Anthraquinones and Their Analogues from Marine-Derived Fungi: Chemistry and Biological Activities

Anthraquinones are an interesting chemical class of polyketides since they not only exhibit a myriad of biological activities but also contribute to managing ecological roles. In this review article, we provide a current knowledge on the anthraquinoids reported from marine-derived fungi, isolated from various resources in both shallow waters such as mangrove plants and sediments of the mangrove habitat, coral reef, algae, sponges, and deep sea. This review also tentatively categorizes anthraquinone metabolites from the simplest to the most complicated scaffolds such as conjugated xanthone–anthraquinone derivatives and bianthraquinones, which have been isolated from marine-derived fungi, especially from the genera Apergillus, Penicillium, Eurotium, Altenaria, Fusarium, Stemphylium, Trichoderma, Acremonium, and other fungal strains. The present review, covering a range from 2000 to 2021, was elaborated through a comprehensive literature search using the following databases: ACS publications, Elsevier, Taylor and Francis, Wiley Online Library, MDPI, Springer, and Thieme. Thereupon, we have summarized and categorized 296 anthraquinones and their derivatives, some of which showed a variety of biological properties such as enzyme inhibition, antibacterial, antifungal, antiviral, antitubercular (against Mycobacterium tuberculosis), cytotoxic, anti-inflammatory, antifouling, and antioxidant activities. In addition, proposed biogenetic pathways of some anthraquinone derivatives are also discussed.


Introduction
Phylogenetically and functionally, fungi are ubiquitous organisms living in associations with almost all viable resources such as plants and animals to complement the nutrient cycling in various ecosystems on Earth [1]. Currently, marine mycology has been somewhat neglected, and most of the fungal diversity normally refers to their terrestrial counterparts. To this respect, although ca. 75% of the Earth's surface is occupied by seas and oceans, harboring small organisms to the largest ones, there are still only about 1000 fungal species that are derived from terrestrial ancestors [2]. Marine-derived fungi are found in diverse habitats and have significant ecological functions. According to Kohlmeyer et al., the filamentous fungi in the marine environment are generally divided into two groups and ecotypes: (i) obligate species, which are originally living in the salt-free waters or estuarine, and (ii) facultative species, which transited from terrestrial and freshwater milieus needing By using suitable cultivation methods, many fungal species have been isolated from the submerged areas such as sea water, sediments, sponges, algae, mangrove plants, etc., that are routinely producing anthraquinone compounds. Some of the predominant fungal strains viz. Aspergillus sp., Penicillium sp., Eurotium sp., Fusarium sp., and Alternaira sp. have been reported in aquatic ecosystems [6]. To a lesser extent, other species that are able to biosynthesize biologically active anthraquinones such as Acremonium sp., Amorosia sp., Chaetomium sp., Cladosporium sp., Guignardia sp., Curvularia sp., Emericella sp., Engyodonitum sp., Geotrichum sp., Gliocladium sp., Halorosellinia sp., Microsphaeropsis sp., Microsporum sp., Monodictys sp., Neosartorya sp., Nigrospora sp., Paecilomyces sp., Phoma sp., Phomopsis sp., Scopulariopsis sp., Sporendonema sp., Stemphylium sp., Talaromyces sp., Thermomyces sp., Trichoderma sp., and Xylaria sp. have also been isolated from the marine environments ranging from decayed plants to living macro-organisms. In the following sections, we have tentatively categorized marine fungal anthraquinones from the simplest to the most complex structures.

