Resorting to a One Strain Many Compounds (OSMAC) approach, the marine Streptomyces sp. BRB081 strain was grown in six different media settings over 1, 2, 3 or 7 days. Extractions of mycelium and broth were conducted separately for each media and cultivation period by sonication using methanol/acetone 1:1 and agitation with ethyl acetate, respectively. All methanol/acetone and ethyl acetate crude extracts were analysed by HPLC-MS/MS and data treatment was performed through GNPS platform using MZmine 2 software. In parallel, the genome was sequenced, assembled and mined to search for biosynthetic gene clusters (BGC) of secondary metabolites using the AntiSMASH 5.0 software. Spectral library search tool allowed the annotation of desferrioxamines, fatty acid amides, diketopiperazines, xanthurenic acid and, remarkably, the cyclic octapeptides surugamides. Molecular network analysis allowed the observation of the surugamides cluster, where surugamide A and the protonated molecule corresponding to the B-E isomers, as well as two potentially new analogues, were detected. Data treatment through MZmine 2 software allowed to distinguish that the largest amount of surugamides was obtained by cultivating BRB081 in SCB medium during 7 days and extraction of culture broth. Using the same data treatment, a chemical barcode was created for easy visualization and comparison of the metabolites produced overtime in all media. By genome mining of BRB081 four regions of biosynthetic gene clusters of secondary metabolites were detected supporting the metabolic data. Cytotoxic evaluation of all crude extracts using MTT assay revealed the highest bioactivity was also observed for extracts obtained in the optimal conditions as those for surugamides production, suggesting these to be the main active compounds herein. This method allowed the identification of compounds in the crude extracts and guided the selection of best conditions for production of bioactive compounds.

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Metabolomic study of marine Streptomyces sp.: Secondary metabolites and the production of potential anticancer compounds

The Mauritian waters harbour micro-organisms that have successfully adapted to their environment; however, the mycobiota associated with algae and sponges have been poorly studied and are awaiting to be identified and exploited. This study focusses on the fungi associated with the sponge Haliclona sp. and two brown algae Turbinaria conoides and Sargassum portierianum from Mauritius. Samples of Haliclona sp., Turbinaria conoides and Sargassum portierianum were collected in the North of Mauritius, and associated fungi were isolated after surface sterilization and plating on artificial media supplemented with seawater. A total of 17 morphologically different isolates were recovered from Haliclona sp., 20 isolates were obtained from Turbinaria conoides, and 16 isolates were from Sargassum portierianum. The selected sponge-associated fungi have been identified as Acremonium sp., Periconia byssoides, Pestalotiopsis sp. Endophytes from Turbinaria conoides were identified as Aspergillus penicilloides, Acremonium sp. and Cordyceps memorabilis, whereas those from Sargassum portierianum were identified as Pseudopithomyces maydicus, Acremonium sclerotigenum and Curvularia lunata. The ITS regions were suitable for identifying fungi up to the genus level. However, species level identification was problematic and should be used with caution.

Molecular characterization of marine fungi associated with Haliclona sp. (sponge) and Turbinaria conoides and Sargassum portierianum (brown algae)

The marine environment is endowed with novel and rich sources of structural and functional metabolites that have prospective biomedical applications. Several marine plants, algae, and invertebrates are known to have a huge number of fungal endophytes. Until now, only a few marine-derived fungal endophytes have been identified as producers of bioactive secondary metabolites compared to the inhabitants of higher plants. These fungal endophytes were recently documented to produce a plethora of signature molecules for exploitation in industry, agriculture, and medicine. Endophytic fungi associated with marine hosts are acquiring increasing importance as promising sources of structural and biological natural products. This review presents knowledge on secondary metabolites isolated from algae in marine habitats, coral and sponge-associated endophytic fungi with a special emphasis on secondary metabolite production. The data presented in this review presents knowledge on secondary metabolites isolated from algae in marine habitats, coral and sponge-associated endophytic fungi.

T. Raja Komaraiah

Papers

Secondary Metabolites from Marine Endophytic Fungi: Emphasis on Recent Advances in Natural Product Research