Natural Products from Actinomycetes Associated with Marine Organisms

The actinomycetes have proven to be a rich source of bioactive secondary metabolites and play a critical role in the development of pharmaceutical researches. With interactions of host organisms and having special ecological status, the actinomycetes associated with marine animals, marine plants, macroalgae, cyanobacteria, and lichens have more potential to produce active metabolites acting as chemical defenses to protect the host from predators as well as microbial infection. This review focuses on 536 secondary metabolites (SMs) from actinomycetes associated with these marine organisms covering the literature to mid-2021, which will highlight the taxonomic diversity of actinomycetes and the structural classes, biological activities of SMs. Among all the actinomycetes listed, members of Streptomyces (68%), Micromonospora (6%), and Nocardiopsis (3%) are dominant producers of secondary metabolites. Additionally, alkaloids (37%), polyketides (33%), and peptides (15%) comprise the largest proportion of natural products with mostly antimicrobial activity and cytotoxicity. Furthermore, the data analysis and clinical information of SMs have been summarized in this article, suggesting that some of these actinomycetes with multiple host organisms deserve more attention to their special ecological status and genetic factors.


Introduction
Actinomycetes are Gram-positive bacteria with a GC-rich linear genome and have proven to be a rich source of secondary metabolites (SMs) of broad structural diversity and biological properties [1]. The ocean has been demonstrated as an ecosystem with many unique forms of actinomycetes [2]. The diversity of marine actinomycetes is determined by the uniqueness of the marine environment: some live freely in seawater, some in the seafloor sediments or sea mud; and some are symbiotic, parasitic, endophytic, or epiphytic with marine organisms [2,3]. Compared with actinomycetes isolated from seawater and sediment samples, recent reports of secondary metabolites from marine actinomycetes associated with a variety of aquatic organisms, including invertebrates such as sponges, corals, ascidians, echinoderms, and vertebrates such as pufferfish, as well as algae and seaweed, have increased significantly [4]. Studies have indicated that multiple active compounds previously isolated from marine invertebrates were possibly produced by their symbiotic microorganisms, especially actinomycetes [5][6][7]. With interactions of the host and having special ecological status, the actinomycetes associated with marine organisms have more potential to produce active metabolites acting as chemical defenses to protect the host from predators and microbial infection.
The objective of this article is to provide an overview of the natural products from actinomycetes associated with marine animals, marine plants, macroalgae, cyanobacteria, and lichens. The present review was not only summarizing the structural classes and biological activities of SMs but also highlighted the taxonomic diversity of actinomycetes,

Natural Products of the Actinomycetes Derived from Marine Animals
Among the 34 animal phyla known on the earth, marine animals account for 33 species, 15 of which are unique to the ocean. Studies have shown that some symbiotic microorganisms are species-specific [8], indicating that marine animals may be rich in microbial resources. And the active substances in marine animals are mostly produced by their associated microorganisms, of which actinomycetes are an important group [5][6][7]. Therefore, actinomycetes associated with marine animals are a flourishing source for novel natural products. (The structure of compounds with 'S' added before the number is shown in supporting information.)

