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

• Main Authors: Conxita Avila, Margherita Gavagnin, María Mercedes Varela, Jaime Rodríguez, Rosa María Nieto, Jennifer Vázquez, Marianna Carbone, Laura Núñez-Pons
• Format: Article
• Language: English
• Published: MDPI AG 2012-08-01
• Series: Marine Drugs
• Subjects: Antarctic colonial tunicates; deterrent activity; sea star <em>Odontaster</em> <em>validus</em> amphipod <em>Cheirimedon</em> <em>femoratus</em> antibacterial activity
• Online Access: http://www.mdpi.com/1660-3397/10/8/1741

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 <em>Aplidium</em> and <em>Synoicum</em> to evaluate the incidence of organic deterrents and their variability. The ether fractions from 15 samples including specimens of the species <em>A.</em> <em>falklandicum</em>, <em>A.</em> <em>fuegiense</em>, <em>A.</em> <em>meridianum</em>, <em>A.</em> <em>millari</em> and <em>S.</em> <em>adareanum</em> were subjected to feeding assays towards two relevant sympatric predators: the starfish <em>Odontaster</em> <em>validus</em>, and the amphipod <em>Cheirimedon</em> <em>femoratus</em>. 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.