*3.2. Alkaloids*

Alkaloids have been identified as a class of nitrogenous organic compounds derived from plants [55,56]; although they are most commonly found in plants, alkaloids can also be isolated from marine organisms and marine microorganisms [57,58].

Chaetoglobosins, which are a large class of secondary metabolites that are cytochalasin alkaloids, have been isolated mainly from the fungus *Chaetomium globosum* [59]. Three previously known chaetoglobosins, chaetoglobosin Fex (**41**), G (**42**), and B (**43**), and one new chaetoglobosin, cytoglobosin X (**44**), were isolated from the fungus *Chaetomium globosum*, associated with the sea cucumber *A. japonicus*, on Chengshantou Island, China [29]. Compound **43** has some inhibitory effects against *Staphylococcus aureus* and methicillin-resistant *Staphylococcus aureus* (MRSA), with MIC values of 47.3 and 94.6 μM, respectively, and weak activity against *Candida albicans* SC5314, *Candida albicans* 17#, *Pseudomonas aeruginosa*, and *Bacillus Calmette–Guérin* (BCG), with MIC values >100 μg/mL for all organisms [29].

Nineteen compounds were isolated from the fungus *Aspergillus fumigatus*, associated with the sea cucumber *S*. *japonicus*, collected near Lingshan Island, Qingdao, China [33]. Among these 19 compounds are seven new prenylated indole diketopiperazine alkaloids, including compound **45**, three spirotryprostatins (C–E) (**46**–**48**), two derivatives of fumitremorgin B (**49** and **50**), and 13-oxoverruculogen (**51**), along with 12 known compounds, including spirotryprostatin A (**52**), 13-oxofumitremorgin B (**53**), fumitremorgin B (**54**), verruculogen (**55**), 3-β hydroxy cyclo-L-tryptophyl-L-proline (**56**), cyclo-L-tryptophyl-Lproline (**57**), tryprostatin B (**58**), tryprostatin A (**59**), N-prenyl-cyclo-L-tryptophyl-L-proline (**60**), fumitremorgin C (**61**), 12,13-dihydroxyfumitremorgin C (**62**), and cyclotryprostatin A (**63**) [41]. Compound **45** showed weak cytotoxicity in HL–60 cells, with an IC50 value of 125.3 μM. Compounds **46**–**51** exhibited some cytotoxicity in MOLT–4 cells, HL–60 cells, A–549 cells, and BEL-7402 cells. Compound **48** showed higher activity in MOLT–4 and A–549 cells than the others, with an IC50 value of 3.1 μM for both cell types. Compound **49** showed higher activity in BEL–7402 cells than the others, with an IC50 value of 7.0 μM. Compound **51** showed higher activity in HL–60 cells than the others, with an IC50 value of 1.9 μM [41]. Compound **53** was also isolated from the fungus *Aspergillus* sp., associated with the sea cucumber *S. japonicus*, collected from Lingshan Island, Qingdao, China [42]. Two new compounds, pseurotin A1 (**64**) and A2 (**65**), as well as pseurotin A (**66**) were also isolated from the fungus *Aspergillus fumigatus*, associated with the sea cucumber *S. japonicus*. Compound **65** exhibited slight cytotoxicity in A549 and HL-60 cells, with IC50 values of 48.0 and 70.8 μmol/L, respectively, and compound **66** showed slight cytotoxicity in HL-60 cells, with an IC50 value of 67.0 μmol/L [60].
