Adjuvants, substances that boost the immune response to an antigen, are critical in vaccine formulation. Different marine-derived products have been shown to be enhancing vaccine efficacy [56-59] and the exploration of marine-derived adjuvants represents a promising frontier in vaccine development. Marine-derived secondary metabolites, with unique chemical structures and diverse biological activities, are may also improve vaccine efficacy through desirable anti-inflammatory and antioxidant properties. COVID-19, caused by the outbreak of the SARS-CoV-2 virus that resulted in a global health crisis, causes systemic infections featuring tissue-damaging cytokine storms. This. prompted the search for the discovery and integration of various molecular and therapeutic approaches, including vaccines, antivirals of synthetic and herbal origin, and immune-modulating treatments. Additions such as forest bathing and other nature-connected practices have also been explored to enhance physical and mental well-being, reduce stress, and boost immune function during the pandemic [60-71]. By harnessing the unique properties of these marine compounds, it is possible to create vaccines that elicit stronger and possibly more durable immune responses, while reducing tissue damage and protecting against oxidative stress. potentially leading to better protection against a wide range of diseases. Polysaccharides, proteins, and lipids extracted from marine algae, sponges, and mollusks have shown promise as potent adjuvants, with one key example being fucoidan [
72], a sulfated polysaccharide from brown seaweed, with demonstrated immunostimulatory properties. Fucoidan can enhance both humoral and cellular immune responses, making it an ideal candidate for a vaccine adjuvant. Sulfated galactans found in red algae [73-75] are also emerging as potent adjuvants with potential to enhance immune responses and provide antiviral protection.
3.1. Marine-derived polysaccharides as potent vaccine adjuvants and antiviral agents
Sulfated galactans, are linear polymers with alternating β-
D-galactopyranose and α-
D-galactopyranose units. Two primary types are identified: carrageenans, which feature a 4-linked α-galactose with a dextro-rotatory (D-) configuration, and agarans, with a levorotatory (L-) 4-linked α-galactose component [75-77]. Carrageenan has shown antiviral properties, because of its property of disrupting the interaction between the virus and host cell receptors and preventing viral entry. Iota-carrageenan, a sulfated polysaccharide from seaweed, at low concentrations (4 μg/mL) reduced cell death caused by the SARS-CoV-2 virus, while high concentrations of κ and λ-carrageenans (400 μg/mL) offered only partial suppression, indicating that carrageenan/iota-carrageenan may be more effectively used during the initial stages of infection [
78]. An iota-carrageenan nasal spray effectively treated common cold symptoms linked to human coronaviruses, reducing symptom recurrence and improving viral clearance compared to placebo treatments [
78]. A nasal spray combining xylometazoline hydrochloride and carrageenan has alleviated nasal congestion and protected the respiratory mucosa from viruses [
79]. Lozenges containing iota-carrageenan have inactivated viral glycoproteins in the mouth, blocking viral effects. Iota-carrageenan also neutralized a SARS-CoV-2 spike pseudotyped lentivirus, suggesting its potential efficacy in COVID-19 prevention [
80]. Marine-derived saponins from sea cucumbers [
81] can boost immunization [
82] and could be incorporated into vaccine formulations as adjuvants. Other adjuvants could come from marine lipids found in microalgae that can form stable emulsions that increase the bioavailability of antigens and facilitate their uptake by immune cells [56, 59]. Molecules of marine origin could also serve as therapeutics or therapeutic support. For example, infection with several viruses including SARS-CoV-2 infection triggers release of reactive oxygen species (ROS), increasing susceptibility to further infection. Therefore, marine algae-derived compounds with strong antioxidant properties like fucoxanthin and fucosterol (
Figure 1) could counter oxidative damage, reduce oxidative stress overall and protect immune cells and host tissues during active infection.
More in detail, fucoxanthin from
Sargassum siliquastrum reduces DNA damage and enhances antioxidant enzyme levels, while fucosterol boosts cellular antioxidant enzymes and protects human hepatic cells from oxidative stress [83, 84].
Table 1 summarizes published information on marine-derived adjuvants and antioxidants, presents future directions, emphasizing the potential of these compounds in antiviral treatments and the need for ongoing research to better understand their mechanisms and full therapeutic potential.
