Secondary Metabolites from Coral-Associated Fungi: Source, Chemistry and Bioactivities

Our study of the secondary metabolites of coral-associated fungi produced a valuable and extra-large chemical database. Many of them exhibit strong biological activity and can be used for promising drug lead compounds. Serving as an epitome of the most promising compounds, which take the ultra-new skeletons and/or remarkable bioactivities, this review presents an overview of new compounds and bioactive compounds isolated from coral-associated fungi, covering the literature from 2010 to 2021. Its scope included 423 metabolites, focusing on the bioactivity and structure diversity of these compounds. According to structure, these compounds can be roughly classified as terpenes, alkaloids, peptides, aromatics, lactones, steroids, and other compounds. Some of them described in this review possess a wide range of bioactivities, such as anticancer, antimicrobial, antifouling, and other activities. This review aims to provide some significant chemical and/or biological enlightenment for the study of marine natural products and marine drug development in the future.


Introduction
Marine organisms, representing approximately 75% of all living organisms, have proven to be a rich source of inspiration for drug discovery, with success rates for marine natural products up to 4 times higher than other naturally derived compounds [1]. Research into the pharmacological properties of marine natural products (MNP) has led to the discovery of many active agents considered worthy of clinical application; to date 14 marine NPs or their derivatives are registered drugs, and another 23 are currently in clinical trials [2]. The annual reviews of marine natural products were reported by the New Zealand group in the Natural Product Reports. These reviews show that marine fungi are currently the most studied marine microorganism phyla, and over the last five years an extraordinary transformation in MNP research continued with a very significant increase in the number of new compounds reported from marine fungi. For example, in 2018, new MNPs reported from marine fungi increased by 38% relative to 2017 [3]. With up to 90% of marine species undescribed, marine fungi can inspire new discoveries and offer many novel solutions to life's problems in the future.
Coral reefs are among the most fragile, biologically diverse and economically important ecosystems on Earth, providing ecosystem services that are vital to human societies and industries through fisheries, coastal protection, new biochemical compounds, and tourism [4,5]. Coral reefs are regarded as one of the most important shelters of microorganisms [6]. Fungi are abundant in the coral reefs and recent studies demonstrate diverse communities associated with coral. Recent advances suggest that fungi associated with

Terpenes
Terpenes have been widely applied in the pharmaceutical, nutraceutical, synthetic chemistry, flavor fragrance, and possibly biofuel industries, and are essential constituents of natural products. Moreover, terpenes are a prime group of essential oils possessing a broad spectrum of antibacterial, antifungal and even antiviral activity [12,13]. Marine fungi are a significant source of terpenes, so it is necessary to carry out further investigation.

Diterpenoids
The well-known compound lovastatin (18) was extracted from a coral-derived fungus Aspergillus terreus. In vitro anti-inflammatory experiments showed that 18 has antiinflammatory activity against NO production and was shown to have a significant inhibitory effect with an IC50 value of 17.45 μM [24]. Meanwhile, compound 18 was an inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme as a lipid-lowering drug [25]. Furthermore, compound 18 also exhibited bioactivities of anti-cancer, prevention and treatment of neurological disorders, and antibacterial effects [26]. Two new harziane diterpene lactones, harziane lactones A and B (19 and 20) and five new harziane diterpenes, harzianones A-D (21)(22)(23)(24) and harziane (25), were identified from the soft coral-derived fungus Trichoderma harzianum XS-20090075. Compounds 19−23 and compound 25 exhibited obvious phytotoxicity against the seedling growth of amaranth and lettuce with a concentration of 200 ppm. Moreover, at the concentration of 200 μg/mL, compounds 19, 21, 22, and 23 completely inhibited seed germination against amaranth. Compared with the positive control glyphosate, these compounds still showed phytotoxicity at a lower concentration of 50 μg/mL [27]. (Figure 2)

Alkaloids
Alkaloids have great development potential as drug scaffolds and scaffold substructures in modern antibacterial chemotherapy. There will be opportunities to accelerate the process of discovering more active alkaloids in the future [37].

Alkaloids
Alkaloids have great development potential as drug scaffolds and scaffold substructures in modern antibacterial chemotherapy. There will be opportunities to accelerate the process of discovering more active alkaloids in the future [37].

Comprehensive Overview and Conclusion
This review concluded that coral-associated fungi are a productive source of structurally diversified secondary metabolites with various bioactivities. In particular, the present review includes 423 metabolites of newly discovered compounds and bioactive compounds with a wide range of biological activities of anticancer, antimicrobial, anti-inflammatory, anti-fouling, and other bioactivities. In addition to the current in vitro bioassays, further clinical studies of these bioactive compounds are required to determine their potential therapeutic applications.