Alkaloids
Alkaloids are a kind of nitrogen-containing organic compounds existing in nature, which have alkali-like properties. They are secondary metabolites with significant biological activities, most of which have complex ring structures and the nitrogen is mostly contained in the ring.
Alkaloids Derived from the Sponge-Associated Actinomycetes Two novel indolocarbazole alkaloids, 4 -N-methyl-5 -hydroxystaurosporine (1) and 5hydroxystaurosporine (2), as well as the known staurosporine (3) (Figure 2) were purified from the culture broth of Micromonospora sp. L-31-CLCO-002, which was associated with marine sponge Clathrina coriacea collected offshore Fuerteventura (Canary Islands). These compounds displayed cytotoxicity against various tumor cell lines [9,10]. The analysis of structure-activity relationships of staurosporine and its derivatives demonstrated that Mar. Drugs 2021, 19, 629 4 of 57 hydroxylation at C-3 of the indolocarbazole moiety led to the increase in anti-proliferative activity, while hydroxylation at C-11 caused a decrease in activity. The results suggested that not only the presence/absence of hydrophilic substitutions but also the position of the alteration within the molecule is significant in the anti-proliferative activities of the various staurosporine analogs [11,12].  Alkaloids Derived from the Coral-Associated Actinomycetes Three thiazole derivatives known as watasemycin A (79) (Figure 3), pulicatin G (S38), and aerugine (80) (Figure 3) were identified together with pyrrole-2-carboxamide (S39) and furan-2-carboxamide (S40) from Streptomyces sp. OUCMDZ  In 2005, an ongoing screening program for in vitro anticancer microbial extracts led to the discovery of two prodigiosin analogs metacycloprodigiosin (4) and undecylprodigiosin (5) (Figure 2) from a CHCl 3 extract of a strain of Saccharopolyspora sp. nov., which was associated with the marine sponge Mycale plumose collected on the coast of Qingdao, China. Both compounds 4 and 5 showed potent in vitro cytotoxicity against five cancer cell lines Mar. Drugs 2021, 19, 629 8 of 57 (P388, HL60, A-549, BEL-7402, and SPCA4), which was the first report on metacycloprodigiosin with anti-cancer activity [13]. The Micromonospora sp. strain M42 obtained from Indonesian sponge Acanthostrongylophora sp. produced manzamine A (6) and 8-hydroxy manzamine (7) (Figure 2), which had demonstrated activities against malaria, tuberculosis, and HIV [9].
Isolation of two rifamycins B and SV (8,9) (Figure 2) was reported in 2006 from the Salinispora sp. strain M403 associated with sponge Pseudoceratina clavata. This is the first recorded source of rifamycins from marine bacteria and Salinispora sp. represents a potential new source of rifamycins outside the genus Amycolatopsis. Rifamycins are a group of antibiotics that belong to the ansamycin family with pronounced activities against Gram-positive bacteria [9,14]. The structure and activity relationship of rifamycins with many different targets have been extensively studied [15,16]. The rifamycin SV has been successfully widely used in the clinical treatment of tuberculosis, leprosy, and mycobacterial infections.
Urauchimycins A and B (10, 11) ( Figure 2) were the first antimycin type antibiotics that possess a branched side chain and the side chain contains an odd number of carbons. The two novel antimycins were obtained from the Streptomyces sp. strain Ni-80 cultivated from an unidentified sponge collected at Urauchicove, Iriomote, Japan. They showed antifungal activity against Candida albicans by inhibiting the morphological differentiation of Candida albicans [9,17].
A novel indole alkaloid streptomycindole (S4) and a known related compound Nphenyl acetyl-L -tryptophan (S5) were isolated from Streptomyces sp. DA22 is associated with Craniella australiensis collected at the South China Sea [26]. In 2011, Rong-Bian Wei et al. reported isolation and identification of two new kijanimicin derivatives lobophorins C (26) and D (27) (Figure 2) from Streptomyces carnosus strain AZS17 associated with the marine sponge Hymeniacidon sp. collected from coastal waters of the East China Sea. Lobophorin C demonstrated potent cytotoxic activity against the human liver cancer cell line 7402 with IC 50 values of 0.6 µg/mL and lobophorin D showed a significant inhibitory effect on the growth of human breast cancer cells MDA-MB 435 with an IC 50 value of 7.5 µM [27].
Streptomyces sp. M7_15 associated with the Puerto Rican Sponge Scopalina ruetzleri produced an angucyclinone derivative frigocyclinone (35) and six new angucyclinone derivatives monacyclinones A-F (36-41) ( Figure 2). Frigocyclinone (35) with an unusual Cglycosidic linked aminodeoxy sugar moiety and monacyclinones A-F exhibited biological activity against rhabdomyosarcoma cancer cells (SJCRH30). Monacyclinones A-F also inhibits Gram-positive bacteria. Among them, monacyclinone F (41) showed the strongest bioactivity against SJCRH30 (EC 50 = 0.73 µM) and Gram-positive bacteria. The analysis of their structures and bioactivities suggested that the aminodeoxysugar subunit, the epoxide groups, and the ketone moiety could all be important for biological activity. [34].
Naphthacene glycoside SF2446 A2 (42) (Figure 2) was isolated from Streptomyces sp. strain RV15 obtained from the sponge Dysidea tupha collected offshore Rovinj, Croatia. The SF2446 A2, previously reported against Gram-positive bacteria and several mycoplasma strains, has proven to possess two novel activities. One is the potential of inhibiting Chlamydia trachomatis and inhibiting the primary infection and progeny formation in a dose-dependent manner. In addition, it can destroy the surface area of Schistosoma mansoni and affect the gonad by impairing the formation of oogenesis and spermatogenesis [35].
In 2016, Sekurova et al. reported isolation and identification of four new deferoxamine analogs (49, S12-S14) with additional acyl and sugar moieties from upon overexpression of Pathway-Specific Regulatory Gene in the Streptomyces albus PVA94-07 associated with sponge collected in the Trondheim fjord (Norway). Compound 49 ( Figure 2) showed 52-56% inhibition of E. coli at 16 µg/mL [42]. Comparing the structure and activity of deferoxamine analogs 49 and S14, it was found that the position of the hydroxyl group in the enamine group affected the antibacterial activity.
Tirandamycins A and B (43,44) were isolated from strain HNM0039 T , a novel Streptomyces sp. named Streptomyces tirandamycinicus sp. nov., which was obtained from a marine sponge collected from the coast of Wenchang, Hainan Province of China. Tirandamycins A and B displayed potent inhibitory activity against Streptococcus agalactiae HNe0 and showed antibacterial activity against Bacillus subtilis GIM1.222 [49].
Two quinocycline antibiotics strains, quinocycline B (kosinostatin) (73) and isoquinocycline B (74) (Figure 2), were separated from the strain 28ISP2-46 T assigned to a novel species Micromonospora ferruginea sp. which was recovered from a mid-Atlantic deep-sea sponge. Compounds 73 and 74 showed antibiotic activity and also inhibited DNA topoisomerase IIα. Isoquinocycline B (74) showed good antibiotic activity not only against the Grampositive strain S. aureus SH1000, but also the Gram-negative strains efflux knockout (KO) mutant strains of K. pneumoniae ATCC10031 and A. baumannii ATCC 17978. It was also active against the human liver cancer cell line HepG2 with an IC 50 of 13.3 µM [57].
Alkaloids Derived from the Coral-Associated Actinomycetes Three thiazole derivatives known as watasemycin A (79) (Figure 3 soft coral sample collected from the South China Sea. Thiazole derivatives 79 and 80 displayed moderate antibacterial activity against S. aureus and three methicillin-resistant strains MRSA082, MRSA111, and MRSA234 [60]. Lobophorin K (81) (Figure 3), a novel metabolites separated from Streptomyces sp. M-207 associated with Lophelia pertusa collected at submarine Avilés Canyon exhibited cytotoxicity against two human tumor cell lines pancreatic carcinoma (MiaPaca-2) and breast adenocarcinoma (MCF-7). It also showed moderate and selective antibiotic activity against pathogenic Gram-positive bacteria such as Staphylococcus aureus [61].
Alkaloids Derived from the Ascidian-Associated Actinomycetes Two novel antitumor antibiotics, lomaiviticins A (85) and B (86) (Figure 4) with unique dimeric diazobenzofluorene glycosides were discovered from Salinispora pacifica LL-37I366 derived from marine ascidian Polysyncraton lithostrotum. Lomaiviticins A and B were demonstrated to be significant DNA-damaging agents with a minimum induction concentration ≤0.1 ng/spot. They also exhibited potent antimicrobial activities against Gram-positive bacteria Staphylococcus aureus and Enterococcus faecium (MICs, 6-25 ng/spot). In addition, lomaiviticin A also displayed cytotoxicity against a number of cancer cell lines with IC 50 values ranging from 0.01 to 98 ng/mL [64][65][66].
Isolation of a new antibiotic arenimycin (92) (Figure 4) was reported in 2009 from Salinispora arenicola strain CNR-647 associated with sea squirt Ecteinascidia turbinate collected at Sweetings Cay, Grand Bahama Island. Arenimycin exhibited potent activities against drug-resistant Staphylococci, some other Gram-positive microanimals and plants, and one Mycobacterium strain. In addition, arenimycin was also active against eukaryotic cell division, which may lead to non-selective cytotoxicity [64,69].

