Role Of Symbiotic Microorganisms In The Biosynthesis Of Sponge
Marine plants and animals are well known to have developed highly specific relationships with numerous microorganisms (Jensen & Fenical 1994). Sponges, perhaps to a greater extent than any other marine invertebrates, harbor extraneous organisms on their surfaces, in their canal systems, and in the intercellular matrix which constitutes a large part of the body. The volume of bacteria in some species can reach 40% of the total cellular content (Vacelet & Donadey 1977; Wilkinson 1978). The question of the role of these microorganisms in the synthesis of compounds of biological interest is currently the subject of intensive research efforts. We have underlined the difficulties encountered in total synthesis of complicated metabolites such as halichondrins and spongistatins, and the possibility of finding cultivable microorganisms is of considerable interest for development of bioactive molecules. Many examples bolster this hypothesis; the first one was okadaic acid, first isolated from the sponge Halichondria okadai (Kadota, 1922) (Tachibana et al. 1981) as the main cytotoxic compound of the extract, and later demonstrated as the toxin responsible for the intoxications due to the marine dinoflagellates (Prorocentrum lima Dodge, 1975, Dinophysis fortii (Pavillard, 1916) concentrated in a variety of filter feeders. Okadaic acid (Fig. 10) is a selective inhibitor of alkaline phosphatase PPA2, allowing this product to be used as a probe for the study of basic cellular phosphorylation processes.
Okadaic acid, like manoalide is now commercially available for biochemical research. Other previously ascribed to invertebrates were later demonstrated to be biosynthesized by bacteria. Dysidea herbacea (Keller, 1889) is a common shallow-water sponge which has been extensively studied. All samples contain terpenoids, and either polychlorinated or polybrominated compounds, but not both (Unson et al. 1994). The dominant endosymbiont is the filamentous cyanobacterium Oscillatoria spongeliae (Schulze, 1892) Gomont, but heterotrophic bacteria are also present. The flow-cytometric separation of the symbionts from the sponge cells showed that chlorinated metabolites such as 13-demethylisodysidenin are located in the cyanobacterial filaments (Unson & Faulkner 1993). Another study revealed that the bromophenylether 1 also occurs exclusively in the Cyanobacteria (Fig. 11). Another example is swinholide. The two monomeric units of the dimeric lactone swinholide, a potent cytotoxic macrolide isolated from the sponge Theonella swinhoei Gray, 1868 (Kobayashi et al. 1989) are obviously similar to scytophycin C obtained from the Cyanobacteria Scytonema pseudohofmanni Bharadwaja, 1934 (Ishibashi et al. 1986).
This similarity has suggested the possibility that swinholide may be produced by a cyanobacteria (Fig. 12). The presence of filamentous bacteria, thought to be cyanobacteria by the authors, was reported for the Okinawan collection of T. swinhoei, and it was suggested that these microorganisms might produce swinholide and bioactive peptides (Kitagawa et al. 1990). However four distinct cell populations were found to be consistently present in T. swinhoei: sponge cells, heterotrophic bacteria, cyanobacteria and filamentous bacteria. Chemical analyses of each cell type showed the macrolide swinholide A to be limited to the heterotrophic bacteria and a cyclic peptide in the filamentous bacteria. No major metabolites were located in the cyanobacteria or in sponge cells (Bewley et al. 1996). Furthermore bacteria collected from sponges (Stierle and Stierle 1992; Shigemori et al. 1992; Imamura et al. 1993; Jayatilake et al. 1996), have
Guyot M. 2000. — Intricate aspects of sponge chemistry. Zoosystema 22 (2) : 419-431.
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