Comprehensive Overview and Conclusions
This review concluded that coral-associated fungi are a productive source of structurally diversified secondary metabolites with various bioactivities. In particular, the present review includes 423 metabolites of newly discovered compounds and bioactive compounds with a wide range of biological activities of anticancer, antimicrobial, antiinflammatory, anti-fouling, and other bioactivities. In addition to the current in vitro bioassays, further clinical studies of these bioactive compounds are required to determine their potential therapeutic applications.
These natural compounds were isolated from several coral-derived fungi covered soft coral, gorgonian coral, hard coral, leather coral, stony coral, and some unknown ones. Among them, soft coral-derived fungi are the dominant producers of natural products, comprising more than 53% of total molecules. And the second, gorgonian-derived fungi, accounts for 37% ( Figure 20). Generally, the epizoic relationship between symbiotic or epiphytic fungi and their host animals or environment induces the production of metabolites. Bioactive compounds are comprised of anticancer (22%), antimicrobial (28%), antifouling (17%), anti-inflammatory (7%), and other biological activities (26%) (Figure 21). Among them, other activities include neuroprotective activity, AChE inhibitory activity, α-glucosidase activity etc. It follows that compound produced by coral-associated fungi reveal great potential bioactivities. These natural compounds were isolated from several coral-derived fungi covered soft coral, gorgonian coral, hard coral, leather coral, stony coral, and some unknown ones. Among them, soft coral-derived fungi are the dominant producers of natural products, comprising more than 53% of total molecules. And the second, gorgonian-derived fungi, accounts for 37% ( Figure 20, Table S7). Generally, the epizoic relationship between symbiotic or epiphytic fungi and their host animals or environment induces the production of metabolites. Generally, marine natural products isolated from coral-associated fungi are dominated by aromatics (35%) and alkaloids (24%), followed by lactones, peptides, terpenes, steroids, and other compounds ( Figure 19). These natural compounds were isolated from several coral-derived fungi covered soft coral, gorgonian coral, hard coral, leather coral, stony coral, and some unknown ones. Among them, soft coral-derived fungi are the dominant producers of natural products, comprising more than 53% of total molecules. And the second, gorgonian-derived fungi, accounts for 37% ( Figure 20). Generally, the epizoic relationship between symbiotic or epiphytic fungi and their host animals or environment induces the production of metabolites. Bioactive compounds are comprised of anticancer (22%), antimicrobial (28%), antifouling (17%), anti-inflammatory (7%), and other biological activities (26%) (Figure 21). Among them, other activities include neuroprotective activity, AChE inhibitory activity, α-glucosidase activity etc. It follows that compound produced by coral-associated fungi reveal great potential bioactivities. Bioactive compounds are comprised of anticancer (22%), antimicrobial (28%), antifouling (17%), anti-inflammatory (7%), and other biological activities (26%) (Figure 21, Table  S1-S5). Among them, other activities include neuroprotective activity, AChE inhibitory activity, α-glucosidase activity etc. It follows that compound produced by coral-associated fungi reveal great potential bioactivities. Structurally, aromatics (36%) and alkaloids (27%) included the major proportions bioactive compounds, followed by lactones (11%) (Figure 22). What is noteworthy is t satratoxin F (364) as a known lactone displayed excellent cytotoxic activities against f human cancer cell lines (MDA-MB-231, C4-2B, MGC803, MDA-MB-468, and A549) w EC50 values less than 39 nM. A previous review indicated that fungi was regarded as best candidate as a source of anticancer agents [186]. In addition, another alkalo aflaquinolone D (111), exhibited notable antifouling activity with EC50 values of 2.8 n Obviously, compounds containing heteroatoms exhibited more potential bioactivities Noteworthy is that several compounds exhibit different bioactivities. As seen in F ure 21, the biological activities mainly focus on anticancer and antimicrobial activity. F thermore, new compounds accounted for 23%, whereas known compounds made up 7 ( Figure 23). New technologies and methods should be applied to improve the discov Structurally, aromatics (36%) and alkaloids (27%) included the major proportions of bioactive compounds, followed by lactones (11%) (Figure 22, Table S1-S5). What is noteworthy is that satratoxin F (364) as a known lactone displayed excellent cytotoxic activities against five human cancer cell lines (MDA-MB-231, C4-2B, MGC803, MDA-MB-468, and A549) with EC 50 values less than 39 nM. A previous review indicated that fungi was regarded as the best candidate as a source of anticancer agents [186]. In addition, another alkaloid, aflaquinolone D (111), exhibited notable antifouling activity with EC 50 values of 2.8 nM. Obviously, compounds containing heteroatoms exhibited more potential bioactivities. Structurally, aromatics (36%) and alkaloids (27%) included the major proportions of bioactive compounds, followed by lactones (11%) (Figure 22). What is noteworthy is that satratoxin F (364) as a known lactone displayed excellent cytotoxic activities against five human cancer cell lines (MDA-MB-231, C4-2B, MGC803, MDA-MB-468, and A549) with EC50 values less than 39 nM. A previous review indicated that fungi was regarded as the best candidate as a source of anticancer agents [186]. In addition, another alkaloid, aflaquinolone D (111), exhibited notable antifouling activity with EC50 values of 2.8 nM. Obviously, compounds containing heteroatoms exhibited more potential bioactivities. Noteworthy is that several compounds exhibit different bioactivities. As seen in Figure 21, the biological activities mainly focus on anticancer and antimicrobial activity. Furthermore, new compounds accounted for 23%, whereas known compounds made up 77% ( Figure 23). New technologies and methods should be applied to improve the discovery Noteworthy is that several compounds exhibit different bioactivities. As seen in Figure 21, the biological activities mainly focus on anticancer and antimicrobial activity. Furthermore, new compounds accounted for 23%, whereas known compounds made up 77% (Figure 23). New technologies and methods should be applied to improve the discovery of new compounds and it is necessary to explore the bioactivities of known metabolites through effective biological screening methods. of new compounds and it is necessary to explore the bioactivities of known metabolit through effective biological screening methods. Figure 23. Distribution of the bioactive compounds between new compounds and known co pounds.
In the last few decades, secondary metabolites gathered from coral-associated fun have shown noteworthy levels in a number of clinical targets, and many of them are stru turally unique and possess remarkable biological and pharmacological properties, su as anticancer and antimicrobial activity. Some of them are lead compounds and potent clinical drug candidates, such as satratoxin F with antitumor activity and aurasperone with radical-scavenging activity against 2,2-diphenyl-1-picryl-hydrazyl. Meanwhile, m rine natural products exhibit diversified bioactivity due to their special chemical stru tures and potential interactions with proteins.
Drug discovery from coral-associated fungi has been a sustainable and intellige methodology that can surmount supply issues through the large-scale fermentation fungi. Furthermore, some key biosynthetic gene clusters in the regulation of fungi expres ing unique skeleton metabolites with high bioactivity remain silent [187]. Therefore, it necessary to apply novel methods and technologies with the purpose of activating t expression of unique secondary metabolites. On the basis of simulating natural cond tions, it is appropriate to add some stimulus.
In conclusion, the present review elucidated chemical structures of 423 compoun obtained from coral-associated fungi, many of them with bioactivities as promising dru lead compounds. These metabolites exhibit diversified structures and various bioactiv ties. In particular, some compounds revealed remarkable activity, even stronger than t positive controls. These findings indicate that these compounds have great potential the treatment of diseases. But the bioactivity of these metabolites was mainly tested vitro; thus, more attention should be paid to the molecular mechanism and further in vi and preclinical studies. In conclusion, there are many coral-associated secondary meta olites with notable biological activity, and numerous drug lead compounds and new m tabolites are still waiting to be discovered.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table  Table S1. Anticancer activity; Table S2. Antimicrobial activities; Table S3. Anti-inflammatory act ities; Table S4. Antifouling activities; Table S5. Other activities; Table S6. Distribution of the co pounds according to chemical structure; Table S7. The strain source of the natural products fro coral-derived fungi.