Other marine natural products [
85] from sea urchins, sea cucumbers, sponges, soft corals, and microalgae exhibited various bioactive properties, including antioxidant, anti-inflammatory, anti-cancer and immune enhancement. Moreover, marine natural products have yielded antimicrobials against coronavirus (SARS-CoV-2 and its variants), tuberculosis,
H. pylori, and HIV, making them promising resources for managing several pathologies [
86]. The anti-inflammatory mechanisms of marine natural products in SARS-CoV-2 infection are particularly notable, as they offer therapeutic benefits with fewer cardiovascular side effects compared to other chemical agents used in COVID-19 treatment. With their ability to target multiple pathways involved in immune regulation and inflammation inhibition, these products hold significant potential for further clinical applications, making the sustainable development of marine ecosystems critical for future biomedical advances [
86]. Dendritic cells (DCs) bridge innate and adaptive immunity by presenting foreign antigens to T-cells and secreting cytokines that activate and coordinate the adaptive response, and lead to long-lasting responses [
87]. Activating DCs is considered a crucial strategy to improve vaccination efficacy. In this context, a novel marine-derived immunomodulatory sulfolipid, β-SQDG18 appeared to be capable of priming human DCs independent of TLR2/TLR4 and trigger an effective immune response in vivo [
88]. β-SQDG18 promotes DC maturation, increases expression of MHC II molecules and the co-stimulatory CD83 and CD86 proteins, along with pro-inflammatory cytokines such as IL-12 and INF-γ that are necessary for antigen presentation to CD4+ T-cells. Mice vaccinated with ovalbumin combined with β-SQDG18 (1:500) generated anti-ovalbumin Ig titers comparable to conventional adjuvants. In a melanoma model, vaccination of C57BL/6 mice with β-SQDG18-adjuvanted hgp10 peptide induced a protective response, slowing tumor growth and extending survival [
88].
3.2. Bioactive compounds from marine sources: antiviral potential and immunomodulatory effects
Marine-derived bioactive molecules, such as griffithsin, and plitidepsin, a cyclic depsipeptide isolated from
Aplidium albicans (
Figure 2), are currently undergoing clinical trials to assess antiviral efficacy [
89]. Several marine phytochemicals can bind with key SARS-CoV-2 drug targets and possess anti-inflammatory and immunomodulatory effects, potentially mitigating COVID-19 complications. The structures of the anti-COVID compounds mentioned in this section are shown in
Figure 2 to illustrate the diversity of marine-derived molecules. These structures encompass different chemical classes of substances, demonstrating the extensive arsenal available from marine sources to combat coronaviruses, particularly SARS-CoV-2. This diversity should serve to inspire new semi-synthetic and clinical studies aimed at optimizing existing natural structures or their combined use in therapy to achieve stronger activities against variants of SARS-CoV-2 or new coronaviruses that may emerge in the future. Molecular docking [
90] and MD simulation studies by Quimque et al. evaluated the binding affinity of marine alkaloids scedapin C and norquinadoline A (
Figure 2) against SARS-CoV-2 targets [
89]. Both compounds demonstrated a high binding affinity for PLpro, a crucial enzyme in SARS-CoV-2 replication and immune response suppression, with the same study suggesting that scedapin C and norquinadoline A may inhibit PLpro, potentially blocking viral replication and activating immune responses. The efficacy of the marine alkaloid fostularin-3, chimyl alcohol, palmitoleic acid, cannabigerolic acid, and acitretin against SARS-CoV-2 was also investigated. Molecular docking and MD simulations revealed that fostularin-3 forms hydrogen bonds and hydrophobic interactions with residues in the Mpro enzyme, a key target for anti-SARS-CoV-2 drugs [
89]. Caulerpin, an alkaloid from various marine algae, showed strong binding to the SARS-CoV-2 main protease (3CLpro) and favorable pharmacokinetic properties, while molecular docking indicated its potential to halt the virus's life cycle and its anti-inflammatory effects by down-regulating pro-inflammatory cytokines involved in COVID-19's hyperinflammatory phase. Also, C-phycocyanin, a pigment from the blue-green algae
Arthrospira platensis [
91], has shown potential to inhibit SARS-CoV-2 non-structural proteins (nsp-8, nsp-7, and nsp-12), with docking studies suggesting its ability to block nsp-12, a key protein in viral replication. Moreover, , nutrient-rich spirulina in nutrients, has been suggested to enhance immune function and reduce inflammation, that could be beneficial for COVID-19 management [
89]. Marine polyphenols, including quercetin from brown algae (genus
Sargassum), exhibit antiviral properties. Consistent with this observation, different studies indicate that quercetin-based treatments may alleviate respiratory symptoms and inflammation associated with COVID-19
in vivo [
89]. However, quercetin is classified as a PAINS (Pan-Assay Interference Compound), which refers to molecules that non-specifically interact with multiple biological targets and can lead to misleading results in drug screening assays [
92,
93]. Due to its promiscuous binding behavior, quercetin's antiviral effects may not always stem from specific interactions with viral proteins, and thus, further studies are needed to confirm its therapeutic potential while accounting for these non-specific effects [
94].
