Volume 51, Issue 3 - Special Issue: Bioactive Compounds in Marine Organisms

We review the occurrence of fungal endophytes in marine organisms, in particular of seaweeds and mangrove plants, and compare them to obligate or marine-derived fungi. The ability of endophytes to produce a wide range of new or novel bioactive compounds is also discussed.

We examined the endophytic fungi associated with the dominant mangrove plant, Kandelia candel , in Mai Po Nature Reserve, Hong Kong. Bark, woody tissue and leaves from four mature healthy trees were sampled for fungal endophytes in April and June, 2001. A total of 880 isolates was obtained, 595 from the June sampling. The highest number of fungal endophytes was cultured from bark (674 isolates) with only 63 from leaves. Three taxa of ascomycetes, 18 of anamorphic fungi and 29 mycelial morphotypes were identified. These results indicate a degree of tissue recurrence with similar endophytic fungal assemblages in bark and wood, and a different assemblage in the leaf samples. Dominant sporulating fungi of K. candel were Phomopsis sp., Pestalotiopsis sp., Guignardia sp. and Xylaria sp., which are cosmopolitan and common endophytic species.

A diverse array of bioactive metabolites have been isolated and characterized from fungi isolated from countries surrounding the South China Sea. From extensive studies of marine fungi of the South China Sea, 25 fungal strains (comprising endophytes, obligate and marine-derived fungi) were screened for bioactive compounds. This resulted in the reporting of 42 new or novel compounds and 35 known compounds (data from 1999–2007). Their biological activities, structural and stereochemical assignments and syntheses of some structurally unique natural products are reported.

Antifouling chemical defense is likely an evolutionary response to the ecological disadvantages of epibiosis, particularly for photosynthetic organisms. Seaweed natural products with antifouling activity can provide effective, environmentally friendly alternatives to currently used antifouling paint booster biocides. The aim of this work was to assess the antifouling potential of natural products from Brazilian littoral seaweeds. Crude organic extracts from 51 populations comprising 42 species of macroalgae from eight locations along the Brazilian coast were tested against a relevant fouling organism in laboratory bioassays, the brown mussel Perna perna. In five cases, antifouling activities of purified compounds were also tested. A total of nine macroalgae were also cultured and tested for the presence of inducible defenses against fouling. Ecologically relevant field tests were performed in 11 cases to confirm laboratory results. Despite the unbalanced number of macroalgae tested among different localities, there seems to be no latitudinal trend of increased antifouling activity towards lower latitudes, where fouling pressure is presumed to be higher. However, there was a clear phylogenetic pattern in antifouling activity, with red macroalgae having the highest proportion (55%) of active species (moderate or strong fouling inhibition), followed by brown macroalgae (14%). Green seaweeds never exhibited strong antifouling activity (≥80% inhibition of byssal attachment relative to controls). Some degree of induced antifouling defense was observed in seven species (78%). These results appear to support known trends of secondary metabolite production among seaweeds and suggest that research efforts should be focused on tropical red macroalgae in the quest for new antifoulants. On the other hand, it seems clear that macroalgal groups, such as green algae, must have mechanisms of defenses against fouling that are not chemical.

Marine macroalgae are particularly prone to epiphytism. The colonisation of their surface may lead to severe perturbations and sometimes to the death of the host. To protect themselves against settling and/or competing organisms, many algae have developed defences against fouling by producing a wide variety of chemically active metabolites. The goal of this study was to investigate potential anti-microfouling activities of two introduced algal species from the Brittany coast (France), Sargassum muticum (Phaeophyceae) and Grateloupia turuturu (Rhodophyceae). Thus, a complete study of the antimicrobial activities of extracts (aqueous, acetone, chloroform, dichloromethane, diethyl-ether, ethyl acetate, ethanol 96°, hexane and methanol) from S. muticum and G. turuturu against marine fouling bacteria (5 strains), fungi (5 strains) and biofilm-associated microphytobenthic strains (5) were performed. The most active extract of G. turuturu was the dichloromethane fraction, while the chloroform extract from S. muticum showed some good results. Our data suggest that these macroalgae can provide useful compounds or templates for industrial anti-microfouling applications.

Specimens of Suberites domuncula that had been cultured in aquaria for 4 weeks were analyzed for their associated fungi. A total of 81 fungal strains belonging to 20 different genera was isolated and identified by morphological and molecular methods. The most frequently isolated taxa were Cladosporium spp., Penicillium spp., Petriella sp., Phialophora spp. and Engyodontium album . Based on chromatographic and mass spectrometric analysis of fungal extracts, as well as on bioassay results, Aspergillus ustus , Penicillium sp., Petriella sp. and Scopulariopsis sp. were selected for in-depth analysis of their natural products. A total of 19 different fungal metabolites, including three new natural products, was isolated and structurally identified. A. ustus yielded two sesquiterpenes, a drimane derivative and deoxyuvidin, as well as a sesterterpene ophiobolin H. The drimane derivative had an ED 50 value against L5178Y cells of 1.9 μg ml -1 in vitro . The crude extract of Petriella sp. was also strongly cytotoxic against the L5178Y cell line. The cyclic tetrapeptide WF-3161 was primarily responsible for the activity; the ED 50 value was <0.1 μg ml -1 . It was identical to the known compound WF-3161 and had been previously isolated from Petriella guttulata. In addition to WF-3161, three further natural products were obtained and unequivocally identified as new derivatives of infectopyrone by one- and two-dimensional NMR spectroscopy and by mass spectroscopy. Of the new compounds, only dihydroinfectopyrone was active against L5178Y cells; the ED 50 value was 0.2 μg ml -1 . Penicillium sp. yielded the largest number of metabolites. Viridicatin, viridicatol, cyclopenin and cyclopenol suppressed larval growth of the polyphagous pest insect Spodoptera littoralis when incorporated into an artificial diet at an arbitrarily chosen concentration of 237 ppm. Viridicatol was the most active compound and had an ED 50 value of ca. 50 ppm. Scopulariopsis sp. yielded three metabolites, including the known acetylcholinesterase inhibitors quinolactacin A1 and A2.

To determine the best sources of novel, biologically active metabolites, both endophytic fungi (plant isolates) and fungi associated with algae were isolated from plants and algae from marine habitats of the North, Baltic and Mediterranean Seas, Atlantic Ocean, and Gulf of Mexico. Following preselection of the isolates according to taxon and metabolic profiles, almost all were active in at least one of the tests for antibacterial, antifungal, and/or herbicidal activities. Metabolites isolated from the culture extracts belonged to diverse structural groups; 42% were previously unknown structures. Compared to fungi associated with algae, endophytic fungi were a better source of novel metabolites and antifungal culture extracts; they produced a higher number of metabolites per fungus. Microsphaeropsis spp. and Coniothyrium spp. synthesized the highest numbers of novel metabolites per isolate, and Geniculosporium , Nodulisporium and Phomopsis the greatest numbers of metabolites per isolate. Based on the proportion of novel to known metabolites, endophytic fungi from marine environments equalled endophytes from terrestrial habitats. Metabolic profiles (HPLC-DAD) of the saprophytic, marine fungi belonging to Dendryphiella spp. from diverse temperate and subtropical locations revealed that geographical source of the isolates had little qualitative effect on secondary metabolite production in this genus.