Alkaloids Derived from the Actinomycetes Associated with Other Marine Animals
Halichomycin (101) ( Figure 5) was a novel structurally unique macrolide with potent cytotoxicity, separated from Streptomyces hygroscopicus OUPS-N92 obtained from the gastrointestinal tract of marine fish Halichoeres bleekeri. Halichomycin exhibited significant cytotoxicity in the P-388 lymphocytic leukemia test system with ED 50 of 0.13 µg/mL [73].
Streptomyces sp. 1053U.I.1a.3b, cultivated from Lienardia totopotens, a new species of conoidean mollusk collected in the Philippines, led to the identification of two novel lobophorins H-I (S51, 107) and three known lobophorins F (108), B (109), C (26). Compounds 107-109 ( Figure 5) and 26 showed strong inhibitory activity to M. tuberculosis and B. subtilis with MIC values ranging from 1.3 to 24 µM. In addition, all these active compounds also showed strong cytotoxicity against the human CEM-TART cancer cell line [78].
A novel hydroxamate metabolite MBJ-0003 (110) (  [87]. The analysis of structure-activity relationships revealed that the carboxyl moiety at C-22 and the position of hydroxylation are significant for both cytotoxic and antibacterial activities. In addition, the cytotoxicities of borrelidins were correlated to the cyano moiety, stereo configurations of the hydroxyl moiety at C-11, and the C-C double bonds. And the steric structural arrangement within the C-17 side chain is important for differentiating cytotoxic and antiangiogenic activities [87][88][89].

Polyketides
Polyketides are a large class of secondary metabolites produced by bacteria, fungi, actinomycetes, or plants. They are synthesized by simple fatty acids under the catalysis of polyketide biosynthase (PKS) through a synthesis pathway similar to long-chain fatty acids. Its structural types include macrolides, aromatic polyketides, polyether, pyranones, and other polyketides.
The identification of a new angucyclinone PM070747 (125) ( Figure 6) was reported from sponge-derived Saccharopolyspora taberi PEM-06-F23-019B obtained from near the coast of Tanzania, together with the known angucyclinone PD116740 (126) ( Figure 6). The two angucyclinones showed antitumor activity and compound 126 was active against leukemia and adenocarcinoma cell lines [91].
Keyicin (112) ( Figure 5), a novel and otherwise unattainable bisnitroglycosylated anthracycline antibiotic, was discovered from the producer Micromonospora sp. co-culturing with Rhodococcus sp. associated with marine invertebrates. The Keyicin was selectively active against Gram-positive bacteria including Rhodococcus sp. and Mycobacterium sp. E. coli-based chemical genomics studies revealed that keyicin's MOA, in contrast to many other anthracyclines, does not invoke nucleic acid damage [81].
Compounds 58, 113-116 ( Figure 5), S21, and S52 were isolated from Streptomyces sp. G278 that was obtained from echinoderm Holothuria edulis collected in Cu Lao Cham-Quang Nam. Compounds 113 and 114 were isolated from a natural source for the first time. Compounds 113 selectively inhibited Enterococcus faecalis. Compound 113 was proven to have antibacterial and antifungal activity and the known metabolites (115, 116, 58) exhibited antimicrobial activity. In addition, compound 115 possessed antifouling activities [82].
The isolation of a new pyrazolidine derivative, 1-acetyl-2-isobutyrylpyrazolidine-4carboxylic acid (S53) was reported in 2018 from a sea anemone-associated actinomycete Streptomyces sp. ZZ406 separated from Haliplanella lineate [83]. An unusual macrodilactone streptoseomycin (117) ( Figure 5) with potent bioactivity against Helicobacter pylori with a MIC value of 2 µg/mL was discovered from Streptomyces seoulensis A01 associated with a marine prawn collected in the Yellow Sea in China. It also exhibited antibacterial activities against a panel of microaerophilic bacteria with MICs in the range of 4-8 µg/mL [84].
In 2020, Zhenbin Zhou et al. reported isolation and identification of three novel borrelidins M-O (121, S61, S62), together with four previously known borrelidins CR1, A, E, and K (122, 123, S63, S64) from Streptomyces olivaceus SCSIO LO13 associated with pulmonated mollusks Onchidium sp. collected at Daya Bay, South China Sea. Borrelidin A (123) ( Figure 5) has a variety of biological activities such as antibacterial, anti-parasite, and cytotoxic activities. It showed significant activity against Micrococcus luteus with an MIC value of <0.5 µM. Borrelidins M (121) and CR1 (122) ( Figure 5) displayed weak cytotoxicity against normal human hepatic cell line L02, but no inhibitory effect on cancer cell lines was detected. In addition, borrelidins M, CR1, and K exhibited moderate activity against Micrococcus luteus with MIC values of 33 µM [87]. The analysis of structure-activity relationships revealed that the carboxyl moiety at C-22 and the position of hydroxylation are significant for both cytotoxic and antibacterial activities. In addition, the cytotoxicities of borrelidins were correlated to the cyano moiety, stereo configurations of the hydroxyl moiety at C-11, and the C-C double bonds. And the steric structural arrangement within the C-17 side chain is important for differentiating cytotoxic and antiangiogenic activities [87][88][89].

