Appelhans, Yasmin (2007) The influence of environmental stress on the fitness of Brazilian macroalgae. (Diploma thesis), Christian-Albrechts-Universität, Kiel, 81 pp.

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Abstract

Grazing and fouling may have a large influence on the survival and distribution of macroalgae. It is therefore essential for seaweeds to find ways to tolerate, avoid or defend against predators and epibionts. Many seaweed species are known to be chemically defended against these threats. The production of chemical components working as defendants, known as secondary metabolites, may be influenced by environmental stress, such as limiting amounts of nutrients and light, and are generally thought to be costly to produce. It is assumed that when energy supply is low, less energy is invested in defence in order to keep constant the amount of energy invested in other life processes, such as growth or reproduction. In this study the level of seaweed chemical defence under a stress gradient was tested indirectly, by measuring the attractiveness of five macroalga species (Codium decorticatum, Osmundaria obtusiloba, Pterocladiella capillacea, Sargassum vulgare and Stypopodium zonale) collected at the Brazilian coast to grazing and fouling organisms when exposed to low-light stress. The species used to test for attractiveness to grazers was the amphipod community dominated by Elasmopus brasiliensis. The attractiveness to macrofouling organisms was tested using the mytilid Perna perna, the attractiveness to microfoulers using seawater with natural concentrations of microorganisms. The seaweeds were kept at six different light levels for the duration of two weeks. By observing the reaction of seaweed defence to low-light stress, and with that at low energy supplies, it was thought to be able to estimate whether chemical defence is costly for macroalgae. None of the investigated species showed the expected linear increase of attractiveness with increasing stress level, but instead a variety of patterns in response to low-light levels were observed. This study therefore found low support for the fact that chemical defence is costly for seaweeds.

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