New 23%
Known 77% Figure 23. Distribution of the bioactive compounds between new compounds and known compounds.
In the last few decades, secondary metabolites gathered from coral-associated fungi have shown noteworthy levels in a number of clinical targets, and many of them are structurally unique and possess remarkable biological and pharmacological properties, such as anticancer and antimicrobial activity. Some of them are lead compounds and potential clinical drug candidates, such as satratoxin F with antitumor activity and aurasperone B with radical-scavenging activity against 2,2-diphenyl-1-picryl-hydrazyl. Meanwhile, marine natural products exhibit diversified bioactivity due to their special chemical structures and potential interactions with proteins.
Drug discovery from coral-associated fungi has been a sustainable and intelligent methodology that can surmount supply issues through the large-scale fermentation of fungi. Furthermore, some key biosynthetic gene clusters in the regulation of fungi expressing unique skeleton metabolites with high bioactivity remain silent [187]. Therefore, it is necessary to apply novel methods and technologies with the purpose of activating the expression of unique secondary metabolites. On the basis of simulating natural conditions, it is appropriate to add some stimulus.
In conclusion, the present review elucidated chemical structures of 423 compounds obtained from coral-associated fungi, many of them with bioactivities as promising drug lead compounds. These metabolites exhibit diversified structures and various bioactivities. In particular, some compounds revealed remarkable activity, even stronger than the positive controls. These findings indicate that these compounds have great potential in the treatment of diseases. But the bioactivity of these metabolites was mainly tested in vitro; thus, more attention should be paid to the molecular mechanism and further in vivo and preclinical studies. In conclusion, there are many coral-associated secondary metabolites with notable biological activity, and numerous drug lead compounds and new metabolites are still waiting to be discovered.