Structures And Activities Of Secondary Metabolites From Four Marine Algicolous Trichoderma Strains

Fungi of the genus Trichoderma have been regarded as the most potential biocontrol agents in agriculture,and hundreds of secondary metabolites with various bioactivities have been discovered from them so far.Although Trichoderma is commonly considered as a terrestrial genus,halotolerant strains have been continuously reported from marine environments.Chemical investigations toward them have been performed since the 1990s,which have attracted great attention for natural product research recently.Marine algicolous Trichoderma spp.are the filamentous fungi inhabiting or associated with marine algae,and they possibly feature unique pathways and structures of secondary metabolites due to their special habitats.To date,only 10 marine algicolous Trichoderma strains have been chemically surveyed.It is meaningful to further explore the structures and bioactivities of secondary metabolites from marine algicolous Trichoderma fungi.Through chemical and biological screening of 130 marine algicolous Trichoderma isolates,four strains including Trichoderma asperellum cf44-2 and A-YMD-9-2 and Trichoderma harzianum A-YMD-2-2 and X-5 were selected as the objects.Their secondary metabolites were isolated by repeated column chromatography on silica gel,RP-18,and Sephadex LH-20 and preparative thin layer chromatography as well as semipreparative HPLC.Their structures were identified by spectroscopic methods,including 1D/2D NMR,IR,UV,ECD,X-ray,and MS.Effects on T.asperellum cf44-2resulted in the isolation and identification of 33 compounds,with sixteen?A1–A16?including one highly transformed ergosterol derivative?A1?with a new carbon skeleton,one ethylated bisobolane diterpene?A2?,two aminoglycosides of bisabolane sesquiterpenes?A3 and A4?,one bisobolane sesquiterpene?A5?,one norbisobolane sesquiterpene?A6?,three cyclonerane sesquiterpenes?A7–A9?,one harziane diterpene?A10?,two naturally occurring monoterpenes?A11 and A12?,one naturally occurring trichodenone?A13?,one chlorine-containing trichodenone?A14?,one diketopiperazine?A15?,and one naturally occurring oxazole derivative?A16?being new ones.Among them,compounds A2–A4 are new structural types.42 compounds including 30 new ones?B1–B30?were characterized from T.asperellum A-YMD-9-2.The new isolates comprise ten nitrogenous cyclonerane derivatives?B1–B10?,seven cyclonerane sesquiterpenes?B11–B17?,four norbisabolane sesquiterpenes?B18–B21?,three norbisabolane aminoglycosides?B22–B24?,two cadinane sesquiterpenes?B25 and B26?,one harziane diterpene?B27?,one diketopiperazine?B28?,and two peptaibols?B29 and B30?.Of those,compounds B1 and B2 featuring a cyclonerane backbone represent an unprecedented family of siderophores.On the other hand,21compounds including one new cyclonerane sesquiterpene?C1?were obtained from the culture of T.harzianum A-YMD-2-2.Additionally,T.harzianum X-5 gave 36compounds,including seven new ones,i.e.harziane diterpene D1,proharziane diterpene D2,cyclonerane sesquiterpenes D3–D5,acorane sesquiterpene D6,and sorbicillin derivative D7.The discovery of D2 verifies the biogenetic pathways of harziane diterpenes.Some isolates were evaluated for antimicroalgal activity against Chattonella marina,Heterosigma akashiwo,Karlodinium veneficum,and Prorocentrum donghaiense and antimicrobial activity mainly against Vibrio parahaemolyticus,V.anguillarum,V.harveyi,V.splendidus,Pseudoalteromonas citrea,and Glomerella cingulata as well as brine shrimp lethality against Artemia salina.The results showed that A1 could remarkably inhibit the four phytoplankton species,and the IC₅₀ values ranged from 0.27 to 0.59?g/mL.Its antifungal activity against the phytopathogenic Glomerella cingulata was also assayed,and the MIC value was determined to be 12?g/mL.Compared to nitrogenous cycloneranes B1 and B2,their ferric complexes exhibit high inhibition of the four phytoplankton species tested(IC₅₀,0.99–10?g/mL).Aminoglycosides A3,A4,and B22–B24 showed potent inhibition of the four microalgae(IC₅₀ 3.7–24.8?g/mL)and four Vibrio bacteria?6.1–7.5 mm zones?.Norbisabolanes B19–B21 with IC₅₀ values ranging from 0.93 to 9.2?g/mL were more active to the four phytoplankton species than B18 and B22–B24,which might correlate with the presence of the phenyl group.Additionally,harziane diterpene B27and proharziane diterpene D2 showed potent inhibition of the four phytoplankton species with IC₅₀ values 1.2–7.7?g/mL.Trichodenones A13 and A14 exhibited growth inhibition of the four Vibrio bacteria,and the latter that contains a chlorine atom was more active?6.5–8.5 mm zones?.The former could inhibit the four phytoplankton species tested,with IC₅₀ values ranging from 4.2 to 8.5?g/mL.In this dissertation,studies on the structures and bioactivities of secondary metabolites from four marine algicolous Trichoderma strains are described.Totally,132 secondary metabolites including 54 new ones were isolated and identified,comprising steroids,diterpenes,sesquiterpenes,monoterpenes,polyketides,diketopiperazines,peptaibols,and alkaloids.Some of them were evidenced to feature antimicroalgal and antimicrobial activities.The discovery of these new bioactive compounds greatly added to the structural diversity of secondary metabolites from marine algicolous Trichoderma,and it will greatly contribute to the development of utilization of marine algicolous Trichoderma in controlling harmful algal blooms and pathogenic microbes.