharides from brown algae can also be used as antioxidants [95]. Keeping in view this much importance, Table 3 describes the functional and cytotoxic properties along with the UAE extracted polysaccharides' use at an industrial level.

Table 3.

Effect of UAE on the functional and cytotoxic properties with their industrial application.

Polysaccharide Type	Algal source	Functional properties		Cytotoxic properties	Industrial relevance		References
		Bioactive properties	Rheological properties		Food Applications	Others Cosmetics
Alginate	Brown algae	−	Soft gel formation, predominant elastic behavior	Non-cytotoxic	−	−	[55]
Carrageenan	Seaweed	−	Improved viscosity	−	Use as a food additive	Pharmaceutical	[86]
						Drugs
						Medical applications
Polysaccharides	Brown algae	Significant antiglycation effect	−	−	−	−	[114]
Fucoidan	Brown algae	Enhanced AOA	Improved EC	−	Development of dietary supplements for humans	−	[116]
					Use in functional foods product development
Polysaccharides	Irish seaweed	Antimicrobial activity against E. coli but not against L. innocua	−	−	Use as an antimicrobial agent in food safety settings	−	[113]
Polysaccharide	Brown algae	Lower AOA	−	−	Use in processed food	Use in pharmaceutical	[100]
					Use in functional food	and chemical industries
Fucoidan	Irish brown seaweed	−	−	Potential anticancer properties	−	−	[104]
Polysaccharides	Brown seaweed	Improved AOA	−	−	Use as a potential natural antioxidant	−	[99]
κ-Carrageenan	Marine red seaweeds	Stable AOA and anti-inflammatory properties	Low viscosity, a proper in situ gelation behavior and marked elastic properties	−	−	−	[117]
Fucoidans	Marine seaweed	Inhibitory effect against P. aeruginos, strong antiviral activity against HSV-2 infection	−	−	−	−	[65]
Carbohydrates	Red algae	−	−	Greater cytotoxicity	−	Use in the delivery of chemotherapeutics −	[115]
Polysaccharides	Marine algae	Potent AOA	−	−	Use as a natural antioxidant	−	[45]
FSPs	Seaweed	No extreme improvement in AOA	−	−	−	−	[79]
Polysaccharides	Brown algae	Improved AOA	High EC	−	Use in dairy	−	[85]
					Use in bakery
					Use in beverages
					Use in processed meat
Fucoidan	Seaweed	Appreciable AOA and macrophage-stimulating capacity	−	Appreciable anti-cancer properties	Potential value for food industry	Use for health maintenance	[75]
Sulfated polysaccharides	Brown seaweed	Potent AOA	−	−	−	−	[78]
Polysaccharides	Brown algae	Improved AOA	−	Improved anti-cancer properties	Use in functional food product development	Use in pharmaceutical industries for development of medicines	[69]
Polysaccharide	Green algae	Strong AOA	−	−	−	−	[91]
Unpurified agar	Red seaweed	Improved AOA	Softer brownish gels with lower gel strength	−	Use as a texture modifier	−	[58]
					Use as thickening agent	−
Polysaccharide	Seaweed	Higher AOA	−	−	Food ingredients in novel food products	−	[60]
Polysaccharide	Red algae	−	−	No cytotoxicity	−	Use in cosmetic products	[110]
Fucoidan	Brown algae	A noticeable impact on AOA	−	−	Use in food industries	Use in pharmaceutical	[67]
						Use in cosmetic industries
Polysaccharide	Macroalgae	Enhanced AOA	−	−	Use as an antioxidant in food products	Use as an antioxidant in pharmaceutical industry	[94]
Polysaccharides	Microalgae	−	−	−	Use in novel functional foods	Use in pharmaceutical substances	[92]
					Use in biologically active food supplements
Polysaccharides	Brown seaweed	High AOA	−	−	Use in functionalproducts	Use in neutraceuticals	[68]
Polysaccharides	Seaweed	−	−	Anticancer properties	−	Natural medicine for the treatment of breast cancer	[98]
Alginate	Brown algae	High AOA	High viscosity	−	−	−	[81]
Sulphated polysaccharides	Green seaweed	High AOA	−	−	−	−	[59]
Alginate	Brown algae	−	−	Improved anticancer properties	−	Anti-cancer drugs development	[53]
Carrageenan	Red seaweed	High AOA	High shear-thinning behaviour and apparent viscosity, high gel strength	−	−	−	[50]
Polysaccharides	Red macroalgae	Lower AOA	−	−	−	−	[96]
Fucoidans	Brown seaweed	High AOA and anti-inflammatory effect	−	−	−	Use in the clinical treatment of several pathologies	[112]
Polysaccharides	Brown seaweed	High AOA and prebiotic activity	−	−	Use as a functional ingredient	−	[2]
Agar	Red seaweed	−	Better gel strength and viscosity	−	Use as a dressing or fat replacer	Use in neutraceuticals	[90]
					−	Use in cosmeceuticals
Fucoidan	Brown seaweed	Improved AOA	−	Potent anti-cancer activity	−	−	[66]
Fucoidan	Brown seaweed	Improved AOA	−	−	Use in food formulations	−	[77]