Polyketides
Polyketides are a large class of secondary metabolites produced by bacteria, fungi, actinomycetes, or plants. They are synthesized by simple fatty acids under the catalysis of polyketide biosynthase (PKS) through a synthesis pathway similar to long-chain fatty acids. Its structural types include macrolides, aromatic polyketides, polyether, pyranones, and other polyketides.
The identification of a new angucyclinone PM070747 (125) (Figure 6) was reported from sponge-derived Saccharopolyspora taberi PEM-06-F23-019B obtained from near the coast of Tanzania, together with the known angucyclinone PD116740 (126) (Figure 6). The two angucyclinones showed antitumor activity and compound 126 was active against leukemia and adenocarcinoma cell lines [91].
In 2014, Min Cheol Kim et al. reported isolation and identification of two novel tetracenedione derivatives nocatriones A (142) and B (143) (Figure 6) from Nocardiopsis sp. KMF-002 was cultivated from an unidentified dark purple marine sponge. Nocatrione A (142) showed a significant protective effect against UVB irradiation in both NHDF cell lines, whereas nocatrione B (143) was active against UVB only in a specific NHDF cell line [99].
Actinokineospora sp. EG49 cultivated from the marine sponge Spheciospongia vagabunda afforded two new actinosporin analogs actinosporins C (144) and D (145) (Figure 6). At 1.25 µM, actinosporins C and D showed a significant antioxidant and protective capacity from the genomic damage induced by hydrogen peroxide in the human promyelocytic (HL-60) cell line. Additionally, two other new antitrypanosomal angucycline-type metabolites actinosporins A (146) and B (147) ( Figure 6) were also discovered from strain EG49 [100,101]. that the ketone functional group at C-4 could enhance the anti-MRSA activity, while the hydroxyl group at C-3 weakened activity. 7-methoxy-2,3-dimethylchromone-4-one (S76) was first isolated from Streptomyces rochei MB037 associated with marine sponge Dysidea arenaria collected at Yongxin Island in the South China Sea. And the co-culture with fungus Rhinocladiella similis 35 derived from gorgonian could enhance its production [54]. The new fridamycins H (154), together with three known actinosporins C, D, and G (144, 145, 155) ( Figure 6) were obtained from sponge-associated Actinokineospora spheciospongiae sp. nov. Among them, fridamycin H (154) exhibited potent antitrypanosomal activity and growth inhibitory activity towards Trypanosoma brucei strain TC221 [52].
Actinosporins A and C (146,144) were isolated from the axenic cultures of strain EG49. Actinosporin A showed anti-trypanosomal activity and actinosporin C exhibited antioxidant activity. New actinosporins E-H (163, S77, 155, 164) ( Figure 6) were produced by EG49 through the activation of cryptic gene cluster by N-acetyl-D-glucosamine (GluNAc). In addition, tetrangulol (165) ( Figure 6) and the same actinosporins E, G, and H were discovered with antimalarial activity from the co-culture of strains Actinokineospora spheciospongiae EG49 and Rhodococcus sp. UR59. Tetrangulol was also reported as an antibiotic in previous research. What's more, the separate cultivation of strain UR59 leads to the isolation of Kaimonolide B (166) (Figure 6) (plant growth inhibitor) and 8,15-Dideoxylankanolide (S78) [59].  (150) showed cytotoxicity against NB4 and HL-60 cells with IC 50 values of 1.7 and 1.8 µM, respectively. Naquihexcin A (149) bears a rare unsaturated hexuronic acid moiety and could inhibit the proliferation of an adriamycin-resistant human breast cancer cell line MCF-7 ADM with IC 50 = 16.1 µM, indicating that the unsaturated hexuronic acid moiety could enhance the activity against the cancer cells [103].
Actinosporins A and C (146,144) were isolated from the axenic cultures of strain EG49. Actinosporin A showed anti-trypanosomal activity and actinosporin C exhibited antioxidant activity. New actinosporins E-H (163, S77, 155, 164) ( Figure 6) were produced by EG49 through the activation of cryptic gene cluster by N-acetyl-D-glucosamine (GluNAc). In addition, tetrangulol (165) (Figure 6) and the same actinosporins E, G, and H were discovered with antimalarial activity from the co-culture of strains Actinokineospora spheciospongiae EG49 and Rhodococcus sp. UR59. Tetrangulol was also reported as an antibiotic in previous research. What's more, the separate cultivation of strain UR59 leads to the isolation of Kaimonolide B (166) (Figure 6) (plant growth inhibitor) and 8,15-Dideoxylankanolide (S78) [59]. Polyketides Derived from the Ascidian-Associated Actinomycetes Ubiquinone Q9 (177) (Figure 8) was isolated from Nocardia sp. strain KMM 3749, a marine actinomycete associated with an unidentified ascidian. This compound inhibited the development of fertilized eggs of Strongylocentrotus intermedius and caused hemolysis of mouse erythrocytes [64,110].
