Natural Products from Antarctic Colonial Ascidians of the Genera Aplidium and Synoicum: Variability and Defensive Role

Abstract

Ascidians have developed multiple defensive strategies mostly related to physical, nutritional or chemical properties of the tunic. One of such is chemical defense based on secondary metabolites. We analyzed a series of colonial Antarctic ascidians from deep-water collections belonging to the genera Aplidium and Synoicum to evaluate the incidence of organic deterrents and their variability. The ether fractions from 15 samples including specimens of the species A. falklandicum, A. fuegiense, A. meridianum, A. millari and S. adareanum were subjected to feeding assays towards two relevant sympatric predators: the starfish Odontaster validus, and the amphipod Cheirimedon femoratus. All samples revealed repellency. Nonetheless, some colonies concentrated defensive chemicals in internal body-regions rather than in the tunic. Four ascidian-derived meroterpenoids, rossinones B and the three derivatives 2,3-epoxy-rossinone B, 3-epi-rossinone B, 5,6-epoxy-rossinone B, and the indole alkaloids meridianins A–G, along with other minoritary meridianin compounds were isolated from several samples. Some purified metabolites were tested in feeding assays exhibiting potent unpalatabilities, thus revealing their role in predation avoidance. Ascidian extracts and purified compound-fractions were further assessed in antibacterial tests against a marine Antarctic bacterium. Only the meridianins showed inhibition activity, demonstrating a multifunctional defensive role. According to their occurrence in nature and within our colonial specimens, the possible origin of both types of metabolites is discussed.

Secondary Metabolites from Polar Organisms

“…The rossinones exhibited antiinflammatory, antiviral and antiproliferative activities [137]. In additon, rossinone B ( 211 ) was proven to take part in the whole-colony chemical defense of Aplidium falklandicum , repelling both sea stars and amphipods [138]. The following year, a biomimetic total synthesis of (±)-rossinone B was achieved through a highly efficient strategy as shown in Scheme 9 [139].…”

Mass spectrometry detection of minor new meridianins from the antarctic colonial ascidiansAplidium falklandicumandAplidium meridianum

“…The results of previous genetic, morphological and chemical studies seem to indicate that A. meridianum and A. falklandicum could be considered as two morphotypes of the species. [17,18] The secondary metabolite profile of Antarctic colonial ascidians of the genus Aplidium brought to light the existence of several unreported molecular formulae related to the meridianin family. [17] In order to unravel the structures of these new analogs, crude organic extracts obtained from Aplidium falklandicum and A. meridianum samples were meticulously analyzed.…”

XAS spectroscopy, sulfur, and the brew within blood cells from Ascidia ceratodes

“…As noted in the Introduction, ascidians are known to deploy an extraordinary array of secondary metabolites, possibly organized for chemical defense [40–42]. This array includes polysulfides, sulfurous alkaloids, and cyclic post-translationally-modified sulfur-containing polypeptides.…”