*3.3. Terpenoids*

Terpenoids, which are widely found in nature and in numerous species, have various structures and are divided into monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), and sesterterpenes (C25) [61]. Although most known terpenoids have been isolated from plants [62], they are also produced by marine microorganisms [63].

Three new pimarane diterpenes, aspergilone A (**67**) and compounds **68** and **69**, one new isopimarane diterpene (**70**), and four known compounds, diaporthin B (**71**), diaporthein B (**72**), 11-deoxydiaporthein A (**73**), and isopimara-8(14),15-diene (**74**), were obtained from the fungus *Epicoccum* sp., associated with the sea cucumber *A. japonicus*, which was collected from Yantai, Shandong Province, China [40,64,65]. Compounds **67**, **68**, and **71** exhibited cytotoxicity in KB cells, with IC50 values of 3.51, 20.74, and 3.86 μg/mL, respectively, and in KBv200 cells, with IC50 values of 2.34, 14.47, and 6.52 μg/mL, respectively [40]. Compounds **70** and **73** exhibited effective inhibitory activities against α-glucosidase, with IC50 values of 4.6 and 11.9 μM, respectively [65].

The fungus *Aspergillus* sp. H30, derived from the sea cucumber *Cucumaria japonica*, which was collected from the South China Sea, produced a meroterpenoid called chevalone B (**75**) that exhibited weak antibacterial activity [36].

Terpene glycosides are a group of natural products with a triterpene or sterol core, and marine diterpene glycosides (MDGs) are a subset of terpene glycosides [66]. Thirty-one new diterpene glycosides, including virescenosides M–R (**76**–**81**), R1–R3 (**82**–**84**), S–X (**85**–**90**), Z (**91**), and Z4–Z18 (**92**–**106**), and three known diterpenic glycosides, virescenosides A (**107**), B (**108**), and C (**109**), together with three known analogues, virescenoside F (**110**), G (**111**), a lactone of virescenoside G (**112**), and the aglycon of virescenoside A (**113**), were isolated from the fungus *Acremonium striatisporum* KMM 4401, associated with the sea cucumber *Eupentacta fraudatrix*, which was collected from Kitovoe Rebro Bay in the Sea of Japan [21,46,67–71]. Compounds **76**, **77**, **79**, and **107**–**109** showed cytotoxic effects on developing eggs of the sea urchin *Strongylocentrotus intermedius* (MIC50 = 2.7–20 μM) [21,67]. Compounds **76**–**81**, **85**–**87**, and **107**–**109**

exhibited cytotoxic activities against Ehrlich carcinoma tumor cells (IC50 = 10–100 μM) in vitro [21,67,68]. Compounds **81** and **85**–**87** showed weak cytotoxic effects on developing eggs of the sea urchin *S. intermedius* (IC50 = 100–150 μM) [68]. At a concentration of 100 mg/mL, compounds **82**–**84** and **91** inhibited esterase activity by 56%, 58%, 36%, and 40%, respectively [46]. The aglycon **113** inhibited urease activity, with an IC50 value of 138.8 μM [71]. Compounds **97**, **98**, **100**, **101**, **104**, and **110**–**113,** at 10 μM, downregulated reactive oxygen species (ROS) production in lipopolysaccharide (LPS)-stimulated macrophages [71]. At 1 μM, compounds **98** and **101** induced moderate downregulation of NO production in LPSstimulated macrophages [71].

#### *3.4. Other Types of Compounds Isolated from Sea-Cucumber-Associated Microorganisms*

Other secondary metabolites, including cyclo-(L-Pro-L-Phe) (**114**), cyclo-(L-Pro-L-Met) (**115**), cyclo-(L-Pro-L-Tyr) (**116**), cyclo-(L-Pro-L-Val) (**117**), cyclo-(L-Pro-L-Pro) (**118**), cyclo-(L-Val-L-Gly) (**119**), and cyclo-(L-Pro-L-Leu) (**120**), have been isolated from the actinomycete *Brevibacterium* sp., associated with the sea cucumber *A. japonicus* [23].

Four compounds, 5-methyl-6-hydroxy-8-methyoxy-3-methylisochroman (**121**), peroxyergosterol (**122**), succinic acid (**123**), and 8-hydroxy-3-methylisochroman-1-one (**124**), were isolated from the fungus *Epicoccum* spp., associated with sea cucumber collected in the Yellow Sea, China [43]. Compound **121** is a pheromone [43] that was also isolated from the fungus *Alternaria* sp., associated with the sea cucumber collected from the Yellow Sea in Weihai, China [27]. The fungus *Alternaria* sp., associated with sea cucumber, also produced a new benzofuran derivative, 4-acetyl-5-hydroxy-3,6,7-trimethylbenzofuran-2(3H)-one (**125**), and a known compound, 2-carboxy-3-(2-hydroxypropanyl) phenol (**126**) [27].

