Gracilaria and its epiphytes: 4. The response of two Gracilaria species toUlva lactuca in a bacteria-limited environment

The responses of Gracilaria lemaneiformis, an easily epiphytized host, and the relatively resistant G. cornea mutant, to the green alga Ulva lactuca w ere studied using biculture experiments with and without antibiotics. Both Gracilaria species grown with and without U. lactuca showed different level s of growth rate, release of hydrogen peroxide and of halogenated hydrocarb ons. These quantitative differences led to a successful response against Ul va lactuca in the case of G. cornea mutant and to a failure in response in the case of G. lemaneiformis. The response of each Gracilaria species to U. lactuca was qualitatively similar to its response to bacteria. This sugges ts the involvement of oligosaccharide elicitors produced in the presence of epiphytes and bacteria. A clear Gracilaria inhibition was demonstrated wit h extracts of the culture medium. It appears that hydrogen peroxide, haloge nated hydrocarbons and oligosaccharides may be components of the inhibitory activity of the extracts. The responses of Gracilaria species to the prese nce of U. lactuca suggest the characterization of a defence response.