Simple Anthraquinones
In general, anthraquinones (9,10-dioxoanthracene or anthracene-9,10-dione) represent the type of pigments possessing a p-quinone moiety as a central ring of the anthracene scaffold. Replacing each hydrogen atom of the benzene rings with simple substituents such as hydroxyl, methoxy, methyl or its oxidation analogs (hydroxymethyl, formyl and carboxyl groups), prenyl group, and other substituents leads to diverse anthraquinoid compounds [11]. Buttachon et al. reported the isolation of emodin (1) (Figure 3) from the culture extract of Aspergillus candidus KUFA0062, isolated from a marine sponge Epipolasis sp., which was obtained from the coral reef at the Similan Island National Park, Phang-Nga province, Thailand [16]. Compound 1 was also obtained from the ethyl acetate (EtOAc) extract of the culture of Penicillium ochrochloron, isolated from the underwater sea sand, which was collected from the North Sea in St. Peter-Ording, Germany [17]. In another study, Wang et al. isolated 1 and questin (2) (Figure 3) from a solid culture extract of A. flavipes HN4-13, obtained from a Lianyungang coastal sediment from Jiangsu Province, China [18]. Liu et al. also isolated 1 from the culture extract of a marine-derived Aspergillus sp. LS57, isolated from a marine sponge Haliclona sp., which was collected at Lingshui, Hainan Province, China [19]. The mycelial extract of A. terreus DTO 403-C9, isolated from the leaves of an unidentified mangrove tree, which was collected at Khanh Hoa Province, Vietnam, furnished 2 and a new naturally occurring 1,2,5-trihydroxy-7-methyl-9,10-anthraquinone (3) (Figure 3) [20]. A DPPH • radical scavenging activity-guided fractionation of the culture extract of A. europaeus WZXY-SX-4-1, isolated from a marine sponge Xestospongia testudinaria, which was collected on Weizhou Island, Guangxi Province, China, resulted in the isolation of 1-methyl emodin (4) and dermolutein (5) (Figure 3) [21]. The culture extract of A. glaucus HB1-19, isolated from a marine sediment collected in Fujian Province, China, furnished 1 and 2, together with physcion (6), catenarin (7), and rubrocristin (8) (Figure 3) [22]. Compound 6 is a common fungal anthraquinone since it was obtained from several sources such as the culture extract of A. wentii EN-48, isolated from a marine brown alga Sargassum sp. [23], the EtOAc extract of the culture of Eurotium repens, isolated from a marine sponge Suberites domuncula, collected near Zelenyi Island (Kuril Islands) [24], the fermentation extract of E. cristatum, isolated from a marine sponge Mycale sp., which was collected from Wonnapa Beach, Bangsaen, Chonburi Province, Thailand [25], the culture broth extract of Microsporum sp. MFS-YL, isolated from a marine red alga Lomentaria catenata, which was collected from Guryongpo, NamGu, PoHang, Republic of Korea [26], and the fermentation extract of Penicillium sp. ZZ901, isolated from a sample of a wild bivalve Scapharca broughtonii (Schrenck), which was collected from the Sea Shoal, China [27].
The culture extract of Eurotium chevalieri KUFA0006, isolated from the inner twig of a mangrove plant, Rhizophora mucronata Poir, which was collected in the Eastern Seaboard of Thailand, yielded 1, 2, 6 and questinol (15) (Figure 3) [31]. Fractionation of the culture extract of an algicolous fungus, Chaetomium globosum, isolated from the inner tissue of a marine red alga, Polysiphonia urceolata, which was collected from the Qingdao coastline, resulted in the isolation of 7 and erythroglaucin (16) (Figure 3) [32].
Fallacinol (17) (Figure 3) was isolated, together with 1 and 15 (Figure 3), from the fermentation extract of Talaromyces stipitatus KUFA 0207, obtained from a marine sponge Stylissa flabelliformis, which was collected at a depth of 10-15 m from the coral reef at Samaesarn Island in the Gulf of Thailand [33].
1-Methyl ether of nalgiovensin (73) (Figure 6) was also isolated, together with 14, 19 ( Figure 3) and 28 (Figure 4), from the MeOH fraction of the mycelial extract of a deep-seaderived fungus, Emericella sp. SCSIO 05240, which was isolated from sediment samples collected from the South China Sea at a depth of 3258 m [78].
Chemical investigation of the culture extract of a marine sponge-associated fungus, Neosartorya spinosa KUFA 1047, led to the isolation of two alkylated anthraquinones, penipurdin A (74) and acetylpenipurdin A (75) ( Figure 6). The absolute configuration of C-2 in 75 was suggested to be the same as that of 74, i.e., 2 S, on the basis of the biogenic consideration [54].
1,3,6-Trihydroxy-7-(dihydroxypropyl)-anthraquinone (76) ( Figure 6) was isolated from a defatted culture extract of a marine sediment-derived fungus, Thermomyces lanuginosus Tsikl KMM 4681. The relative configurations of C-15 and C-16 of a diol side chain in 78 were determined by the observed correlations in the NOESY (Nuclear Overhauser Effect Spectroscopy) spectrum and the value of the coupling constant of the vicinal protons, as well as the presence of magnetically non-equivalent methyl groups of its acetonide (76a) ( Figure 6). The absolute configurations of C-15 and C-16 in 76 and 76a were established as 15R,16S by comparison of their calculated and experimental ECD spectra [66].
Averantin (85) was isolated, together with 81 and 82 (Figure 7), from the culture extract of A. versicolor, isolated from a marine sponge, Petrosia sp., which was collected at the depth of 20 m at Jeju Island, Korea [84]. Compounds 82 and 85 ( Figure 6) were also isolated from a culture broth of a marine-derived Penicillium flavidorsum SHK1-27 by bioassay-guided isolation approach [85].
Averythrin (87) (Figure 7) was obtained, together with 80 and 85, from the culture broth extract of A. versicolor INF 16-17, isolated from the inner tissue of an unidentified marine clam [88]. Compounds 81, 85, and 87 ( Figure 7) were also obtained from the culture extract of A. versicolor A-21-2-7, isolated from a deep-sea sediment from the South China Sea [89]. Compound 87 was also obtained from the fermentation extract of a mangrove endophytic fungus, Aspergillus sp. 16-5C, which was isolated from the leaves of a mangrove tree, Sonneratia apetala, collected at Hainan Island, China [90].
Aspergilol I (92), SC3-22-3 (93), and coccoquinone A (94) (Figure 7) were isolated, together with 81 and 82, from the culture broth extract of A. versicolor SCSIO-41502, which was obtained from marine sediment samples collected from the South China Sea. The absolute configuration of C-16 in 92 was determined as S by comparison of its circular dichroism (CD) spectrum with that of the previously described (1 S)-7-chloroaverantin, while the absolute configuration of C-19 was established by the modified Mosher's method. Moreover, the absolute configuration of C-16 in 93 and 94 was also determined as S by comparison of their CD spectra and optical rotations ([α] 25 D − 30.6 • for 93 and −11.1 • for 94) with those of (1 S)-7-chloroaverantin [92].
The culture extract of Aspergillus sp., isolated from the inner part of a fresh tissue of a gorgonian, Dichotella gemmacea, which was collected from the South China Sea, furnished 8-O-methylaverufin (117) and 8-O-methylaverufanin (118) (Figure 9), in addition to 104, 106, 107, and 110. The relative configuration of 110 was established by 1 H-1 H coupling constants and analysis of NOESY correlations, whereas the absolute configurations of its stereogenic carbons were proposed as 1 R,2 S,5 S on the basis of the biogenic consideration as well as by comparison with those of 107, whose stereostructure was unambiguously established [103]. Versicolorin A (119) (Figure 9), together with 100, 104, 107, 109 and 118, were isolated from the culture extract of a marine-derived Penicillium flavidorsum SHK1-27 by bioassay-guided isolation approach [85].
Emodacidamides A (128), B (129), D (130), E (131), and H (132) (Figure 10), anthraquinones with amino acid-containing amide side chains, were obtained from the culture extract of a deep-sea sediment-derived Penicillium sp. SCSIOsof101. The absolute configurations of the amino acid residues were determined by Marfey's method or by a combination of Marfey's method with chiral-phase HPLC analysis. L-Val was identified as the amino acid in the amide side chain of 128 and 129, whereas L-Ile was the amino acid of the amide side chain of 130 and 131, and L-Ala was identified for 132 [46].
The chlorinated anthraquinones containing amide side chain, viz. emodacidamides C (150), F (151), and G (152) (Figure 11), were also reported from the fermentation extract of a deep-sea sediment-derived fungus, Penicillium sp. SCSIOsof101. The absolute configurations of the amino acids in the amide side chains were assigned by Marfey's method and chiral-phase HPLC analysis as L-Val in 150, L-Ile in 151, and L-Leu in 152 [46].

