Defense induction in marine macroalgae: Its prevalence, capabilities and limitations

Herbivory is an important biotic factor affecting algal biomass, abundance, and distribution. Especially in marine habitats, herbivores often consume 50 -100% of the macroalgal production. Algae are not only passive player in this interac-tion but have evolved a variety of strategies to cope with herbivory. One strat-egy is the production of chemical metabolites that function as defense against consumers. These metabolites can either be continuously present at bioactive concentrations (constitutive defense), or be produced ‘on demand’, i.e. when appropriate cues indicate presence of consumers (inducible defense). Ecologi-cal theory postulates that inducible defense should be favoured when the de-fense incurs costs to algal fitness, when the presence of herbivores is variable or when increased chemical variability enhances its effectiveness. In vascular plants, the occurrence and mechanisms of inducible defense are well documented. However, the prevalence of defense induction, the cues triggering in-duction, the interactions with other factors like stress, and the temporal dynam-ics of defense induction in macroalgae are generally not well understood. This thesis investigated the prevalence of defense induction and the nature of the inducing cues in the macroalgal species Fucus serratus, Fucus evanescens (Phaeophyceae), Delesseria sanguinea, Phyllophora pseudoceranoides, and Furcellaria lumbricalis (Rhodophyta) (Chapter I). These species are widely dis-tributed perennial seaweeds of the western Baltic and represent important habi-tats for many associated species. Also, the efficiency of two proposed induction cues, direct grazing and waterborne cues, were investigated. All tested red algae, D. sanguinea, P. pseudoceranoides and F. lumbricalis, in-duced defense in response to direct grazing. The brown algae F. evanescens and F. serratus also showed induced effects but these were not always signifi-cant in different types of feeding assays. This study supports recent findings that defense regulation in marine ...