Phenolic contents of two brown algae, Turbinaria ornata and Sargassum mangarevense on Tahiti (French Polynesia): interspecific, ontogenic and spatio-temporal variations

To elucidate the possible implications of phenolic content in survival strategies of two tropical brown fucaleans, Turbinaria ornata and Sargassum mangarevense, we determined total phenolic contents in relation to ontogenic stages, and then followed spatio-temporal variations. Samples were collected at different sites and seasons. Both species exhibited low phenolic contents with, however, some differences: levels were higher in the algae from grazer- and nutrient-rich sites, and during the austral summer. Moreover, adults produced more phenolic compounds than immature stages. In addition, Turbinaria ornata had higher levels than Sargassum mangarevense. Parent thalli may exert an indirect protection of easily-grazed recruits which settle in close vicinity. The differences between species in protection of recruits would, therefore, derive from the respective species morphologies. A mechanical protection can be hypothesized in S. mangarevense because of its soft flexible thallus: plants can sweep around their base and, therefore, would need to produce less phenolics as a repellent than Turbinaria. On the other hand, due to its relatively tough texture, together with its upright-thallus, Turbinaria has a greater need to protect recruits by chemicals. The low phenolic content observed from thalli collected at the outer barrier reef may be explained by an exudation of phenolics after dessication.