A review of literature conducted using a systematic search was employed using PRISMA guidelines. The following were used as inclusion criteria: (i) the study had to be focused only on primary research, such as experimental studies that looked into marine natural products as a source of antioxidants; (ii) it had to have used marine natural products, such as coral, seaweed, marine sponges, and other marine invertebrates, to look into anti-oxidant activity in humans; and (iii) the main outcome of interest was the antioxidant activity of marine natural products as measured by a recognized assay, e.g., ABTS, DPPH, FRAP, ORAC, etc., (iv) it had to be focused solely on antioxidant assays employing ethanolic and/or methanolic extracts, (v) the study had to be written in English, (vi) it had to be published between 2002 and 2022, and (v) had to be open-access, and peer-reviewed articles published in scientific journals. The study’s inclusion criteria were all met by a total of 18 papers.

The search engine Science Direct, Google Scholar, PubMed, and Mendeley, were used to obtain the most relevant articles by using the words “marine natural products,” “marine natural resources,” and/or “marine products” combined with “antioxidant,” “antioxidants,” and “antioxidant activity. Only English, open-access peer-reviewed articles with full text were considered, preferably published between 2002 and 2022.

The search engine Science Direct, Google Scholar, PubMed, and Mendeley, were used to obtain the most relevant articles by using the words “marine natural products,” “marine natural resources,” and/or “marine products” combined with “antioxidant,” “antioxidants,” and “antioxidant activity. Only English, open-access peer-reviewed articles with full text were considered, preferably published between 2002 and 2022.

After removing duplicates from a total of 1244 independently downloaded papers, 1093 published articles remained. 37 articles’ full-text versions were retrieved after their titles and abstracts were screened for further assessment. There are 19 articles with methods that still need to meet the inclusion criteria. In the end, only 18 qualified articles were used in this review, as illustrated in Figure 1.

The analysis’s results indicate the level of distribution of MNP studies as a potential source of antioxidants in terms of (a) publication year, (b) marine sources, and (c) marine species. Figure 2 below provides information on the frequency distribution of the studies based on the classified feature based on publication year.

Results revealed that out of the 18 qualified studies, only a few published studies focused mainly on the MNPs as a source of antioxidants (2002-2012, 0; 2013, 2; 2014, 2; 2015, 3; 2016, 3; 2017,0; 2018,1; 2019, 1; 2020, 2; 2021, 3; 2022, 1). These indicate that the chemistry of marine sources, in general, has yet to be fully embraced by the field of MNPs, at the expense of its potential as antioxidants based on the inclusion criteria developed for the purpose.

The systematic review findings highlight the significant potential of marine natural products as a rich source of antioxidant compounds. As presented in Figure 4, although marine algae are the most studied and considered a rich source of natural antioxidants (Muthiyan et al., 2018), marine fungi have the highest number of species possessing high antioxidant potential.

The data analysis showed that MNPs have much potential as a source of antioxidant compounds. Almost all the research included in the analysis mentioned MNPs’ antioxidant activity. The abundance of antioxidants in various marine species, such as algae, fungi, sponges, and other invertebrates, is shown in Table 1.

Marine macroalgae, also known as seaweed, is used as food in many countries. As a result, there is much interest in finding bioactive macroalgal metabolites to employ them as active components in developing functional foods, nutraceuticals, and pharmaceuticals (Mohapatra et al., 2013). In this context, searching for natural antioxidants in macroalgae is relevant to ongoing scientific interest and societal needs.

The fresh samples of Chlorococcum humicola in the study of (Kavitha & Palani, 2016); (Lakmal et al., 2014) were obtained from the Rameshwaram seashore in Tamil Nadu. The ethanol extracts showed considerable antioxidant capability in reducing power, DPPH, superoxide, and nitric oxide radical scavenging assays. Therefore, the algae C. humicola can be a substantial source of vital compounds that the pharmaceutical industry can utilize to produce medicines. Based on the GC-MS study’s findings, 14 compounds were present, and their nature varied greatly. The analysis revealed that the 1-propene, 3-(2-cyclopentenyl)-2-methyl-1,1- diphenyl exhibited the most significant peak area (Figure 5). According to the analysis, flavonoids, alkaloids, and steroids are the metabolites responsible for antioxidant activity.

The marine macroalga Avrainvillea erecta (Berkeley) A. Gepp and E.S. Gepp (Family Dichotomosiphonaceae) that was discovered in the South China Sea off the southwest coast of Tinggi Island, Malaysia, was investigated by (Chai et al., 2015). The findings showed that A. erecta’s methanol extract and solvent fractions have the ferric reducing ability as well as scavenging 2,2-diphenyl-1picrylhydrazyl (DPPH), nitrogen oxide (NO), and hydrogen peroxide (H2O2). On the other hand, DPPH scavenging activity is correlated with the total phenolic contents.