Tetrahydroanthraquinones
The culture extract of a soft coral-associated fungus, Aspergillus tritici SP2-8-1, furnished aspetritone B (168) ( Figure 15). The relative configurations of the stereogenic carbons (C-2 and C-3) in 168 were established by NOESY correlations from H-1 to H-3 and H-2 to H ax -4, while the absolute configurations were established as 2R,3S by comparison of the calculated and experimental ECD spectra [28].
The solid-rice culture extract of a mangrove endophytic fungus, Stemphylium sp., yielded altersolanol C (171) (Figure 15), together with 170, altersolanol A (172), auxarthrol C (173), and 2-O-acetylaltersolanol B (174) ( Figure 15). The absolute configurations of the stereogenic carbons in 173 were established as 1R,2R,3R,4R,1aS,4aR by X-ray analysis of the product resulting from the epoxide ring-opening reaction to obtain a suitable crystal for X-ray crystallography. The absolute configurations of the stereogenic carbons in 174 were established as 2R,3S by X-ray analysis of a crystal obtained from a hydrolysis reaction, followed by a preparation of its 2,3-O-acetonide [50]. Compounds 170, 172 and 173 ( Figure 15) were also isolated from the solid-rice culture extract of a gorgonian-associated fungus, S. lycopersici [51].

Tetrahydro-5,8-anthraquinones
Chemical investigation of the culture extract of a soft coral-associated fungus, Aspergillus tritici SP2-8-1, resulted in the isolation of aspetritone A (181) ( Figure 16). The relative configurations at C-1, C-2 and C-3 were established based on NOESY correlations, while their absolute configurations were determined as 1S,2S,3R by comparison of the calculated and experimental ECD spectra [28].
The culture extract of Aspergillus sp. strain 05F16, isolated from an unidentified alga collected in the coral reef at Manado, Indonesia, yielded bostrycin (182) (Figure 16) [110]. Compound 182 was also isolated from the culture extract of a mangrove endophytic fungus strain no. 1403, collected from the South China Sea [111].