(AOA) Antioxidant activity; (FSPs) fucose-sulphated polysaccharides; (EC) emulsifying capacity.

6. Structure-activity relationship, industrial scale-up, and safety of UAE-derived polysaccharides

UAE, along with increasing the yield of polysaccharides, changes the structural properties (branching, molecular weight, monosaccharide composition, and sulfation levels) of polysaccharides. Consequently, there occur changes in bioactive properties such as anti-tumor, antioxidant, anticoagulant, antithrombotic, immunoregulatory, antiviral, and anti-inflammatory effects. The AOA is inversely proportional to the Sulfate level and molecular weight. While it also depends on the polysaccharides' structure, protein value, and position of sulfate groups [50,59,102,103]. For instance, the UAE decreased the molecular weight of agar of seaweed H. pannosa, which in turn exhibited higher AOA than CE-treated seaweed [90]. Similarly, when CE preceeded by ultrasound-induced depolymerization of fucoidan from Irish brown seaweed F. vesiculosus was done, it resulted in increased antitumor activity, due to structural changes linked with the biological activity [104]. ]. In another study, in green algae Ulva lactuca, UAE contributed to an enhanced radical scavenging activity by DPPH and ABTS assays than CE, resulting in reduced molecular weight [105]. The US-assisted enzymatic extraction of green algae Ulva spp. presented a higher amount of sulfate levels (21 %) and up to 786 kDa of molecular weight. While species containing a low molecular weight had higher AOA and anticoagulant properties [106]. UAE was optimized, and it extracted microalgal polysaccharides of Dictyosphaerium sp. containing a maximum molecular weight of 40 kDa, with enhanced radical scavenging activity [91].

UAE enhanced yield with minimized solvent use and processing duration, but the scale-up of the US-assisted extraction process and managing its energy cost pose a problem. Therefore, future investigation is needed to report the impact of the change in raw material, downstream techniques, for example, heating and drying, and cost exploration for cost-efficient scalability with low energy consumption and successive carbon dioxide release. Techno-economic and life cycle assessment (LCA) tell us that the UAE can be environmentally advantageous, but an energy optimization approach is needed for economic feasibility and sustainability at the scale-up level [107,108]. Detailed LCA results further highlight its need, for example, Ulva sp. integrated UAE process reported global warming potential (GWP) of 29.50 kg CO₂ equivalent /g of polysaccharide, human toxicity potential (HTPnc) of 48.80 kg 1,4 DCB, and terrestrial ecotoxicity potential (TETP) of 26.40 kg 1,4 DCB/g of polysaccharides [109]. The toxicity and safety of UAE-derived polysaccharides also require thorough investigation. Algal polysaccharides are considered to be safe, but the US can cause their structure modification, resulting in a negative effect on bioaccessibility as well as immunogenicity. In a study, the cytotoxicity of extracted alginic acid from S. angustifolium was evaluated before its usage in drug development [53]. Similarly, cytotoxicity of red Alga Solieria chordalis after UAE was assessed before application in cosmetics [110].

7. Challenges, future recommendations, and conclusion

The process conditions, such as temperature, pH, and extraction time, as well as the generation of by-products, can affect the polysaccharides’ yield and quality. A future investigation is necessary on the optimization of these parameters in industrial applications. For example, time and temperature are used for food-grade antioxidant polysaccharides, pH optimization for anticoagulant polysaccharide behavior in the pharmaceutical industry, and optimized by-products generation for application in cosmetics.

