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 macroalgae is a rather common phenomenon, especially in the Baltic Sea.My further experiments focused on the bladderwrack Fucus vesiculosus. Its populations in the Baltic have declined strongly from the 1970s. This decline has been attributed to eutrophication leading to light limited conditions and associated changes in competitive interactions and grazing pressure. However, these statements are based on observational data without support of experi-mental evidence. This study demonstrated that F. vesiculosus in the Kiel Fjord is already light limited in 2 m water depth and its lethal depth limit lies between 4 - 6 m depth (Chapter II). Epibionts enhance the negative effect of reduced light conditions. The reduced depth distribution and consequently the massive biomass loss of the Fucus-population in the Baltic may be enhanced by the det-rimental effects of herbivores which stresses the ecological importance of algal resistance to consumption. Algal physiology and alga-herbivore interactions can be affected by abiotic factors. However, the interactions of environmental stress and defense, in particu-lar defense induction, have not been assessed so far. Climatic models predict a water temperature increase of ca. 4°C for the Baltic. This would lead to water temperatures comparable to those at Fucus’ southern distribution limit. They can, thus, be expected to represent a physiological stress for Fucus. To assess the interactive effects of reduced light and increased temperatures on the de-fensive performance of F. vesiculosus, I conducted a two-factorial experiment, which showed that reduced light conditions reduced growth rates significantly but had no effect on inducible defense at ambient temperatures (15°C), and that the predicted summer warming suppressed induced defensive responses completely. This study is one of the first that shows that abiotic stress can affect alga-herbivore interactions and therefore indirectly macroalgal population dy-namics. The coevolution of algal defense and herbivore tolerance of the defenses can result in high species specificities of the interactions. Temporal dynamics of induction and relaxation of defense may therefore be tuned the typical activity patterns of locally important herbivore species. This project identified the tempo-ral dynamics of antifeeding defense regulation in a marine macroalga, F. vesiculosus in response to its main consumer, the isopod Idotea baltica. Time lag of induction and relaxation of induced feeding resistance were assessed. F.vesiculosus induced defense 10 days after the onset of grazing by the isopod Idotea baltica. Defenses were relaxed within 2 - 4 days after cessation of grazing. Thus, defense seems to be deployed sparingly and only induced in longer periods of high grazing pressure to avoid substantial loss of tissue. This thesis gives new insights in the prevalence and the variability of induced resistance and its interactions with other biotic and abiotic factors, a study field which this thesis has started to examine. Also, I present a possible scenario what effects the predicted climate changes may have on the interactions of F. vesiculosus and its consumer I. baltica and how these changes may consequently affect the Fucus-populations in the Baltic.