The polyketide griseorhodin A (178) (Figure 8) was found to be biosynthesized by Streptomyces sp. JP95, which is associated with marine ascidian Aplidium lenticulum collected at Heron Island, Queensland, Australia. Griseorhodin A, a member of the rubromycin family, is an inhibitor of human telomerase [64,111].
A novel gram-positive antibiotic anthracimycin B (171) and anthracimycin (172) (Figure 7) were isolated from Streptomyces cyaneofuscatus M-169 associated with a gorgonian coral collected in the Avilés Canyon. Anthramycin has significant activity against four Gram-positive bacteria (MSSA, MRSA, vancomycin sensitive Enterococcus faecium, and vancomycin sensitive Enterococcus faecalis) with MIC values less than 0.03 µg/mL. And anthracimycin B was also active against these four Gram-positive bacteria. In addition, anthracimycin displayed anti-tubercular activity against Mycobacterium tuberculosis [107].
Isolation of three novel glycosylated macrolides iseolides A-C (174-176) (Figure 7) were reported in 2020 from Streptomyces sp. DC4-5 is associated with a stony coral Dendrophyllia. Iseolides showed potent antifungal activity against a plant pathogen Glomerella cingulata, as well as human pathogens Candida albicans and Trichophyton rubrum with MICs in the range of 0.19-6.25 µg/mL [109].
Polyketides Derived from the Ascidian-Associated Actinomycetes Ubiquinone Q9 (177) (Figure 8) was isolated from Nocardia sp. strain KMM 3749, a marine actinomycete associated with an unidentified ascidian. This compound inhibited the development of fertilized eggs of Strongylocentrotus intermedius and caused hemolysis of mouse erythrocytes [64,110].
The Streptomyces sp. PTY087I2 associated with styela canopus collected from Bastimentos Park, Bocas del Toro, Panama produced three naphthoquinone derivatives granaticin (197), granatomycin D (198), and dihydrogranaticin B (199) (Figure 8). Coculture of Streptomyces sp. PTY087I2 with human pathogens such as Bacillus subtilis, MSSA, and MRSA, respectively, resulted in increased production of these three antibiotics. In addition, co-culture resulted in greatly enhanced biological activity against the above three Gram-positive human pathogens [64,118].  The polyketide griseorhodin A (178) (Figure 8) was found to be biosynthesized by Streptomyces sp. JP95, which is associated with marine ascidian Aplidium lenticulum collected at Heron Island, Queensland, Australia. Griseorhodin A, a member of the rubromycin family, is an inhibitor of human telomerase [64,111].
The Streptomyces sp. PTY087I2 associated with styela canopus collected from Bastimentos Park, Bocas del Toro, Panama produced three naphthoquinone derivatives granaticin (197), granatomycin D (198), and dihydrogranaticin B (199) (Figure 8). Co-culture of Streptomyces sp. PTY087I2 with human pathogens such as Bacillus subtilis, MSSA, and MRSA, respectively, resulted in increased production of these three antibiotics. In addition, co-culture resulted in greatly enhanced biological activity against the above three Gram-positive human pathogens [64,118].
Compounds 200-203, 204a, and 204c (the acyl products of 204) suppressed the proliferation of cancer cell lines, and metabolites 203 is the most active compound with IC50 values ranging from 0.59 to 3.39 mM. The 11-hydroxycurvularins 200 and 201 also showed antibacterial activity inhibiting the growth of Escherichia coli [119]. Compound 205 (Figure 9), which was isolated from cucumber-associated Streptomyces sp. G278 selectively inhibited Enterococcus faecalis (MIC: 256 μg/mL). And the known metabolite 301 (Figure 9) with antibacterial and antifungal properties was also discovered from strain G278 [82].
Isolation of two novel 3,4,6-trisubstituted α-pyrone derivatives violapyrones H (210) and I (211) were reported together with known violapyrones B (212) and C (213) (Figure 10), from Streptomyces sp. 112CH148 associated with starfish Acanthaster planci collected at Chuuk, Federated States of Micronesia. Violapyrones (210-213) showed growth inhibitory activity against cancer cell lines at concentrations less than 26.12 µg/mL. Wherein compound 210 showed the highest cytotoxic activity against the HCT-15 cell line with a GI 50 value of 1.10 µg/mL. Additionally, violapyrones B and C were demonstrated to have antibacterial activities. Therefore, it may be noteworthy that each compound has structural similarities, but showed different activities. The results suggested that the length of the aliphatic side chain and the position of the methyl group affected the activity. Furthermore, violapyrones having an isomethyl group in the alkyl side chain showed better activity than others [122]. In