Anthrones
Anthrone derivatives reported from marine-derived fungi occur as complex structures with the anthrone or modified anthrone scaffolds. These compounds can be considered to derive from a condensation of the anthraquinone scaffold, such as physcion (6) and catenarin (7) (Figure 3) with polyketides of diketopiperazine derivatives.
Further investigation of the mycelial extract of A. glaucus HB1-19, isolated from the marine sediment-surrounding mangrove roots collected in Fujian Province (China), by the same authors led to the isolation of aspergiolides C (192) and D (193) (Figure 17), two spiro [5,5]undecane scaffold-containing anthrones. Although 192 and 193 possess a stereogenic center at a spiro junction of the ring system (C-19), both compounds displayed no optical rotation and CD effects. Therefore, both compounds were assumed to be a 1:1 mixture of enantiomers. By using HPLC with a Lux-Amylose-2 column, each compound gave a baseline-separated peaks in a 1:1 ratio for both compounds, confirming their racemic nature. Of these peaks, HPLC-CD spectra were recorded in the stopped-flow mode and the resulting opposite CD curves confirmed the assumption that the two peaks represent their enantiomers. Comparison of the online and calculated CD spectra and the configurations of both enantiomers of 192 and 193 were established [115].
Three pairs of anthrone-based racemic spirocyclic diketopiperazine enantiomers, variecolortins A (194), B (195) and C (196) (Figure 18), were obtained from Eurotium sp. SCSIO F452, isolated from the South China Sea sediment samples. Compounds 194-196 represented a 6/6/6/6 tetracyclic cyclohexene-anthrone skeleton. The relative configurations of the stereogenic carbons in 194 were unambiguously determined as (12R,21S,32R) by X-ray analysis. However, the lack of optical rotation of 194 suggested its racemic nature. The enantiomers were subsequently separated by a chiral HPLC to give (+)-194 and (−)-194. Conversely, the relative configurations of 195 and 196 were established by NOESY experiments. In each compound, the diagnostic NOESY correlations between NH-11 and H-21b, as well as between OH-22 and H-21a, resulted in the identification of αand β-orientations, respectively. In addition, the geometry of the ∆ 8 double bond was assigned as Z-configuration via the deshielding effect of H-8 caused by the carbonyl group on the β-vinyl proton. The baseline ECD curves of 195 and 196 revealed that they were racemic mixtures. Therefore, 195 and 196 were separated by a chiral-phase HPLC, and the calculated ECD spectra for the individual enantiomer assigned them as 12S,22R-195 and 12S,22R-196, which were in agreement with the experimental ECD spectra of (+)-195 and (−)-196, respectively.  Hydroxyviocristin ( Figure 18) was proposed to be a biosynthetic precursor of (±)-194, while physcion (6) (Figure 3) was proposed as a biosynthetic precursor of (±)-195 and (±)-196 [116]. The proposed biosynthetic pathways leading to the formation of 194-196 are depicted in Figure 18.

Tetrahydro-9-hydroxyanthrones
Terahydro-9-hydroxyanthrones are considered to derive from a reduction of the carbonyl group on C-10 of tetrahydroanthraquinones to a hydroxyl group. This group of anthraquinone derivatives are widely isolated from culture extracts of marine-derived fungi.
The culture extract of an algicolous Aspergillus sp. strain 05F16 furnished tetrahydrobostrycin (197) and 1-deoxytetrahydrobostrycin (198) ( Figure 19). The relative configurations of the stereogenic carbons of 197 were assigned by analysis of the 1 H-1 H coupling constants and NOESY correlations [110].  (Figure 19). The relative stereochemistry of 199 was determined by analysis of 1 H-1 H coupling constants as well as by NOESY correlations. The large coupling constant value (J = 8.8 Hz) between H-9 and H-9a revealed a trans orientation. The important NOE correlations were observed between H-9 and H-4a, indicating a co-facial orientation of the two protons, while the NOE correlations between H-2 and H-9a showed that they were on the opposite sides of the molecule. The absolute configurations of the stereogenic carbons of 199 were established as 2S,4aS and 9R,9aS by X-ray analysis. The relative and absolute configurations of the stereogenic carbons of 200 and 201 were deduced to be the same as those in 199. However, the measured ECD spectrum of 201 suggested the R absolute configuration at C-9 in 199-201. The 1 H and 13 C NMR data revealed that 202 is a C-4 epimer of 201. The absolute configurations of the stereogenic carbons in 202 were determined as 2R,4S,4aR,9R,9aS by comparison of its calculated and experimental ECD spectra. The relative configuration of 203 was established on the basis of NOESY correlations, while the absolute configurations of its stereogenic carbons were established as 2S,4S,4aS,10S,9aS by comparison of the calculated and experimental ECD spectra [117].