Two depsidones, emeguisin A (**127**) and aspergillusidone C (**128**), were isolated from the fungus *Phialemonium* sp., associated with the sea cucumber *H. nobilis*, collected in South China [38].

Three compounds, (+)-butyrolactone IV (**129**), butyrolactone I (**130**), and terrelactone A (**131**), were isolated from the fungus *Aspergillus terreus*, associated with the sea cucumber *A. japonicus*, collected from the Yellow Sea in China [47]. Compounds **129** and **130** showed moderate antiangiogenic activity when evaluated using a zebrafish assay. The inhibition ratio of compound **129,** at a concentration of 100 μg/mL, was 43.4% and that of compound **130,** at a concentration of 10 μg/mL, was 28.7% [47].

Nine known compounds, 2,4-dihydroxy-6-methylaceto-phenone (**132**), pannorin (**133**), 2-hydroxy-4-(3-hydroxy-5-methylphenoxy)-6-methylbenzoic acid (**134**), 3,3-dihydroxy-5,5-dimethyldiphenyl ether (**135**), aloesone (**136**), aloesol (**137**), acremolin (**138**), cyclo-(L-Trp-L-Phe) (**139**), and cyclo-(L-Trp-L-Leu) (**140**), were isolated from the fungus *Aspergillus* sp. S-3-75, associated with the sea cucumber *H. nobilis*, which was collected from the Antarctic [35].

Cerebroside (**141**) was isolated from the fungus *Alternaria* sp., associated with sea cucumber from the sea near Zhifu Island in Yantai, China [28].

Three known compounds, streptodepsipeptide P11B (**142**), streptodepsipeptide P11A (**143**), and valinomycin (**144**), and one novel valinomycin analogue, streptodepsipeptide SV21 (**145**), were produced by the actinobacteria *Streptomyces* sp. SV 21, isolated from the sea cucumber *S. vastus* in Lampung, Indonesia [72]. Compounds **142**–**145** exhibited antifungal activity against *Mucor hiemalis*, with MIC values of 16.6, 8.3, 2.1, and 16.6 μg/mL, respectively. These four compounds also exhibited antifungal activity against *Ruegeria glutinis,* with MIC values of 33.3, 8.3, 4.2, and 16.6 μg/mL, respectively. Compounds **144** and **145** showed activities against the Gram-positive bacterium *Staphylococcus aureus*, with MIC values of 4.2 and 16.6 μg/mL, respectively. Compound **145** showed activity against the Gram-positive bacterium *Bacillus subtilis*, with an MIC value of 33.3 μg/mL. Compounds **143**–**145** showed pronounced antiinfectivity effects against hepatitis C virus (HCV). Compound **142** showed weak antiinfectivity effects against HCV [72].

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**Figure 2.** *Cont.*


**Figure 2.** *Cont.*

**Figure 2.** *Cont.*

**Figure 2.** *Cont.*

**Figure 2.** *Cont.*

**Figure 2.** Chemical structures of the 145 compounds isolated from sea-cucumber-associated microorganisms.

#### *3.5. Summary of the Natural Products Isolated from Microorganisms Associated with Sea Cucumbers*

From 2000 to 2021, 145 natural products were isolated from microorganisms associated with sea cucumbers. The numbers of compounds isolated in 2008, 2014, and 2020 were significantly higher than the numbers isolated in other years (Figure 3). The compounds isolated from sea-cucumber-associated microorganisms are mainly polyketides, alkaloids, and terpenoids (Figures 4 and 5), which account for 28%, 18%, and 32% of the total isolated compounds, respectively (Figure 4). Most of these compounds were isolated from seacucumber-associated fungi (Figure 4), and many of them have demonstrated bioactivities, including cytotoxicity, antimicrobial, enzyme-inhibiting, antiviral, and antiangiogenic activities, and the downregulation of ROS and NO production (Figure 6).

**Figure 3.** Natural products isolated from sea-cucumber-associated microorganisms from 2000 to 2021.

**Figure 4.** Percentage distribution of the natural products isolated from sea-cucumber-associated microorganisms.

**Figure 5.** Natural products isolated from sea-cucumber-associated microorganisms.

**Figure 6.** Percentage distribution of the bioactivities of the natural products isolated from sea-cucumber-associated microorganisms.