Polyketides Derived from the Actinomycetes Associated with Marine Vertebrates
Further investigation for metabolites of this strain has led to the isolation of three additional novel cytotoxic metabolites designated as halichoblelide A-C (223-225) ( Figure  11). Halichoblelide A was active against the murine P-388 cell line (ED50 0.63 μg/mL), and 3), and good stability in human liver microsomes [83]. A novel flavonoid derivative flavoside A (S98) and the known angucyclinone PD116740 (126) were isolated from the sea urchin-derived Streptomyces sp. HDa1 [86].

Polyketides Derived from the Actinomycetes Associated with Marine Vertebrates
Further investigation for metabolites of this strain has led to the isolation of three additional novel cytotoxic metabolites designated as halichoblelide A-C (223-225) ( Figure 11). Halichoblelide A was active against the murine P-388 cell line (ED 50 0.63 µg/mL), and all three novel metabolites exhibited significant cytotoxicity against the 39 human cancer cell lines [125,126].

Peptides
Most peptides from actinomycetes are circular and contain further rare structural elements, such as chromophore or unusual amino acids.

Peptides
Most peptides from actinomycetes are circular and contain further rare structural elements, such as chromophore or unusual amino acids.
A diketopiperazine (231) (Figure 12) with weak cytotoxic activity, known as a synthetic compound before, was first reported to be discovered from Nocardiopsis strain HB383 [92]. The new teleocidin analog JBIR-31 (232) (Figure 12 And the oxidation level of the β-ring proline residue influenced both cytotoxic and antibacterial activity. The single most important structure-activity factor in the peptide moieties is, however, the integrity of the cyclic structure. In addition, the configuration of 4-hydroxyl group points has a strong influence on activity [141,142]. Streptomyces sp. Call-36 isolated from sponge Callyspongia sp. collected in the Red Sea was reported to produce a new diketopiperazine actinozine A (252), cyclo(2-OH-D-Pro-L-Leu) (253), cyclo(D-Pro-L-Phe) (254) (Figure 12), and cyclo(L-Pro-L-Phe) (S123). Compounds 252 and 253 displayed potent activity against S. aureus and were moderately active against C. albicans. Compound 254 exhibited moderate and selective activity against HCT-116 with an IC50 of 32.7 μM, while cyclo (L-Pro-L-Phe) (S123) was inactive, indicating that the D/L configuration of Pro had an important effect on the activity [143].
Four new D-type actinomycin analogs actinomycins D1-D4 (247-250) and actinomycin D (251) (Figure 12) were discovered from Streptomyces sp. LHW52447 associated with Phyllospongia foliascens obtained from the Xisha Islands in the South China Sea. Actinomycins D1 (247) and D2 (248) introduced an oxazole unit into the central phenoxazinone chromophore and exhibited more potent activities against three strains of MRSA with MIC values of 0.125-0.25 µg/mL than that of actinomycins D3-D4 (MIC = 0.5-1.0 µg/mL), which indicated that the incorporation of the oxazole unit would enhance the antibacterial activity. In addition, the cytotoxicity evaluation against human lung WI38 embryonal fibroblasts suggested that the incorporation of oxazole unit could decrease the cytotoxicity of actinomycins on human normal cells [140]. The SAR studies had indicated that amino acid substitutions in different positions of the peptides influenced biological potencies. And the oxidation level of the β-ring proline residue influenced both cytotoxic and antibacterial activity. The single most important structure-activity factor in the peptide moieties is, however, the integrity of the cyclic structure. In addition, the configuration of 4-hydroxyl group points has a strong influence on activity [141,142].
Peptides Derived from the Coral-Associated Actinomycetes Thiocoraline (235) is a thiodepsipeptide antitumor antibiotic isolated from Micromonospora sp. L-13-ACM2-092 is associated with a soft coral collected in the Indian Ocean off the coast of Mozambique. Thiocoraline had an inhibitory effect on DNA polymerase α. In addition, it displayed potent cytotoxicity and strong activity against Gram-positive bacteria [145,146].

Peptides Derived from the Actinomycetes Associated with Other Marine Animals
Five cyclic depsipeptides with unusual structures salinamides A, B (266, 267) ( Figure  15) and salinamides C-E (S127-S129) were discovered from Streptomyces sp. CNB-091 obtained from the surface of jellyfish Cassiopeia xamachana collected in the Florida Keys. Salinamides A and B exhibited moderate antibiotic activity against Gram-positive bacteria. Additionally, the results of phorbol ester-induced mouse ear edema assay showed that salinamides A and B displayed significant topical anti-inflammatory activity using [149,150]. In 2014, salinamide F (268) (Figure 15), a new bicyclic depsipeptide, was also reported to be separated from this strain, possessing significant RNAP-inhibitory and antibacterial activity like salinamide A [151].