Studies by Song et al. [
95] assessed the anti-SARS-CoV-2 activities of the above-mentioned marine sulfated polysaccharides, including sea cucumber sulfated polysaccharide (SCSP), fucoidan from brown algae, iota-carrageenan from red algae, and chondroitin sulfate C from sharks. SCSP demonstrated the strongest inhibitory activity, potentially blocking viral entry into host cells, whereas fucoidan has shown promise in reducing inflammation and enhancing vaccine responses, warranting further research [
95]. Molecular docking studies evaluated briarane-type diterpene excavatolide M from gorgonian (
Briareum excavatum) for its ability to bind SARS-CoV-2 TMPRSS2 and the potential of illimaquinone, a marine sponge metabolite, against SARS-CoV-2 target proteins, including papain-like protease, compared to standard antiviral drugs [
89]. An in silico study identified esculetin ethyl ester from marine sponge
Axinella cf. corrugata as a compound with strong binding affinity to SARS-CoV-2 protease N3, while seaweed lectins, including griffithsin (
Figure 3) from
Griffithsia sp., inhibited various enveloped viruses
in silico [
89]. Griffithsin has shown the ability to block SARS-CoV spike glycoprotein and prevent viral entry into host cells which recalled clinical trials aimed at investigating its potential against HIV and SARS-CoV-2.
Overall, we can conclude that marine-derived bioactive molecules have promising potential applications for combating COVID-19. Some, like spirulina, are already in use [
96], while others, such as griffithsin and plitidepsin (
Figure 2), have entered clinical evaluation. Ongoing studies are crucial to further explore their therapeutic potential and determine their efficacy against viruses from the beta coronavirus family [
89].
Table 2 summarizes marine-derived compounds identified through molecular docking studies and other approaches [
89] for their specific interactions with SARS-CoV-2 proteins. Scedapin C, norquinadoline A, and fostularin-3 show potential in inhibiting key viral enzymes such as PLpro and Mpro, which are crucial for viral replication. Additionally,
caulerpin and
C-phycocyanin exhibit anti-inflammatory properties alongside their ability to block viral replication. Others, such as marine-derived polysaccharides like fucoidan and iota-carrageenan, target viral entry, while
griffithsin shows promise in blocking the spike glycoprotein. These compounds offer potential as therapeutic agents against COVID-19 by targeting specific proteins involved in the virus’s life cycle.
Beyond predictive in silico studies, preclinical trials have also highlighted marine compounds' therapeutic and prophylactic potential [
107]. Various concentrations of fucoidan, specifically RPI-27 and RPI-28, extracted from
Saccharina japonica have demonstrated antiviral activity against SARS-CoV-2 in Vero cells. RPI-27 significantly inhibited infection (EC
50 = 0.08 μM), showing greater efficacy compared to RPI-28 (EC
50 = 1.2 μM). Carrageenans, may prevent SARS-CoV-2 entry through the nasal cavity by interacting with the virus's positively charged membrane. Additionally, plitidepsin showed 90% inhibitory activity at 0.88 nM against SARS-CoV-2, far exceeding the efficacy of the antiviral remdesivir [
107]. Another marine compound, gallinamide A, from the cyanobacteria
Schizothrix, inhibited cathepsin L and significantly reduced viral load. Furthermore, the above-mentioned lectin griffithsin has shown high specificity for viral glycoproteins, effectively inhibits early-stage viral infection in a dose-dependent manner, and completely protected SARS-CoV-2-infected rats treated with it (100% survival). The already discussed esculetin ethyl ester has also been shown to inhibit the SARS-CoV 3CLpro/Mpro enzyme, further highlighting the antiviral potential of marine-derived compounds [
107]. Remarkably, marine natural products [108-112] are particularly advantageous over synthetic chemicals [
113] in treating COVID-19 due to fewer adverse cardiovascular effects. An example is pseudopterosin from the Caribbean Sea whip, which has been used to prevent skin irritation [
114]. Marine organisms have endured for millions of years, developing metabolites that enable them to survive harsh conditions and some of these metabolites, found in microalgae, exhibit antiviral properties and help combat various diseases, with microalgae being abundant in amino acids, saccharides, vitamins, minerals, and metabolites that support immune health.