9,10-Dihydroxyanthracenes
Anthrininone A (233) (Figure 21) was obtained from the culture extract of a deep-sea sediment-derived fungus, Altenaria tenuissima DFFSCS013. The absolute configurations of its stereogenic carbons were established as 4R,6S,7R,15R,17S,18R by a single-crystal X-ray diffraction analysis using Cu Kα radiation [63]. The proposed biosynthetic pathway leading to a formation of a hexacyclic spiro-fused ring system in 233 was shown in Figure 21

2-Aza-anthraquinones
2-Aza-anthraquinones consist of a naphthoquinone moiety fused with a pyridine ring. These compounds are synthesized in nature by either fungi or lichens. Van Wagoner et al. reported the isolation of scorpinone (234) ( Figure 22) from the extract of a rare fungus, Amorosia littoralis, collected from an inertial sediment in the Bahamas. The biosynthetic pathway of 234 was studied using [2-13 C]-acetate and [1,2-13 C]-acetate, and was followed to verify if its biosynthesis was similar to that of bostrycoidin (235) ( Figure 22). The labeling results showed that a linear heptaketide is a precursor in the biosynthesis of 234, and consequently the incorporation of a nitrogen atom produced 2-aza-anthraquinones [120]. A. littoralis gen. sp. nov., also isolated from the littoral zone in the Bahamas, was capable of producing 234 ( Figure 22) and caffeine [121]. Chemical investigation of the CHCl 3 -MeOH extract of cultured mycelia of Bispora-like tropical fungus, collected from the intertidal zone surrounding the Bahamas Island, also led to the isolation of 234 ( Figure 22) [122].

