Secondary Metabolite Defenses Against Pathogens and Biofoulers

Competition for space and resources is intense in benthic marine environments (McClintock and Baker 2001). In these habitats, macroalgae constitute a seemingly ideal substrate for growth of microorganisms and other epibionts, presenting these organisms with a living space rich in organic material. Some associations between macroalgae and microbes are mutualistic, benefiting both host and symbiont. For example, algal-associated bacteria may produce metabolites that protect hosts from biofouling (e.g., Boyd et al. 1999; Armstrong et al. 2001), and recent reports have indicated that epibiotic hydroids may enhance growth of the kelp Macrocystis pyrifera (Hepburn and Hurd 2005). Other algal associates are clearly detrimental to hosts, as evidenced by reports of algal disease and fouling-associated fitness costs (e.g., D’Antonio 1985; Correa 1997; Ruesink 1998). Red spot disease in the commercially valuable kelp Laminaria japonica (Sawabe et al. 1998) is caused by Pseudoalteromonas bacteriolytica bacteria; similarly, white rot disease in the kelp Nereocystis luetkeana is caused by an Acinetobacter sp. bacterium (Andrews 1977). Some bacteria act as secondary pathogens, accelerating disease progression following attack of a primary pathogen (Correa et al. 1994). Fungi can also act as seaweed pathogens, including Lindra thallasiae, an Ascomycete, which causes raisin disease in Sargassum spp. brown algae and Thalassia disease in seagrasses (Kohlmeyer 1971; Andrews 1976; Porter 1986). In addition to bacterial and fungal pathogens, some species of endophytic multicellular algae, cyanobacteria, and amebas have been implicated as causes of disease in macroalgae (Andrews 1977; Correa et al. 1993; Correa and Flores 1995). In addition to pathogens, micro- and macrofoulers negatively impact a variety of macroalgal hosts. Biofouling by the diatom Isthmia nervosa is related to declines in growth and reproduction of the red alga Odonthalia floccose (Ruesink 1998). Biofoulers may indirectly reduce algal fitness by increasing drag and susceptibility to tissue breakage in turbulent water and by increasing herbivore attraction (Dixon et al. 1981; D’Antonio 1985; Wahl and Hay 1995).