Peptides Derived from the Actinomycetes Associated with Other Marine Animals
Five cyclic depsipeptides with unusual structures salinamides A, B (266, 267) ( Figure 15) and salinamides C-E (S127-S129) were discovered from Streptomyces sp. CNB-091 obtained from the surface of jellyfish Cassiopeia xamachana collected in the Florida Keys. Salinamides A and B exhibited moderate antibiotic activity against Gram-positive bacteria. Additionally, the results of phorbol ester-induced mouse ear edema assay showed that salinamides A and B displayed significant topical anti-inflammatory activity using [149,150]. In 2014, salinamide F (268) (Figure 15), a new bicyclic depsipeptide, was also reported to be separated from this strain, possessing significant RNAP-inhibitory and antibacterial activity like salinamide A [151].
ZZ406. Valinomycin (230) was active against the proliferation of different glioma cells and downregulating the expressions of glioma metabolic regulators [83]. In addition, the valinomycin was also isolated from sponge-derived Streptomyces sp. strains 22 and 34 that have been mentioned above. Compound 274 (Figure 15) was discovered from the sea urchin-associated Streptomyces sp. HDa1 displayed weak activity against the Gramnegative bacteria Vibrio harveyi with an inhibition zone of 1.5 mm [85].
Microeunicellols A (280) ( Figure 16) and B (S135), two novel eunicellin diterpenoids, were reported in 2020 from the culture of Streptomyces albogriseolus SY67903 associated with the gorgonian Muricella sibogae collected at the South China Sea. Microeunicellol A exhibited cytotoxicities against several human cancer cell lines [158]. Mengxuan Chen et al. reported isolation and identification of a new compound S130 and the known valinomycin (230) in 2018, from sea anemone-associated Streptomyces sp. ZZ406. Valinomycin (230) was active against the proliferation of different glioma cells and downregulating the expressions of glioma metabolic regulators [83]. In addition, the valinomycin was also isolated from sponge-derived Streptomyces sp. strains 22 and 34 that have been mentioned above. Compound 274 (Figure 15) was discovered from the sea urchin-associated Streptomyces sp. HDa1 displayed weak activity against the Gramnegative bacteria Vibrio harveyi with an inhibition zone of 1.5 mm [85].
Microeunicellols A (280) ( Figure 16) and B (S135), two novel eunicellin diterpenoids, were reported in 2020 from the culture of Streptomyces albogriseolus SY67903 associated with the gorgonian Muricella sibogae collected at the South China Sea. Microeunicellol A exhibited cytotoxicities against several human cancer cell lines [158]. Figure 17) were discovered from Actinomadura sp. SBMs009 is derived from the sponge Suberites japonicus. They showed anti-inflammatory activity based on NF-kB inhibition and glucocorticoid receptor-protein binding properties. Among them, bendigole F exhibited the highest activity against translocation of GFPlabeled NF-kB into the nucleus of hamster ovary CHO cells in vivo with an IC 50 of 71 µM. The three sterols displayed activity against the glucocorticoid receptor translocation and bendigole D was the most potent. Bendigole D showed mild cytotoxicity against the L929 murine aneuploid fibrosarcoma with an IC 50 (Figure 17), a novel metabolite isolated from Streptomyces seoulensis IFB-A01, showed a significant inhibitory effect on NA in a dose-dependent manner, and the IC50 value was 3.92 μmol/L [153].
New nucleoside derivative (S143) and two known compounds S144 and S145 were isolated from Streptomyces microflavus strain No. HVG29 is associated with the marine sponge Hymeniacidon perlevis collected from the coast of Dalian (China). Compounds S143-S145 were the first time to isolate deoxyuridine structures from S. microflavus associated with sponges [161].
Streptomyces sp. NIO 10068 derived from a marine sponge produced cinnamic acid (296) (Figure 18), which was active against QS (quorum sensing)-mediated virulence factors in Pseudomonas aeruginosa. Cinnamic acid was proven to possess previously undescribed QS antagonist properties. In addition, it is demonstrated to display bactericidal activity in the present study [135].
Isolation of a new oxaphenalene derivative (S151) was reported in 2017 from Streptomyces griseorubens sp. ASMR4 is associated with an unidentified soft coral collected in the Red Sea at the Hurghada coast, Egypt. Additionally, along with metabolite S144, seven other known metabolites S152-S158 were also discovered from the strain ASMR4 [164].
Isolation of four metabolites ethyl plakortide Z (305) (Figure 18), seco-plakortide Z (S167), actinopolysporin B (S168), and acanthosterol G (S169) was reported in 2021 from sponge-associated Streptomyces sp. RM66. Peroxidessethyl plakortide Z was active against solid tumor and L-1210 leukemia cell lines in vitro [58]. Two new anti-inflammatory compounds lobophorins A ( Figure 19) and B (306, 109) were produced by an unidentified actinomycete strain CNC-837 obtained from the surface of the Caribbean brown alga Lobophora variegate. The new compounds, distantly related to antibiotics of the kijanimicin class, are potent inhibitors of topical PMA-induced edema in the mouse ear assay when administered either topically or IP [167].

Alkaloids Derived from the Green Algae Associated Actinomycetes
A unique indolizinium alkaloid streptopertusacin A (312) ( Figure 19) and a novel N-arylpyrazinone derivative streptoarylpyrazinone A (S170) were separated from the seaweed-associated Streptomyces sp. HZP-2216E obtained from sea lettuce Ulva pertusa, a traditional Chinese medicine. They are both existing as zwitterion and streptopertusacin A showed moderate activity against the growth of MRSA [170,171].
Shan-Shan Su et al. reported the discovery of a novel compound (S191) in 2013 together with a known compound (S192) from lichen-derived Streptomyces cavourensis YY01-17 [173]. And a novel bioactive antimicrobial kocumarin (338) (Figure 20) was produced by Kocuria marina CMG S 2 associated with brown macroalga Pelvetia canaliculata attached to the rocks of Sonmiani Beach and demonstrated prominent and rapid growth inhibition against all tested fungi and pathogenic bacteria [182]. Three known compounds benzoic acid (S158), hydrocinnamic acid (S193), and (E)-cinnamic acid(S194) were

Alkaloids Derived from the Actinomycetes Associated with Lichens
Isolation of a novel metabolite S177 was reported in 2013 from Streptomyces cavourensis YY01-17 separated from the lichens grown in the Antarctic area [173].
Desertomycin G (323) (Figure 19) was separated from Streptomyces althioticus MSM3 associated with intertidal macroalgae Ulva sp. collected at Cantabrian Sea. Desertomycin G displayed potent antibiotic activities against several clinically relevant pathogens and moderate activity against relevant Gram-negative clinical pathogens. Additionally, it affects the viability of tumor cell lines, such as human breast adenocarcinoma (MCF-7) and colon carcinoma (DLD-1), but not normal mammary fibroblasts [176].
Polyketides Derived from the Cyanobacteria-Associated Actinomycetes Isolation of antibiotic Bisanthraquinones (326-328) ( Figure 19) was reported from Streptomyces sp. N1-78-1 associated with unicellular cyanobacteria parasitized on the tunic surface of Ecteinascidia turbinate collected in La Parguera, Puerto Rico. The metabolites 326 and 327 potently inhibited the growth of MRSA. All compounds were moderately active against HCT-116 human colon tumor cells [112].