Dimeric anthraquinones
The compounds of this group include two anthraquinoid units, one anthraquinone and one tetrahydroanthraquinone, two tetrahydroanthraquinones, one anthrone and one tetrahydro-5,8-anthraquinone, or one anthraquinone and one seco-anthraquinone, linked together by C-O-C or C-C bonds.  2,2 -Bis-(7-methyl-1,4,5-trihydroxyanthracene-9,10-dione) (239) ( Figure 24) was obtained from the fermentation extract of a marine sponge-associated fungus, Talaromyces stipitatus KUFA 0207 [33]. (SK11). The relative stereochemistry of 243 was determined as 6S*,7R*,8S*,8aS*,10R*,10aR* and 6 R*,7 S* by the 1 H-1 H coupling constant values as well as by NOESY correlations, while the absolute configurations of the stereogenic carbons were established as 6S,7R,8S,8aS,10R,10aR and 6 R,7 S by comparison of calculated and experimental ECD spectra. The planar structure of 244, elucidated by high-resolution mass spectrometry (HRMS) and 1D and 2D NMR analyses, was the same as that of alterporriol C. The relative configurations of the stereogenic carbons of 244 were also established by 1 H-1 H coupling constants and the correlations ob-served in the NOESY spectrum as 5 R*,6 S*,7 R*,8 S*. Since 244 displayed the specific rotations [α] 27 D + 75 • and +208 • ; c 0.02, in EtOH, it was suggested to be an atropisomer of alterporriol C. Comparison of the calculated and experimental ECD spectra of 244 revealed the absolute configuration of its stereogenic carbons as 5 R,6 S,7 R,8 S. Thus, the axial configuration of 244 was identified as aS, also called M helicity [62].
The previously described cytoskyrin A (252) ( Figure 24) was isolated from the culture broth and mycelial extracts of a marine sponge-associated fungus, Curvularia lunata [64].
The Since the experimental ECD spectrum showed significant CEs at 269 (∆ε 35.79) and 285 (∆ε −36.06) nm, the aR (also defined as P helicity) configuration at C-6-C-6 was assigned to 264. However, the calculated ECD spectra did determine the absolute configuration of C-1-C-4. The 1D and 2D NMR analysis revealed that the planar structure and relative stereochemistry of 265 were the same as those of 264. However, the experimental ECD spectrum of 265 was quasi-mirror image of 264, indicating that 265 is an atropisomer of 264. The relative configurations of the stereogenic carbons in 266 were determined as 1 R*,2 R*,3 S*,4 S* on the basis of the 1 H-1 H coupling constants and NOESY correlations, whereas the absolute configurations of the C-1/C-6 chiral axis was assigned as aR, based on the similarity of its ECD spectrum to that of 264. Moreover, the ECD spectrum of 262 also assigned the configuration of a C-5/C-5 chiral axis as aR [124].
Antibacterial activity-guided fractionation of the culture extract of an unidentified marine red alga-derived fungal strain F-F-3C led to the isolation of rubellin A (267), 14acetoxyrubellin A (268), and 14-acetoxyrubellin C (269) (Figure 25). The structures of 268 and 269 were elucidated by 1D and 2D NMR analysis and comparison of their NMR data with those of the previously reported 267; however, the relative and absolute configurations of their stereogenic carbons were not described [44].
Eurotone A (278) (Figure 26) was isolated from the culture extract of a marine sedimentderived fungus, Eurotium sp. SCSIO F452. X-ray diffraction analysis not only confirmed its planar structure, elucidated by 1D and 2D NMR analysis, but also determined the relative configuration of its stereogenic carbons as 10S*,10 S*. However, the crystal of 278 occupied a Pccn space group, indicating its racemic nature, which was also supported by its lack of optical activity. Separation of (±)-278 by chiral HPLC yielded (+)-278 and (−)-278, whose absolute configurations were established as 10S,10 S and 10R,10 R, respectively, by comparison of their calculated and experimental ECD spectra [70]. The proposed biosynthetic pathway of 278 from physcion (6) ( Figure 3) as a precursor was depicted in Figure 26.
Compounds 29, 267, 268, and 269 ( Figure 25), isolated from a red alga-associated fungal strain F-F-3C, showed antibacterial activity against pathogenic bacteria E. coli and S. aureus at a concentration of 50 µg/disk with inhibition zones ranging from 13 to 15.5 mm [44]. Compound 48 ( Figure 5), isolated from the culture extract of a sea urchin-derived Monodictys sp., at a concentration of 2.5 µg/disk, inhibited the growth of B. subtilis and E. coli, with the inhibition zones of 7 and 8 mm, respectively [60].
Compounds 53 ( Figure 5) and 252 (Figure 24), isolated from the culture extract of a marine sponge-associated fungus, Curvularia lunata, at a concentration of 5 µg/mL, inhibited the growth of S. aureus ATCC 25923, E. coli ATCC 25922 and E. coli HBI 101 in the agar plate diffusion assay with the same inhibition zones of 8.5, 9.0, and 8.0 mm, respectively. Both 53 and 252 were also active against B. subtilis 168, with MIC values of 7.5 and 8.0 mm, respectively [64].
Compounds 156, 157 (Figure 13), and 231 (Figure 20), isolated from an algiclous fungus, Eurotium cristatum EN-220, were evaluated for their antibacterial activity. Compound 156 inhibited the growth of E. coli with MIC value of 32 µg/mL, while 157 was inactive, indicating that the methyl group at C-3 is essential for bioactivity in 156. Compound 231 showed weak activity against E. coli, with MIC value of 64 µg/mL. The positive control, chloramphenicol, showed MIC value of 4 µg/mL [95].
Compound 126 (Figure 10), isolated from a sea fan-derived fungus, Penicillium citrinum PSU-F51, showed moderate antibacterial activity against S. aureus ATCC25923 and MRSA S. aureus SK1, with a MIC values of 16 µg/mL. Vincomycin was used as a positive control and showed MIC value of 1 µg/mL [37].
Compounds 237 and 238 (Figure 23), isolated from a marine clam-associated fungus, A. versicolor, selectively inhibited S. aureus (inhibition zones 14 and 19 mm) at a concentra-tion of 30 µg/well by a radial dilution assay. The positive control, tetracycline, displayed an inhibition zone of 30 mm at a concentration of 30 µg/well [88].
Compound 182 (Figure 16), isolated from a mangrove endophytic fungus strain no. 1403, inhibited, in a yeast-based assay on Saccharomyces cerevisiae, cell proliferation through the cell cycle at G1 phase, leading to cell death in a time-and dose-dependent manner [111].
The fermentation extract of Fusarium equiseti, isolated from a brown alga, Padina pavonica, potently inhibited the HCV NS3-NS4A protease with an IC 50 value of 27.0 µg/mL. Compounds 28 (Figure 4), isolated from this extract, also inhibited HCV NS3-NS4A protease with an IC 50 value of 10.7 µg/mL, which was comparable to the positive control, HCV-I 2 (IC 50 = 1.5 µg/mL). Conversely, the co-isolate 29 was void of activity. It was suggested that the substituent CH 2 OH at C-3 is essential for the bioactivity of 28 [40].
Compounds 192 and 193 (Figure 17), isolated from modified cultures of a mangrove sediment-derived fungus, Aspergillus glaucus HB 1-19, were examined for its activity against the pathogens of leishmaniasis and African sleeping sickness. Compound 192 showed no activity against Leishmania major (promastigote form) or Trypanosoma cruzi (IC 50s > 50 mm) but weak activity against T. brucei brucei and L. donovani (amastigote form) with IC 50 values of 29 and 17 µM, respectively, while 193 had no activity against both parasites (IC 50 > 50 µM) [115].
Compound 77 (Figure 6), isolated from a mangrove endophytic fungus, Fusarium sp. ZZF60, exhibited cytotoxicity against human larynx carcinoma (Hep2) and HepG2 cancer cell lines, in a cell-based MTT assay, with IC 50 values of 16 and 23 µmol/L, respectively [79].  [89]. Compound 107, isolated from a gorgonian-associated fungus, Aspergillus sp., showed significant growth inhibitory effects on K562 and HL-60 cell lines, in the MTT assay, with IC 50 values of 0.87 and 1.46 µM, respectively [103]. Further in vitro antitumor activity investigation revealed that 107 caused a significant induction in cell cycle arrest at G 2 /M transition in K562 cell line in a concentration-and time-dependent manners (IC 50 = 12.6 µM) [99].
Compound 189 (Figure 16), isolated from a mangrove endophytic fungus no. 1403, exhibited a potent cytotoxicity against KB and KBv200 cells in the MTT assay, with IC 50 values of 19.66 and 19.27 µM, respectively. Compound 189 caused apoptosis in KB and KBv200 cells through non-related reactive oxygen species (ROS) generation in mitochondria and activation of caspase-8 in death receptor pathways [113].
Compounds 240 and 241 (Figure 24), isolated from a mangrove endophytic fungus, Altenaria sp. ZJ9-6B, displayed cytotoxicity against MDA-MB-435 (higher metastasizing cells) and MCF-7 (lower metastasizing cells) cell lines (by MTT assay), with IC 50 values of 26.97 and 29.11 µM (for 240), and 13.11 and 20.04 µM (for 241), respectively [61]. Compound 241 was further investigated for its underlying mechanism for cytotoxicity in MCF-7 cells. It was found that 241 mainly induced cell necrosis, and only a portion of cells was in the state of apoptosis. Compound 241 also caused a significant increase in ROS production, a significant increase in intracellular calcium and alteration of cell morphology of the MCF-7 cells, which is characteristic of apoptosis [135]. Compound 158 (Figure 13), isolated from a mangrove endophytic fungus, Stemphylium sp. 33231, showed a moderate lethality effect in brine shrimp lethality assay, with LD 50 = 10 µM [50].

