Structures And Activities Of Secondary Metabolites From Four Marine Algicolous Fungi

Marine algicolous fungi mainly refer to filamentous fungi which reside in the inner tissues as endophytes or on the surfaces as epiphytes of marine algae.This thesis reviewed the types of new compounds produced by the metabolism of marine algicolous fungi from 2015-2021,among which terpenoids and steroids accounted for the major proportion of 53%,with terpenoids accounted for the largest proportion,followed by polyketides accounting for 29%,thus showing that terpenoids and polyketides are the major secondary metabolites of marine algicolous fungi.Moreover some of the products have significant biological activity and have the potential to be developed into novel pharmaceuticals and pesticides.Therefore,it is important to investigate their chemical structures and biological activities.A total of 119 strains of algicolous fungi were isolated from Rhodomelaceae algae,their biological activities were screened.Based on the screening of biological activities,Trichoderma citrinoviride A-WH-20-3,Trichoderma atroviride RR-dl-3-9,Byssochlamys spectabilis RR-dl-2-13,and Fusarium fujikuroi RL-ylw-2-8 were selected as the subjects for the study of secondary metabolites.The separation and purification of secondary metabolites and their structural identification are performed by a combination of various modern chromatographic techniques such as column chromatography,HPLC,and PTLC,and modern spectroscopic techniques,such as NMR,IR,UV,MS,ECD and X-ray.In total,122 compounds were isolated and identified,including 23 new compounds,which contained steroids,terpenoids,polyketides,diketopiperazines,lipids,and alkaloids.Efforts on T.citrinoviride A-WH-20-3 resulted in the isolation and identification of 26 compounds,including two new cyclonerane sesquiterpenes(TC1 and TC2),one new isocoumarin derivative(TC3)and one new triol(TC4).TC3 is the first 3-phenylisocoumarin derivative from Trichoderma.39 compounds were characterized from T.atroviride RR-dl-3-9,including two new bisabolane derivatives(TA1 and TA2),one new steroid derivative(TA3),and two new lauric acid derivative(TA4 and TA5).B.spectabilis RR-dl-2-13 gave 32 compounds,including four new carotane sesquiterpenoids(BS1-BS4),two new nor-sesquiterpenoids(BS5 and BS6),and five new polyketides(BS7,BS8-BS10b).The 2,15 epoxide in the structures of compounds BS1-BS4 is relatively rare in carotene sesquiterpenes,BS7 is a structurally unique furan fatty acid derivative that possesses an aldehyde group and a large conjugated unit,and BS8 features a hemiketal group.Additionally,26 compounds including three new lauric acid derivative(FF1-FF3)were obtained from the culture of F.fujikuroi RL-ylw-2-8.The strains of T.citrinoviride A-WH-20-3 and F.fujikuroi RL-ylw-2-8 were selected for co-culture screening.The composition of secondary metabolites under co-culture conditions were determined by HPLC analysis and bioactivity analysis.The secondary metabolites were isolated by repeated column chromatography and the structures were identified by spectroscopic methods A total of 20 compounds were isolated from the co-cultured strains.Compared with the individual cultures,the structural types of the compounds were observed to be different,and new diterpenes and sesterterpenes were synthesized,thus confirming that the co-culture conditions can initiate the microbial "silent" genes to generate new natural products.Comprehensive evaluation of biological activity showed that the isocoumarin derivative TC3 showed significant inhibitory effects against Chattonella marina,Heterosigma akashiwo,and Prorocentrum donghaiense with MIC values of 4.4-9.1μg/m L,and low toxicity to Artemia salina.The lauric acid derivative TA4 and TA5 showed inhibition of the four microalgae and four Vibrio bacteria.The polyketides BS7 and BS8 showed inhibitory activity against V.harveyi and V.parahaemolyticus with MIC values of 13-25 μg/m L.Additionally,compound BS8 also showing inhibitory activity against phytopathogenic fungi with MIC values of 13-50 μg/m L.The evaluation of the activity of the crude extracts and the compounds under co-culture conditions showed that co-culture metabolism produced more anti-microalgal and antibacterial active substances compared to the individual culture.In this thesis,the composition of secondary metabolites of four strains of marine algicolous fungi were analyzed to identify bioactive molecules with anti-microalgae,anti-pathogenic bacteria and low toxicity.The co-culture study on the secondary metabolites and their activities confirmed that the co-culture conditions can activate the "silent" genes of microorganisms to promote the production of more active secondary metabolites,which provides a scientific basis for the large-scale production of active natural products and the biotransformation of drug lead molecules.