Data Analysis of the Secondary Metabolites from Actinomycetes Associated to Various Hosts
A total of 536 metabolites have been discovered from 155 actinomycetes associated with various marine hosts belonging to 22 genera. Among them, alkaloids (37%), polyketides (33%), and peptides (15%) comprise the largest proportion of secondary metabolites, while Streptomyces (68%), Micromonospora (6%), and Nocardiopsis (3%) are the dominant producers ( Figure 21). Figure 22 showed the distribution of secondary metabolites currently identified in different host-related actinomycetes. The majority of the secondary metabolites were isolated from the actinomycetes associated with sponges (47%), ascidians (11%), and corals (9%), as well as brown algae (5%). Furthermore, the Sankey diagram and histogram were done to show the distribution of secondary metabolites produced by actinomycetes with various genera derived from different hosts, which is convenient for readers to have an overall understanding of the current secondary metabolites from marine organism-associated actinomycetes (Figures 23 and 24).
Approximately 64% of the SMs displayed various biological activities, especially antimicrobial activity and cytotoxicity ( Figure 25). Interestingly, some of these active metabolites with multiple biological properties deserve more attention (Table S7); for example, metabolites with cytotoxicity usually have antibacterial or antiparasitic activities, and some metabolites showed antibacterial activity can also act as enzyme inhibitors. This study advances the knowledge of these actinomycetes in respect to the metabolic potential of medicinal lead compounds. Isolation of two antibacterial benzaldehydes 2-hydroxy-5-(3-methylbut-2-enyl) benzaldehyde (336) and 2-hepta-1,5-dienyl-3,6-dihydroxy-5-(3-methylbut-2-enyl) benzaldehyde (337) (Figure 20) were reported from Streptomyces atrovirens PK288-21 separated from the rhizosphere of the brown algae Undaria pinnatifida. The compound 336 is a new benzaldehyde derivative, and metabolite 337 was the first time reported in the genus Streptomyces. The two compounds were active against Edwardsiella tarda and Streptococcus iniae [181].

Data Analysis of the Secondary Metabolites from Actinomycetes Associated to Various Hosts
A total of 536 metabolites have been discovered from 155 actinomycetes associated with various marine hosts belonging to 22 genera. Among them, alkaloids (37%), polyketides (33%), and peptides (15%) comprise the largest proportion of secondary metabolites, while Streptomyces (68%), Micromonospora (6%), and Nocardiopsis (3%) are the dominant producers ( Figure 21). Figure 22 showed the distribution of secondary metabolites currently identified in different host-related actinomycetes. The majority of the secondary metabolites were isolated from the actinomycetes associated with sponges (47%), ascidians (11%), and corals (9%), as well as brown algae (5%). Furthermore, the Sankey diagram and histogram were done to show the distribution of secondary metabolites produced by actinomycetes with various genera derived from different hosts, which is convenient for readers to have an overall understanding of the current secondary metabolites from marine organism-associated actinomycetes (Figures 23 and 24).
Approximately 64% of the SMs displayed various biological activities, especially antimicrobial activity and cytotoxicity ( Figure 25). Interestingly, some of these active metabolites with multiple biological properties deserve more attention (Table S7); for example, metabolites with cytotoxicity usually have antibacterial or antiparasitic activities, and some metabolites showed antibacterial activity can also act as enzyme inhibitors. This study advances the knowledge of these actinomycetes in respect to the metabolic potential of medicinal lead compounds.

Clinical Information of the Secondary Metabolites
Molecules with excellent activities, which have been in clinical applications or have entered clinical trials, were listed in Table 1. For example, rifamycin SV (9) is the earliest rifamycin antibiotic used in clinical application. Tetrodotoxin (105) has been widely used as an analgesic, sedative, antispasmodic, and local anesthetic in clinics. And daunomycin (307) has a good effect on acute myeloid leukemia. In addition, for these drugs already in clinical use, more clinical trials are underway for new diseases or new usages. (More detailed information can be found on the website ClinicalTrials.gov accessed on 25 October 2021).
ar. Drugs 2021, 19, x FOR PEER REVIEW Figure 21. The structural distribution of metabolites from the actinomycetes is divid

Clinical Information of the Secondary Metabolites
Molecules with excellent activities, which have been in clinical applications entered clinical trials, were listed in Table 1. For example, rifamycin SV (9) is th rifamycin antibiotic used in clinical application. Tetrodotoxin (105) has been wid as an analgesic, sedative, antispasmodic, and local anesthetic in clinics. And dau (307) has a good effect on acute myeloid leukemia. In addition, for these drugs a clinical use, more clinical trials are underway for new diseases or new usage detailed information can be found on the website ClinicalTrials.gov. October 25,

Discussion
Metabolites from actinomycetes associated with marine organisms have proven to be an abundant source for the isolation of multiple potent bioactive metabolites with diverse structures. In this review, we attempt to discuss the significance of the special ecological status and genetic factors of these actinomycetes with multiple hosts. The chemical ecology underlying hosts-actinomycetes interactions provide a great opportunity for the discovery of novel drugs. During the co-evolution, these actinomycetes and their specific hosts constructed a coordinated and relatively independent micro-ecological environment, in which SMs can be tolerated by the host and are the active inhibiting specific external invasion. Therefore, actinomycetes associated with various marine hosts play an important ecological role in producing novel medicinal active compounds. Currently, there are relatively few studies on these actinomycetes, but many secondary metabolites have been isolated with excellent bioactivities. Some of these metabolites have been used in clinical applications or have entered clinical trials where they are expected to become new drugs. There is no doubt that further exploration can be a useful strategy for discovering novel marine natural products. These actinomycetes, however, are difficult to be cultured under experimental conditions. Therefore, in-depth exploration of the ecology of these actinomycetes to continuously optimize culture conditions is crucial for further research. Meanwhile, the use of advanced bioinformatics technology for gene detection of uncultured actinomycetes and heterologous expression of the collected biosynthetic gene clusters will be another important pathway for research on SMs of marine organism-associated actinomycetes.