Therefore, the antioxidant activity of A. erecta extract revealed in the study is due to flavonoids and other phenolic components. The information showed flavonoids, phenols, and tannins are the main chemical components.

Fresh marine seaweeds were gathered from sample sites at the Beruwela coral reef in the Southern Province of Sri Lanka for the study by (Lakmal et al., 2014). Among the methanolic extracts of the identified species such as red algae (i.e., Chondrophycus ceylanicus, Gelidiella acerosa, Gracilariacorticata), green algae (i.e., Chaetomorphacrassa, Caulerpa racemosa) and brown algae (i.e., Sargassum cassifolium); C. racemosa showed a significant average radical scavenging activity against DPPH (34.34%), alkyl (85.17%) and hydroxyl (81.16%), respectively. Green seaweed has the highest phenolic content and antioxidant activity of the three macroalgae categories (Mohamed et al., 2012). Alkaloids, saponins, flavonoids, tannins, and glycosides are some of the phytochemicals found in the species (Jayaseelan et al., 2012)

Red algae of the Chondrus crispus species from the Red Sea’s coasts near Jeddah City were gathered for Alkhalaf’s (2021) investigation. According to the findings, the methanolic extract made from the red algae species C. crispus had considerable antioxidant activity in the current investigation, and it successfully scavenged ABTS and DPPH free radicals in a dose-dependent manner. The extract’s overall antioxidant capacity was also remarkable. The presence of flavonoids and polyphenols is thought to contribute to the antioxidant properties of several species of red algae, and the potency of this ability is correlated with the concentration of these bioactive chemicals (Zhang et al., 2019). Since flavonoids and phenols were also discovered in C. crispus extract, the antioxidant effects displayed by the extract in the current investigation could be attributed to these bioactive components.

Seaweed samples (Leelavathi & Prasad, 2014) were collected from the Gulf of Mannar. Cymodeace rotundata, Acanthopora spicifera, Ulva lactuca, Ulva reticulate, Turbinaria conoides, Gracillaria edulis, Kappaphycus alvarezii, Gracillaria crassa, Gracillaria foliifera, and Cymodeace serrulata were the specimens identified. Petroleum ether and methanol were used to produce the extracts. By observing the decline in absorbance at 517 nm, the DPPH assays were performed to determine the antioxidant properties of seaweed. The highest overall antioxidant activity was demonstrated by C. rotundata, G. crassa, and C. serrulata in the methanol extract compared to other samples. Red algae samples were collected from the Chabahar coastlines at the coordinates (25° 18′ 53′′N; 60° 37′ 41′′E) in the Oman Sea, Iran, for the study of (Karimzadeh & Zahmatkesh, 2021). The red seaweed Laurencia synderiae was studied, and the results showed that different extracts (methanolic, chloroform, and ethyl acetate) have dose-dependent antioxidant activities. Future medication development is predicted to involve the significant identified compounds that have a strong biological impact on red algae, L. snyderiae. The most prevalent compounds are hexadecanoic acid (19.81%; Figure 6), oleic acid (8.34%; Figure 7), squalene (6.96%; Figure 8); and tetradecanoic acid (6.86%; Figure 9); as well as glycerin (8.62%;Figure 10), xylitol (8.46%; Figure 11 ), and squalene (8.96%; Figure 12). The red algae L. snyderiae has strong bioactive components that it can employ as a natural antioxidant and source of additional resources. Total phenolic and flavonoid concentrations are among its phytochemical components.

The green macroalga Spongomorpha indica was gathered from the Visakhapatnam coastal area (Rajasekaran, 2022). The phytochemical tests revealed the presence of potent active constituents such as alkaloids, tannins, steroids, and flavonoids, by which further GCMS analysis was conducted, and seven components were determined. The results of the physicochemical parameters tests were under WHO guidelines. Nonadecanoic acid (Figure 13) had the most significant peak area, around 47.006%, and the most prolonged retention period, 19.050.

In the DPPH assay model, S. indica demonstrated a considerable dose-dependent reduction in the case of the DPPH radical. All the data demonstrated the highest levels of antioxidant activity, with superoxide scavenging activity displaying the most significant outcomes. Based on the findings, it was established that S. indica had powerful active ingredients with potent antioxidant activity.

