Chemo-ecological studies on hexactinellid sponges from the Southern Ocean.

Hexactinellids (glass sponges) are an understudied class with syncytial organization and poor procariotic associations, thought to lack defensive secondary metabolites. Poriferans, though, are outstanding sources of bioactive compounds; nonetheless, a growing suspicion suggests that many of these chemicals could be symbiont-derived. In Polar latitudes, sponges are readily invaded by diatoms, which could provide natural products. Hexactinellids are typical of deep waters; but in Antarctica, they dominate the upper shelf providing shelter and food supply to many opportunistic mesograzers and macroinvertebrates, which exert strong ecological pressures on them. Aiming to examine the incidence of defensive activities of hexactinellids against consumption, feeding experiments were conducted using their lipophilic fractions. Antarctic hexactinellid and demosponge extracts were tested against the asteroid Odontaster validus and the amphipod Cheirimedon femoratus as putative sympatric, omnivorous consumers. Hexactinellids yielded greater unpalatable activities towards the amphipod, while no apparent allocation of lipophilic defenses was noted. After chemical analyses on the lipophilic fractions from these Antarctic glass sponges, quite similar profiles were revealed, and no peculiar secondary metabolites, comparable to those characterizing other poriferans, were found. Instead, the lipidic compounds 5α(H)-cholestan-3-one and two glycoceramides were isolated for their particular outspread presence in our samples. The isolated compounds were further assessed in asteroid feeding assays, and their occurrence was evaluated for chemotaxonomical purposes in all the Antarctic samples as well as in glass sponges from other latitudes by NMR and MS. Characteristic sphingolipids are proposed as chemical markers in Hexactinellida, with possible contributions to the classification of this unsettled class.

Factors promoting marine invasions: a chemoecological approach.

The Mediterranean Sea is losing its biological distinctiveness, and the same phenomenon is occurring in other seas. It gives urgency to a better understanding of the factors that affect marine biological invasions. A chemoecological approach is proposed here to define biotic conditions that promote biological invasions in terms of enemy escape and resource opportunities. Research has focused on the secondary metabolite composition of three exotic sea slugs found in Greece that have most probably entered the Mediterranean basin by Lessepsian migration, an exchange that contributes significantly to Mediterranean biodiversity. We have found toxic compounds with significant activity as feeding deterrents both in the cephalaspidean Haminoea cyanomarginata and in the nudibranch Melibe viridis. These findings led us to propose aposematism in the former and dietary autonomy in producing defensive metabolites in the latter case, as predisposing factors to the migration. In the third mollusk investigated, the anaspidean Syphonota geographica, the topic of marine invasions has been approached through a study of its feeding biology. The identification of the same compounds from both the viscera of each individual, separately analyzed, and their food, the seagrass Halophila stipulacea, implies a dietary dependency. The survival of S. geographica in the Mediterranean seems to be related to the presence of H. stipulacea. The initial invasion of this exotic pest would seem to have paved the way for the subsequent invasion of a trophic specialist that takes advantage of niche opportunities.

A new cytotoxic tambjamine alkaloid from the Azorean nudibranch Tambja ceutae.

The chemical investigation of Azorean nudibranch mollusk Tambja ceutae led us to isolate a new member of the tambjamine family, tambjamine K (1). The bryozoan Bugula dentata, prey of the nudibranch, was also analyzed and found to contain compound 1 in very small amounts together with known blue pigment 2 and tambjamines A (3) and B (4). The structure of tambjamine 1 was elucidated by the interpretation of the spectroscopic data as well as by the comparison with related compounds. Compounds 1 and 2 possess antiproliferative activity, in particular, tambjamine K (1) displayed high cytotoxicity against both tumor and non-tumor mammalian cells.

Rare casbane diterpenoids from the Hainan soft coral Sinularia depressa.

A series of nine casbane diterpenes, compounds 5-13, exhibiting either cis or trans ring junctions were isolated from the Hainan soft coral Sinularia depressa. The structures of this group of compounds, the basic member of which was named depressin (5), were established by detailed spectroscopic analysis. In addition, the absolute configuration of the main metabolite, 10-hydroxydepressin (7), and of its epimer, 1-epi-10-hydroxydepressin (8), was determined by a combination of conformational analysis and the modified Mosher's method. A stereochemical relationship between all isolated molecules was investigated by analyzing their circular dichroism profiles. Antiproliferative and antibacterial activities of the depressins were also evaluated.

Illudalane sesquiterpenoids of the alcyopterosin series from the Antarctic marine soft coral Alcyonium grandis.

Chemical investigation of the lipophilic extract of the Antarctic soft coral Alcyonium grandis led us to the finding of nine unreported sesquiterpenoids, 2-10. These molecules are members of the illudalane class and in particular belong to the group of alcyopterosins, illudalanes isolated from marine organisms. The structures of 2-10 were determined by interpretation of spectroscopic data. Repellency experiments conducted using the omnivorous Antarctic sea star Odontaster validus revealed a strong activity in the lipophilic extract of A. grandis against predation.

Unusual C21 linear polyacetylenic alcohols from an Atlantic ascidian.

Four novel straight-chain polyacetylenic alcohols were isolated from a marine ascidian (Phylum Chordata, subphyllum Urochordata) collected off Vigo, along the Atlantic coast of northwestern Spain. The chemical structures, which exhibit an uncommon dienyne group, were characterized by spectroscopic methods, mainly mono- and bi-dimensional NMR. This is the first finding of acetylenic lipids from an organism belonging to phylum Chordata.

Occurrence of a taurine derivative in an antarctic glass sponge.

The n-butanol extract of an Antarctic hexactinellid sponge, Anoxycalyx (Scolymastra) joubini, was found to contain a taurine-conjugated anthranilic acid, never reported so far either as a natural product or by synthesis. The compound was inactive against human cancer cells in an in vitro growth inhibitory test, and also showed no antibacterial activity.

Structure and cytotoxicity of phidianidines A and B: first finding of 1,2,4-oxadiazole system in a marine natural product.

Two indole alkaloids, phidianidines A (1) and B (2), exhibiting an uncommon 1,2,4-oxadiazole ring linked to the indole system, have been isolated from the marine opisthobranch mollusk Phidiana militaris. The structures of the two metabolites have been elucidated by spectroscopic techniques. Phidianidines exhibit high cytotoxicity against tumor and nontumor mammalian cell lines in in vitro assays.

Sesquiterpene metabolites of the antarctic gorgonian Dasystenella acanthina.

The ethereal extract of the Antarctic gorgonian Dasystenella acanthina was found to contain three main nonpolar and relatively transient sesquiterpene metabolites (1-3), which were isolated and submitted to spectral analysis. Two of them were identified as the previously reported compounds trans-beta-farnesene (1) and isofuranodiene (3), whereas the third metabolite, the furanoeudesmane 2, was unknown. The structure elucidation of this new sesquiterpene was solved mainly on the basis of its spectral properties and correlation with known compounds.