Technological improvements are also needed in the optimization of the process parameters, including temperature, energy consumption, and raw material collection, to enhance the polysaccharides extraction efficiency and selectivity. The emphasis should be directed towards US and MAE combined extraction, to establish industrial feasibility for assessment of energy trade-offs, cost-benefit analyses, and solvent recycle potential. Studies should also focus on improving purification and characterization to improve bioactivity and components investigation, as well as further exploration of the UAE mechanism of polysaccharides. A broad investigation is required on the chemical and physicochemical studies through analytical techniques such as HPLC, Near Infrared Spectroscopy (NIR), and X-ray Fluorescence (XRF) to understand how much the chemical composition of the extracted polysaccharide affects biological characteristics, as well as to examine various components of the extracted polysaccharides. It will also facilitate interaction between molecular characterization and AOA. The detailed compositional analysis (sulfate content, molecular weight) via HPLC coupled with techniques like FTIR and NMR is critical for linking composition and bioactivity. For example, Zhang et al. [111] optimized perilla seed meal polysaccharides via RSM. These polysaccharides revealed strong antioxidant yields tied to molecular characteristics analyzed via Gas Chromatography–Mass Spectrometry (GC–MS), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance Spectroscopy (NMR).

For public acceptability and product safety, future studies should also explore sensory analysis for food applications, cytotoxicity, and immunogenic methods of polysaccharides such as alginates and fucoidans, extracted by UAE, to be applicable in cell culture and animal systems. Future studies should also be emphasized to screen anti-nutritional factors or contaminants during the US application. Studies also highlight the need for future investigation on the AOA of the extracted polysaccharides and their use as antioxidants. Studies should also be conducted on the internal diffusivity factors, for example, solvent polarity and particle size, as well as on antithrombotic mechanisms of sulfated polysaccharides, to make them effective for therapeutic use. Research should also investigate possible interaction among the structure and activity of the polysaccharide by linking sulfate position, length of chain, and anticoagulant activity. Principal component analysis has revealed varying yields of fucose and glucans across different extraction techniques, highlighting the need for separate extraction methods in biorefinery. Future research should investigate the therapeutic applications of R. intricata polysaccharides to advance their clinical applicability, mainly for cancer, cardiovascular diseases, and wound-recovery systems.

The US has emerged as a green, fast, and efficient extraction method for the recovery of polysaccharides from non-conventional sources, micro and macroalgae. It increases the extraction yield with less energy, cost, and time consumption. It also exhibits promising effects on the bio-functional properties, including gelling properties, viscosity, as well as antioxidant, antimicrobial, anti-glycation, and anticancer activities. Its synergism with other methods, for example, enzymatic processes, MAE, and DESs, supports green biorefinery approaches, which further improve the recovery of these valuable compounds with enhanced properties. The extracted polysaccharides from marine biomass can be promising candidates in the agriculture, food, cosmetics, pharmaceutical, and nutraceutical sectors. These embrace substantial potential as a valuable antioxidant and functional ingredient in the food sector. Future research is required to optimize UAE conditions, safety, purification, characterization, and valorization of polysaccharides. Further studies should also focus on structure–activity relationships, AOA, and internal diffusivity factors. The future study should focus on optimization of the key process parameters to increase the extraction efficiency and antioxidant activity. There is also a need to explain the effect of ultrasonic power on the structural properties of polysaccharides and to devise methods that can reduce the use of energy in large-scale processes. The future development of purification methods and structure–activity data will also be essential to the successful application of UAE-derived polysaccharides as a functional and antioxidant ingredient in many industries.

CRediT authorship contribution statement

Tehmina Naseem: Writing – review & editing, Writing – original draft, Investigation, Conceptualization. Khushi Ali: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Conceptualization. Nisha Zahid: Writing – review & editing, Writing – original draft. Syed Ali Hassan: Writing – original draft, Validation, Investigation, Data curation, Conceptualization. Gholamreza Abdi: Writing – review & editing, Writing – original draft, Supervision, Software, Project administration, Conceptualization. Seydi Yıkmış: . Rana Muhammad Aadil: Writing – review & editing, Writing – original draft, Software, Resources, Conceptualization, Methodology.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.