Distinct authors have proven that seaweeds from the genus Sargassum have high potential to serve as bio-source of antioxidant compounds. To measure their antioxidant potential, a primary screening is usually carried out using different chemical methods including, among others, radical scavenging activity, protection against lipid peroxidation, metal-ion chelating ability, and reducing capacity, which allow the evaluation of the compounds’ mechanistic intervention, concentration effectiveness, and synergistic effects.

Radical scavenging is one of the most typical mechanisms of antioxidant activity and can be tested via a wide variety of in chemico methods. DPPH and ABTS⁺ radicals are the most commonly used due to their stability, reproducibility, and simplicity. However, although they are useful for an initial screening of the extracts’/compounds’ antioxidant activity, they have low biological relevance since they are synthetic and do not occur in biological systems [68]. For that, radicals, such as NO^•, O₂^•− or HO^•, are a more suitable approach since these are biologically produced during normal cellular activities [72].

Reducing power is another common method that has been used as a comparative tool among foods and algae. It essentially measures the antioxidant activity of a compound through its ability to stabilize radicals by donating electrons, usually involving the reduction potential of transition metals, such as iron (Ferric Ion Reducing Antioxidant Power, FRAP) or copper (Cupric Reducing Antioxidant Capacity, CUPRAC) [73].

Free radicals can also originate from heavy and transition metals, namely mercury, lead, arsenic, and iron, leading to diseases associated with oxidative stress. Therefore, another mechanism of estimating the antioxidant activity of Sargassum compounds is via their capacity to chelate transition metals, forming complex structures which decrease the metal reactivity and eases their excretion from the body.

Other systems, such as β-carotene bleaching or lipid peroxidation, have been reported on compounds extracted from Sargassum seaweeds, all of them employed for chemically screening their antioxidant activity of compounds at certain conditions.

Clearly, phenolic compounds, in particular phlorotannins, appear as the major and most well studied group of compounds contributing to the antioxidant properties of Sargassum seaweeds. Indeed, positive correlations between phenolic content and antioxidant activity have been reported in many studies using whole algae, their parts, extracts, or fractions [74,75,76,77]. Notably, studies on phlorotannins isolated from Sargassum have shown that compounds, such as triphlorethol B, tetraphlorethol C, and pentaphlorethol A, can be more effective than certain antioxidant references such as ascorbic acid or resveratrol at preventing lipid peroxidation or scavenging of O₂^•− radicals [78]. The phenolics group is not, however, the solo contributor for the antioxidant properties of Sargassum seaweeds.

Other molecules, such as pigments have been described for their strong radical scavenging and reducing power properties. This is the case of the pigment-rich extracts retrieved from S. cristaefolium, which were shown to exert promising dose-dependent antioxidant properties via DPPH and FRAP assays [79]. Likewise, a positive correlation between the antioxidant activity and the carotenoid content of methanol and ethanol extracts from S. siliquosum and S. polycystum was described on ABTS^+•, DPPH^• and FRAP [80]. More recently, a carotenoid-rich extract obtained from S. polycystum was found to be the most active on DPPH^• and ORAC assays compared with those obtained from two other seaweeds, including Euchema denticulatum (red) and Caulerpa lentillifera (green), most likely due to its high content in fucoxanthin [81]. In fact, within the group of carotenoids, fucoxanthin clearly stands out as one of the most prominent compounds. Due to its structural features, specifically the presence of an allenic bond and a 5,6-monoepoxide motif, fucoxanthin has a strong proton donating capacity that translates into an exceptional antioxidant activity [82]. Indeed, several authors have described strong antioxidant activities in multiple chemical assays (e.g., DPPH^•, ABTS^+•, and FRAP) for different Sargassum spp.-derived extracts that contained significant concentrations of fucoxanthin [82,83,84]. Moreover, Raji et al. [85] reported that fucoxanthin isolated from S. wightii exhibited great radical scavenging properties with IC₅₀ values if 79.55 µM and 75.99 µM in DPPH^• and ABTS^+• assays.