Inhibition of Trypsin Activity
Compounds 1 and 4 (Figure 3), isolated from a green alga-derived fungus, A. versicolor, displayed a non-competitive inhibitory activity against the human trypsin, with IC 50 values of 450.5 and 50.1 µg/mL, respectively. The soybean trypsin-chymotrypsin was used as a positive control and showed trypsin inhibitory activity with an IC 50 value of 0.01 µg/mL [34].
Compound 28 (Figure 4), isolated from the culture extract of an algicolous fungus, Fusarium equiseti, inhibited trypsin activity with an IC 50 value of 48.5 µg/mL, which was comparable with the positive control (soybean trypsin-chemotrypsin inhibitor; T-I, IC 50 = 0.01 µg/mL). Compound 29 (Figure 4), isolated from the same extract, did not show any inhibitory activity. Therefore, it was proposed that the CH 2 OH group at C-3 of the anthraquinone scaffold was essential for the bioactivity of 28 [40].

Anticoagulant Activity
Compounds 179 and 180 (Figure 15), isolated from a marine sediment-derived fungus, Sporendonema casei HDN16-802, were assayed for anticoagulant activity and showed moderate inhibition of thrombin and Factor Xa, with inhibition ratios of 47.8% and 51.5%, respectively. The positive control, argatroban, showed an inhibition ratio of 65.0% [109].
Compounds 87 (Figure 7), 104, 107, 109 and 115 (Figure 9), isolated from a deep-sea sediment-derived fungus, A. versicolor, were assayed for antioxidant capacity by a Trolox equivalent antioxidant capacity (TEAC) assay. Compounds 87, 104, 107, 109, and 115 scavenged the 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid radical cations (ABTS •+ ), which were approximately equivalent to that of trolox (1.0 mmol/L). These compounds were further evaluated for their capacity to regulate the nuclear factor E2-related factor 2 (Nrf2), a transcription factor that responds to oxidative stress by binding to the antioxidant response element (ARE) in the promoter of genes coding for antioxidant enzymes and proteins for glutathione synthesis, and its activity can be measured by ARE-driven luciferase reporters using HepG2C8 cells, stably transfected with AREluciferase reporter plasmids. Compounds 87, 104, 107, 109, and 115, at a concentration of 10 µmol/L, caused significant induction of luciferase 1.41−1.58-folds more than that of the blank control (DMSO), and approximately half of the positive control, tBHQ (tertiary butylhydroquinone), at a concentration of 50 µmol/L [89].