Cyanophyta algae, particularly Spirulina (
Arthrospira platensis), demonstrate potent antiviral activity. Clinical trials with Spirulina involving 30 patients showed promising results in reducing viral infections [115, 116]. Marine microalgal polysaccharides like naviculan from
Navicula directa (
Bacillariophyta) and polysaccharides A1 and A2 from dinoflagellate
Margalefidinium polykrikoides (formerly
Cochlodinium polykrikoides) have shown antiviral efficacy against HIV-1 and influenza type A virus [
117]. Nutrient-rich microalgae such as
Chlorella (
Chlorophyta) have boosted immune responses and are used in cancer treatments. In particular, hydrophilic extracts from
Chlorella have shown potential for lowering blood sugar, reducing hyperlipidemia, and improving immunity, while the green microalga
Haematococcus lacustris (formerly
Haematococcus pluvialis) is rich in astaxanthin [
118], which enhances IgA, IgG, and IgM immunoglobulin production by activating T-helper cells, improves immune response via NK cells, and reduces stress-related inflammation [
119].
Navicula directa has shown antiviral activity against Herpes simplex Virus 1 (HSV-1) and Herpes simplex Virus 2 (HSV-2). Additionally,
Arthrospira platensis has exhibited antiviral effects against mumps, influenza, HIV, polio, and measles viruses, while
Nostoc flagelliforme (
Cyanobacteria) has been effective against influenza A and herpes viruses [
117].
3.3. Other marine-derived antimicrobials
With the term "antimicrobials," we refer to a broad category of substances that kill or inhibit the growth of microorganisms. This includes antibiotics, antifungals,; antivirals,; and antiparasitics,. Several noteworthy antimicrobials have been identified from marine sources, particularly sponges and their associated bacteria (
Figure 4,
Table 3).
Mycalamide A and Mycalamide B, antiviral compounds derived from a New Zealand sponge (
Mycale sp.), were first reported for their antiviral activity in the late 1980s. These compounds also exhibit antitumor properties and inhibit protein synthesis, showcasing activity against murine coronavirus A59, HSV, and polio viruses. Mycalamide A has additionally demonstrated the ability to inhibit influenza virus replication [
120]. Vidarabine, a purine nucleoside analogue isolated from the sponge
Cryptotethya crypta, gained prominence as one of the first successful antiviral agents licensed in 1977. Despite being largely replaced by acyclovir due to its toxicity and poor solubility, vidarabine remains significant for its effectiveness against herpes simplex virus, cytomegalovirus, and the varicella zoster virus (VZV) [
121]. It is particularly valuable for treating acyclovir-resistant strains of HSV and VZV and is still used in ophthalmic procedures in the European Union [
122]. Trisindoline, identified in extracts of
Callyspongia siphonella, is responsible for significant antibacterial and cytotoxic activities. Compounds such as 5-bromotrisindoline and 6-bromotrisindoline were isolated through bioactivity-guided fractionation, exhibiting effective antibacterial properties against
S. aureus and
Bacillus subtilis [
123]. Andrimid, a peptide antibiotic originating from
Hyatella sp., displays a broad spectrum of antibacterial activity against both Gram-positive and Gram-negative bacteria, including methicillin-resistant
Staphylococcus aureus (MRSA) and bacterial pathogens including
Salmonella enteritidis,
Vibrio harveyi, and
Yersinia ruckeri [
124]. This compound was isolated from a bacterial strain associated with the sponge and has also been sourced from
Pseudomonas fluorescens. The widespread occurrence of andrimid among delta-Proteobacteria suggests a role for horizontal gene transfer in its dissemination [
125]. PM181104, a thiazolyl cyclic peptide antibiotic derived from the marine sponge
Spirastrella inconstans var. digitata, exhibits potent antibacterial activity against MRSA and other bacterial pathogens. Characterized by various analytical methods, PM181104 effectively inhibits bacterial protein synthesis and has demonstrated minimal inhibitory concentrations (MIC) against both resistant and sensitive strains of
S. aureus and
Enterococcus. Notably, it has shown non-toxicity to mammalian cell lines, positioning it as a promising therapeutic agent. In in vitro testing against a variety of organisms, PM181104 revealed strong inhibitory action on
S. aureus with an MIC range of 0.008 to 2.048 µg/mL, and its efficacy was comparable to that of standard antibiotics in various in vivo models of infection [
126]. Aurantoside K, sourced from
Melophlus sp., has shown inhibitory activity against various pathogenic fungi, including wild-type and amphotericin B-resistant
C. albicans. Its antifungal efficacy extends to
Cryptococcus neoformans, demonstrating potential against a range of fungal pathogens [
127].
Marine-derived antimicrobials hold significant promise for enhancing vaccine development and efficacy. Notably, compounds such as Puupehedione, isolated from the Verongid sponge, demonstrate diverse beneficial properties, including antimicrobial, and immunomodulatory activities [120, 128] that enhance the immune response, potentially improving vaccine effectiveness. Furthermore, specific marine compounds could be incorporated into formulations to target pathogens directly or strengthen immune recognition.