Apart from carotenoids, meroterpenoids have also been described as relevant contributors for the antioxidant properties of the Sargassum seaweeds. Among them, sargachromenol (SCM), sargaquinoic acid (SQA), and sargahydroquinoic acid (SHQA) stand out the most, with several authors reporting promising antioxidant activity measured via ABTS^+•, DPPH^•, FRAP, OH^•, and O₂^•− assays for extracts containing these compounds [52,56]. In fact, after isolating SCM and SQA from S. micracanthum, Ham and coworkers observed that these two meroterpenoids displayed strong DPPH^•− scavenging activities, with the latter being slightly better than the former (49.3 versus 100.2 µM) [86]. Concordantly, the scavenging effects of SCM, SQA, and SHQA isolated from S. serratifolium on DPPH^• (IC₅₀ = 8.0, 15.3 and 5.9 µg/mL, respectively) and OH^• (IC₅₀ = 0.26, 0.27 and 0.27 µg/mL, respectively) were found promising and even superior to that of the commercial antioxidant reference butyl hydroxytoluene (IC₅₀ = 40.4 and 0.9 µg/mL, for DPPH^• and OH^•, respectively) [56]. Likewise, upon isolation from S. thunbergii, Seo et al. noticed that these three compounds were as good peroxynitrite (ONOO^-) scavengers as L-ascorbic acid or penicillamine [87].

Derivatives of these compounds were shown to exert excellent antioxidant properties as well. According to Jang et al., sixteen different sargachromanols isolated from S. siliquastrum exhibited significant radical scavenging activity in the range of 87–91% at the concentration of 100 µg/mL [88]. Moreover, mojabanchromanol, isolated from the same species, showed equal or even better results than BHT, L-ascorbic acid, or α-tocopherol on TBARS and DPPH^• assays [89]. Identical observations were described for thumbergol A and B, isolated from S. thunbergii, which were also as effective as BHT, L-ascorbic acid and α-tocopherol at scavenging DPPH^•, ONOO⁻, and O₂^•− [60]. In turn, superior antioxidant activities comparing with L-ascorbic acid and α-tocopherol were described for other chromene derivatives retrieved from S. micracanthum, including 2-geranylgeranyl-6-methylbenzoquinone and its hydroquinone derivative, performing 28 to 300 times better inhibitory properties on lipid peroxidation than the referred standard compounds [50]. Two other derivatives from 2-geranylgeranyl-6-methyl-1,4-benzohydroquinone isolated from the same species were also found very active against lipid peroxidation, showing an inhibitory effect 40 times stronger than that of α-tocopherol [51].

Apart from these compounds, there are other less studied molecules that can still can contribute importantly for the antioxidant activity of Sargassum seaweeds. This is the case of (+)-epiloliolide, which was isolated from S. naozhouense and, despite not exerting as good DPPH^• scavenging activity as ascorbic acid, it displayed a quite interesting IC₅₀ value of 17 mM [54]. In turn, four terpenoids isolated from S. wightii, including 2α-hydroxy-(28,29)-frido-olean-12(13),21(22)-dien-20-propyl-21-hex-40′(Z)-enoate, 2α-hydroxy-(28,29)-frido-olean-12(13),21(22)-dien-20-prop-2(E)-en-21-butanoate, 2α-hydroxy-8(17),12E,14-labdatriene, and 3β,6β,13α-trihydroxy-8(17),12E,14-labdatriene exhibited DPPH^• and ABTS^+• scavenging effects comparable with that of BHT, and significantly better activities in the range of approximately 2–5 times comparing with α-tocopherol [64].

However, one should bear in mind that, for complex samples such as crude extracts, the antioxidant activities are a result of the interactions occurring between multiple compounds that can either lead to synergistic or antagonistic effects. Therefore, the presence of certain compounds individually recognized as antioxidants in a given sample may not translate into the expected antioxidant properties.

Following in chemico screening experiments, the use of biological models is essential to further corroborate the antioxidant activity of a compound and elucidate its mechanisms of action. The screening of the antioxidant activity of Sargassum spp. secondary metabolites in cell and animal models has thus been reported by a few authors. In general, oxidative stress inductors, such as H₂O₂, carbon tetrachloride (CCl₄), tert-butyl hydroperoxide (t-BHP), and 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH), are applied, while the protective effects of secondary metabolites are estimated through monitorization of oxidative stress markers, such as the levels of intercellular reactive oxygen species (ROS), lipid peroxidation, enzymes, and transcription factors involved in redox homeostasis. 

As in chemical studies, distinct authors have emphasized the oxidative stress-protective abilities of Sargassum spp. phenolic-rich extracts in biological systems., e.g., Pinteus et al. [111] demonstrated that two purified fractions obtained from the S. muticum extract (rich in phenolics, including phlorotannins), namely the MeOH fraction and 50:50 MeOH:DCM fraction, were able to reduce the H₂O₂—induced elevation of intracellular ROS in MCF-7 cells by 70% and 56%, respectively. Likewise, phenolic compounds together with sulfated polysaccharides were reported as the major contributors to the antioxidant activity of a S. polycystum Celluclast-assisted extract, which was demonstrated to significantly decrease the cell death brought on by H₂O₂-induced oxidative stress in zebrafish embryos to normal levels [112].

In a different in vivo model, a polyphenol-rich extract of S. pallidum containing mainly phlorotannins was reported to decrease lipid peroxidation by 37%, increase the levels of the antioxidant enzyme superoxide dismutase (SOD) levels by 47%, and restore GSH to the control levels in CCl₄-induced oxidative stress in Wistar rats [113]. Following an identical behavior, a non-identified polyphenol-rich extract of Sargassum spp. led to an increase in GPx levels (8.14 ± 4.49 nmol/g) in the liver of seaweed-treated animals when compared to the non-treated CCl₄-induced Wistar rats (9.88 ± 1.07 nmol/g) [113,114]. Furthermore, after administrating a phlorotannins-rich extract from S. hemiphyllum to CCl₄-exposed Kunming mice, Zhao et al. [115] observed a statistically significant increase in SOD, catalase (CAT), glutathione peroxidase (GPx), and total antioxidant capacity (TAOC) in the serum, kidneys, liver, and brain (p < 0.05), which was accompanied by a significant decrease in lipid peroxidation in their livers (p < 0.05).

Terpenoids constitute another important group of compounds of Sargassum origin that have shown important antioxidant properties in cell and animal models. Among these, the carotenoid fucoxanthin, isolated from a methanolic extract of S. siliquastrum, was proven to significantly decrease ROS production in H₂O₂-stimulated Vero cells at 5 µM (p < 0.05), 50 µM and 100 µM (p < 0.01) [116]. Other terpenoids, such as meroterpenoids, extracted or isolated from several Sargassum spp., also demonstrated interesting protective effects on oxidative stress induction. Indeed, after treating t-BHP-stimulated HepG2 cells with a meroterpenoid-rich extract of S. serratifolium (mainly containing SHQA, SCM, and SQA), Lim et al. [57] showed a statistically significant ROS and lipid peroxidation reduction in a dose-dependent manner (p < 0.05). In addition, sargachromanols D, E, and K, and three other chromanols isolated from S. siliquastrum at 5 µg/mL, significantly restored GSH in H₂O₂-stimulated HT1080 cells [59].

A similar trend was described for (−)-loliolide, a different type of monoterpenoid, which was able to significantly reduce ROS levels (p < 0.01) on AAPH-stimulated Vero cells and zebrafish embryos at concentrations of 12.5 and 25 µg/mL. Interesting inhibition of lipid peroxidation was observed in a different study using zebrafish embryos as well [44]. Notably, an indole derivative, namely indole-6-carboxaldehyde, isolated from S. thunbergii, was found interfere strongly with the cellular antioxidant defenses of V79-4 cells, triggering a significant increase in the expression heme oxygenase-1 (HO-1), and most importantly, in nuclear factor-erythroid 2-related factor 2 (Nrf2), an important transcription factor that regulates the expression of intracellular antioxidant proteins [62].

Multiple diseases are frequently linked to oxidative stress. Considering this, many in vitro and in vivo experiments of Sargassum spp. revealed a variety of protective properties against diseases linked to the antioxidant activity associated with the secondary metabolites present in this brown seaweed.

UV irradiation leads to increased inflammation and skin damage through the induction of ROS. In UV-B irradiated HaCaT cells, i.e., immortalized aneuploid keratinocytes from adult human skin, extracts and/or compounds from different Sargassum species have been shown to decrease ROS production, lipid peroxidation, and increase antioxidant defense through induction of SOD and CAT activity. Accordingly, Piao et al. [117] and Han et al. [118] demonstrated that ethanolic and methanolic extracts from S. muticum and S. horneri, respectively, triggered a significant increase in SOD and CAT activity (p < 0.05), and a decrease in ROS and lipid peroxidation in UV-B-exposed HaCaT cells at all tested concentrations (S. muticum—12.5 µg/mL, 25 µg/mL, 50 µg/mL, 100 µg/mL, and S. horneri—31.6 µg/mL, 62.5 µg/mL, and 125 μg/mL, respectively). The authors also found a good correlation between their results observed and the high percentage of polyphenols commonly present in the extracts. S. horneri extract contained 7.45 ± 0.29% of total phenolics and 0.29 ± 0.07% of total flavonoids. In a different model using UV-irradiated zebrafish embryos, a polyphenol-rich ethanolic extract from S. thunbergii, demonstrated a decrease (at 0.8 µg/mL, 1.2 µg/mL, and 1.6 μg/mL concentrations, respectively) in ROS production by 71%, 71%, and 85%, respectively [119]. Fucoxanthin and the meroterpenoid tetraprenyltoluquinol chromane, isolated from S. siliquastrum and S. muticum, respectively, showed a similar trend in human dermal fibroblasts exposed to the same radiation [53,120]. Other terpenoids, such as (−)-loliolide and sargachromanol (SC) E, isolated from S. horneri, caused a reduction in ROS production induced by UV irradiation, in HaCaT cells and human dermal fibroblasts, respectively. Thus, (−)-loliolide only caused a significant inhibition at 12.5 µg/mL and 25 μg/mL, while SC E in all concentrations tested (5 µg/mL, 10 µg/mL, and 20 µmol/L) [45,47].

The effects of two S. virgatum extracts with high contents of polyphenols on spermatogenesis and infertility of male Wistar rats exposed to γ-irradiation were shown to increase the antioxidant enzymes SOD, CAT, GPx, and GSH at 100 mg/kg body weight (p < 0.05). At 400 mg/kg, the ethanolic extract displayed the best effect in the increased antioxidant enzymes: 86.38% for SOD, 80.87% for GPx, 90.04% for CAT, and 81.55% for GSH compared with the controls [121]. In a different approach, fucoxanthin, isolated from S. glaucescens, also stimulated these enzymes’ activity in plasma and testicles from Syrian hamsters, which were lowered by cisplatin exposure, a chemotherapeutic agent that is usually associated with infertility [122].

Accumulation of particulate matter in the lungs is another common cause that triggers oxidative stress and can lead to serious tissue damage and cardiopulmonary complications [123]. In this context, the treatment of particulate matter-exposed HaCaT and murine lung epithelial cells with polyphenol-rich S. horneri ethanol extracts exhibited promising effects towards counteracting ROS generation and lipid peroxidation, while notably improving SOD and CAT activities at 62.5 µg/mL and 125 µg/mL [123,124]. Similarly, in particulate matter exposed HaCaT cells and zebrafish embryos, a fucoxanthin-rich extract of S. fusiformis caused a reduction in several factors associated with inflammatory responses, including ROS production, reaching statistical significance at 25 µg/mL, 50 µg/mL, and 100 µg/mL [125]. Other compounds, such as (–)-loliolide, isolated from S. horneri, or fucosterol isolated from S. binderi, demonstrated interesting activities as well. S. horneri revealed a great capacity to boost cell viability through ROS reduction in fine dust-exposed HaCaT cells [46], while S. binderi not only caused a dose-dependent (12.5 µg/mL, 25 µg/mL, 50 µg/mL, and 100 µg/mL) increase in SOD and CAT activity, but also triggered a significant increase in HO-1 levels and Nrf2 nuclear translocation [126].

The synthetic drug methamphetamine (MA) has a neurodegenerative effect on the human brain and can increase the generation of ROS through dopamine oxidation. In fact, a hydroethanolic extract of S. angustifolium rich in polyphenols, including gallic acid, protocatechuic acid, gentisic acid, and hydroxybenzoic acid, led to a decrease in ROS generation on MA-exposed SH-SY5Y dopaminergic cells with doses of 80 and 160 μg/mL [127]. Similarly, fucoxanthin isolated from S. horneri prevented the effects of MA on ROS, significantly reducing their production (p < 0.05) and causing a significant increase in HO-1 expression in PC12 cells at 3 µM [128].

Researchers frequently induce inflammatory reactions in RAW 264.7 cells by exposing them to lipopolysaccharide (LPS). It can stimulate membrane-bound NADP oxidase and consequently, the generation of ROS. Ethanol, methanol, and ethyl acetate S. serratifolium extracts, whose major compounds identified were the meroterpenoids SHQA, SC, and SQA, were reported to dose-dependently (1.25–10 µg/mL) reduce LPS-induced ROS in LPS-stimulated macrophages [56]. The same trend was observed with SCM and SHQA isolated from S. horneri and S. macrocarpum, respectively [48,49]. In turn, an acetone:ethanol extract of S. glaucescens rich in fucoxanthin caused a decrease in LPS-induced ROS and O₂^•− production in the same cell line at concentrations ranging from 25 µg/mL to 100 µg/mL [122]. The anti-inflammatory properties of fucoxanthin are further evidenced by in vivo studies in rodents [129,130], including scenarios of induced asthma [131] and induced colon inflammation [132].

ROS are produced in high quantity in the mitochondria in scenarios of hyperglycemia and in the case of other metabolic disorders such as diabetes. In this sense, Lee et al. [133] treated glucose-stimulated INS-1 cells with an extract of S. sagamianum with the extract at 100 µg/mL and observed a reduction of ROS (by 122%) and lipid peroxidation, attributing these effects to the presence of secondary metabolites, such as phlorotannins and plastoquinones. A hydroethanolic extract of S. fusiformis rich in several secondary metabolites such as flavonoids, fucoxanthin, and fucosterol, also afforded a statistically significant increase in SOD and CAT activities accompanied by a notable decrease in lipid peroxidation in a high-fat diet and streptozotocin-induced ICR mice [134]. In fact, studies using diabetic mice showed that fucoxanthin lowers fasting glucose levels, increases plasma insulin levels and, when administered in tandem with metformin, induces the regeneration of pancreatic β-cells [135,136]. Usually diabetic mice have altered serum lipid profiles, with elevated values of triglycerides and cholesterol, which are reduced when treated with fucoxanthin [135]. These anti-diabetic and lipid profile regulating properties of fucoxanthin are the target of a clinical trial in patients with metabolic syndrome, currently under phase II [137].

Three phlorotannins isolated from S. carpophyllum were identified as anti-allergic agents. The compounds inhibited the activation of mast cells, the cells of the immune system responsible for the release of histamine, particularly when triggered by an allergen [138]. In addition to inhibiting the formation of ROS, phlorotannins are promising candidates for novel anti-allergic drugs. Choi et al. [54] showed that SHQA, isolated from S. serratifolium, prevented the activation of effector cells in allergic responses and caused a statistically significant reduction in ROS formation, once again demonstrating the antioxidant properties of meroterpenoids present in Sargassum spp.

In addition to the previously reported disease-counteracting effects, S. angustifolium hydroethanolic extracts rich in phenolic compounds were shown to dose-dependently (at 20 mg/kg, 40 mg/kg, and 80 mg/kg) improve defense against oxidative stress in rats, through the notable increase in the total antioxidant capacity in rats’ blood, and the decrease in lipid peroxidation brought on by hypertension and dyslipidemia, respectively [139,140].