THE ECOLOGY OF BACTERIALLY MEDIATED BLEACHING IN A CHEMICALLY DEFENDED SEAWEED

As global climates warm and habitats are modified, disease is emerging as a major threat to biodiversity. Understanding the causes and consequences of disease and the influence of environmental change on both host organisms and pathogens is critical for the conservation of important ecosystems. Current rates of environmental change are unprecedentedly rapid, due largely to anthropogenic disturbances, further necessitating the study of disease dynamics. In this thesis, I assessed the causes and consequences of a bleaching phenomenon afflicting a chemically-defended macroalga, Delisea pulchra. Broadly, the aims of this study were to characterise patterns of bleaching in D. pulchra and to assess whether they related (directly or indirectly) to environmental factors, either via influences on algal chemical defences and/or bacterial pathogens. Additionally, I aimed to understand the consequences of disease for D. pulchra and its trophic interactions. Bleaching was more common in summer when water temperatures were elevated and was also more prevalent in shallow water habitats than at depth. Bleached algae supported significantly different microbial communities on their surfaces and also had depleted chemical defences (furanones) compared to co-occurring, healthy conspecifics. Bleaching was more likely at high temperatures when algal chemical defences were low or absent, and microorganisms were abundant and this was evident both in natural populations and in experimental manipulations. Inoculation with a candidate pathogen, Ruegeria sp. R11 caused bleaching in novel field inoculation experiments, but infection and bleaching were moderated by algal defences. Exposure to excessive solar radiation indirectly increased algal susceptibility to bleaching by causing a precursor condition (‘fading’), which had depleted furanones. Association with shadeproviding kelps prevented fading and bleaching in D. pulchra individuals. Bleaching had significant, sub-lethal performance consequences for affected algae and also altered trophic interactions between this seaweed and locally abundant macroherbivores. This work provides strong evidence that bleaching in D. pulchra is the result of an environmentally-mediated bacterial infection(s) that occurs when algal defensive chemistry is low. Understanding how environmental change and disease affect habitat-forming organisms like macroalgae is crucial for the study and management of important natural ecosystems under increasing stress.