Seaweeds were gathered from the station in Rameshwaram on the southern Indian coast of Olaikuda (N.Lat. 09°18.300’ and E. Long. 079°20.096’) for the study by (Elangovan et al., 2019) The methanolic extracts of all the test species, including green algae Enteromorpha intestinalis and red algae Gracillaria edulis, exhibit high antioxidant activity (DPPH). The stated activity results from the high concentration of total chlorophylls that are lipid soluble, particularly Chl-a and related compounds, as well as the high concentration of total carotenoids.

In the study by (Muthiyan et al., 2018), marine fungus strains were obtained from biological samples taken in the intertidal zone of Dalian, China (Utkina, 2013), from shark gill tissues captured in the East China Sea, and from various corals in the Zhanjiang Sea region of China. The methanolic extracts of Aspergillus flavipes, Hypocreaceae, Sterigmocystis sp., Penicillium digitatum, Penicillium polonicum, Penicillium chrysogenum, and Penicillium corylophilum were some of the strain samples that showed DPPH free radical scavenging activity (scavenging ratios > 30% at 200 g/ml). Thus, found to exhibit antioxidant activity. The said antioxidants were assigned as Aspernolide E (Figure 14), butyrolactone I (Figure 15), a phenolic derivative, and possibly unidentified compounds.

The Aspergillus versicolor SH0105 fungus strain (Yang et al., 2020) was isolated from a deep- sea sediment sample from the Mariana Trench at a depth of 5455 m. The bioactive assay revealed that A. versicolor demonstrated a substantial reduction of Fe3+ and significant DPPH radical scavenging activity, which were more potent than ascorbic acid. This suggests discovering chemical entities with antioxidant activities from the marine medicinal microbial resources. Aspermutarubrol/ violaceol-I (Figure 16) was identified as the chemical responsible for the antioxidant activity.

Many sedentary marine organisms, such as sponges, have acquired the capacity to create a variety of toxic substances to defend themselves against predators or to compete with other marine species (Paul et al., 2006). Sponges are receiving the most interest from the pharmaceutical industry because they produce the most bioactive secondary metabolites among all marine organisms. Alkaloids, steroids, terpenes, peptides, macrolides, and polyketides are only a few marine sponges’ structurally diverse natural compounds (Dias et al., 2012).

The marine sponge Zyzzya fuliginosa (order Poecilosclerida) is a rich source of alkaloids with a pyrrolo[4,3,2-de]quinoline structure (Utkina, 2013). The antioxidant properties of zyzzyanones A to D (1-4) and makaluvamines C, E, G, H, and L (Figure 16) were tested, which were isolated from the ethanolic extracts of the Australian marine sponge Z. fuliginosa. According to research on their effectiveness as radical scavengers, zyzzyanones and makaluvamines have moderate antioxidant activity, indicated by a phenolic function in the molecules.

The marine sponge Hyrtios erectus was procured from the North Bay of the South Andaman Sea for the study of (Muthiyan et al., 2018). According to the findings, the methanolic extract of H. erectus exhibited antioxidant activity against DPPH free radicals, superoxide anions, and hydroxyl radicals. With 50 µg/mL sponge extract, more than 50% inhibition (half inhibitory concentration) was observed. However, there is a need for further functional characterization of the bioactive compounds to determine the compound responsible for the activities.

Marine sponge species were gathered in Pecaron Situbondo, East Java, Indonesia, for the study of Abdillah et al. (2013). Results showed that only Aaptos suberitoides exhibited vigorous antioxidant activity in an assay using the DPPH method, as evidenced by an IC50 value of less than 30 mg/mL. In contrast, Fascaplysinopsis reticulata, Acanthella sp, Petrosia contignata, and Xestospongia exigua displayed moderate antioxidant activity, with an IC50 value of less than 100 mg/mL. The IC50 value for Callyspongia sp. and Xestospongia sp. was more significant than 100 mg/ml. The research has shown the high potential of Xestospongia sp, F. reticulata, Callyspongia sp, Petrosia contignata, and Aaptos suberitoides for developing bioactive chemical isolation as antioxidant agents.

Aquatic species’ capacity for adaptation and survival in various habitats relies on their physical and chemical adaptations (Thakur et al., 2005). Bryozoans, mollusks, andothermarineinvertebrates all have soft bodies and sedentary lifestyles, making a chemical defense system necessary for survival. When introduced into their aqueous habitat, these compounds are rapidly diluted. The compounds must be very potent to be effective (Karimzadeh & Zahmatkesh, 2021). Numerous researchers have investigated these environments in quest of biologically active molecules due to the high potency of chemicals used in aquatic defense systems and the requirement that they be water-soluble.