Other Biological Activities
Using calcium imaging assay, 233 (Figure 21), isolated from a deep-sea sedimentderived fungus, Altenaria tenuissima DFFSCS013, effectively stimulated intracellular levels of calcium flux in HEK293 (human embryonic kidney) cells, at a concentration of 10 µM, in the calcium imaging assay. However, 233 did not show any effect at a concentration less than 10 µM [63].
In order to enhance a readability of this review, we have summarized the anthraquinoid metabolites and their derivatives, obtained from the marine environment in Table 1. This includes the names and numbers of the isolated compounds, the names of fungal producers, the sources from which the fungi were obtained, the reported biological/pharmacological activities and the references. Table 1. Anthraquinone metabolites and their analogues reported from marine-derived fungi.

Compound Fungus Species/Strain No. Source of Marine-Derived Fungi Bioactivity
Ref.

Concluding Remarks and Future Perspectives
This review shows that polyketides are the predominant metabolites reported from marine-derived fungi. Altogether, we have reported 296 specialized metabolites belonging to the anthraquinone class and their derivatives, which were isolated from 28 marine fungal strains, and less-studied fungal species highlighting the chemical diversity and their myriad biological/pharmacological properties. In general, these compounds exhibited a wide range of biological activities, including antibacterial and antibiofilm formation, antifungal, antiviral, antiparasitic, anti-inflammatory, enzyme inhibitory, antioxidant, anticoagulant, anti-angiogenesis, anti-obesity, anti-fouling, algicidal, insecticide and cytotoxic activities. More specifically, members of the genera Aspergillus, Penicillium, Eurotium, and Fusarium are the most prolific sources of anthraquinones and their derivatives. Among the isolated anthraquinones, 112 were from Aspergillus, 37 from Penicillium, 36 from Altenaria, 26 from Stemphylium, 23 from Eurotium, 19 from Fusarium, 14 from Trichoderma, 13 from Acremonium, 11 from Talaromyces, 10 from Nigrospora, and the rest of anthraquinones are from other fungal resources ( Figure 28). Members of the genera Aspergillus and Penicillium are found to be more versatile in terms of secondary metabolite biosynthesis, producing various types of anthraquinones viz. hydro-, alkylated, halogenated, seco-, furano and pyrano derivatives. Sulphated anthraquinoids and anthraquinones fused with xanthones and chromones have been also reported in species of Penicillium, while the glycosylated anthraquinones were reported from algicolous and mangrove endophytic fungi of the genera Fusarium and Stemphylium, which have a close symbiotic relationship with the hosts, indicating that they can adjust the biosynthetic pathways to each other. Bianthraquinones are found predominantly in Altenaria and Stemphylium species, while the anthraquinonexanthones are more preponderant in Acremonium and Engyodontium species, suggesting the species-specific metabolites. Another interesting observation is the elasticity of the biosynthetic capacity of fungi, for instance, cytoskyrin anthraquinone has been reported from the fungus Curvularia sp., which is associated with sponges. The influence of the fungal habitats, the organisms with which they are associated, the type of culture media and biotic and abiotic stressors can influence their capacity to biosynthesize a myriad of specialized metabolites with unique structural features, which ultimately can manifest different biological/pharmacological activities. The advantage of fungi in terms of secondary metabolite production over other organisms is their capacity to produce a large quantity of interesting compounds by fermentation. These compounds can be used as a scaffold for medicinal chemistry study. Given a versatility of the anthraquinoid scaffolds for their biological activities, it is legitimate to think that varying the side chains of the anthraquinoid scaffolds could render compounds with unique structures and efficient biological/pharmacological activities. Therefore, searching for marine-derived fungi from different niches, with different pressure, temperature and light intensity such as from thermal vent, deep-sea, polar habitats, and different animal hosts, can be promising to find structurally unique and biologically relevant compounds. Another perspective is the development of new culture media, which can allow for unculturable marine-derived fungi, which do not grow in normal media to thrive. In addition, taking advantage of the plasticity of the enzymology of the biosynthetic pathways of fungi, the addition of natural or synthetic amino acids to the culture media should be another challenging avenue to obtain compounds of unknown values. Funding: This work is partially supported by national funds through the FCT-Foundation for Science and Technology with the scope of UIDB/04423/2020 and UIDP/04423/2020, and by the structured program of the R&D&I ATLANTIDA Platform for the monitoring of the North Atlantic Ocean and tools for the sustainable exploitation of marine resources (reference NORTE-01-0145-FEDER-000040), supported by the North Portugal Regional Operational Program (NORTE2020) through the European Regional Development Fund (ERDF).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable.