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As global climates change, the incidence and severity of diseases in natural ecosystems also appears to be increasing. This has been linked to increasingly stressful conditions, which can lead to more susceptible hosts and environmental effects on pathogen abundance and virulence. Recently, environmentally-mediated diseases have affected a diverse range of both terrestrial and marine organisms. The ecological impacts of diseases are likely to be more severe when they affect habitat-forming organisms like trees, corals and seaweeds, as any impacts could cascade throughout entire communities. In marine environments, organisms are exposed to persistently high densities of potentially pathogenic microorganisms, which can affect habitat-formers like corals and seagrasses. On temperate rocky reefs, the dominant habitat-formers are seaweeds. Here we review recent work on a chemically defended seaweed, and a bleaching phenomenon common in natural populations near Sydney, Australia. The prevalence of bleaching is positively correlated with water temperature and negatively correlated with concentrations of secondary metabolites in the alga, which are known to inhibit bacteria. Bleaching is associated with a shift in the composition of microbial communities on algal surfaces and can be induced via exposure to ambient seawater microbes and cultures of putative pathogens. Direct consequences of bleaching include reductions in algal growth and fecundity. Bleaching also has indirect ecological effects on the alga, with bleached individuals attracting higher densities of herbivores and preferential consumption of bleached tissues. This environmentally-mediated bleaching phenomenon appears to be the result of complex interactions between increasing ocean temperatures, host defences and pathogen virulence and has significant implications for this habitat-forming organism and the community it supports.

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