Seaweed Resistance to Microbial Attack: A Targeted Chemical Defense Against Marine Fungi

Pathogenic microbes can devastate populations of marine plants and animals. Yet, many sessile organisms such as seaweeds and sponges suffer remarkably low levels of microbial infection, despite lacking cell-based immune systems. Antimicrobial defenses of marine organisms are largely uncharacterized, although from a small number of studies it appears that chemical defenses may improve host resistance. In this study, we asked whether the common seaweed Lobophora variegata is chemically defended against potentially deleterious microorganisms. Using bioassayguided fractionation, we isolated and characterized a 22-membered cyclic lactone, lobophorolide (1), of presumed polyketide origin, with sub-M activity against pathogenic and saprophytic marine fungi. Deterrent concentrations of 1 were found in 46 of 51 samples collected from 10 locations in the Bahamas over a 4-year period. Lobophorolide (1) is structurally unprecedented, yet parts of the molecule are related to tolytoxin, the scytophycins, and the swinholides, macrolides previously isolated from terrestrial cyanobacteria and from marine sponges and gastropods. Until now, compounds of this structural class have not been associated with marine macrophytes. Our findings suggest that seaweeds use targeted antimicrobial chemical defense strategies and that secondary metabolites important in the ecological interactions between marine macroorganisms and microorganisms could be a promising source of novel bioactive compounds…

Atypical milliliter of seawater contains 103 fungal cells, 106 bacteria, and 107 viruses, including pathogens that cause widespread mortalities and microbes that initiate fouling of host surfaces

Thus, marine plants and animals are continually exposed to high concentrations of potentially harmful microbes.

1. Microbial pathogens cause black band disease in stony corals
2. Caribbean sea fan mortality
3. coralline lethal orange disease in coralline algae
4. green spot rotting disease in the alga Porphyra
5. red spot disease in the kelp Laminaria
6. and raisin disease in the brown alga Sargassum,

to name just a few.

In the 1930s, a slime mold wasting disease eliminated almost all of the eelgrass Zostera marina on the Atlantic coasts of North America and Europe, with continuing negative effects on softsediment communities, waterfowl populations, and scallop fisheries (8, 9). In the 1980s, an unidentified pathogen killed 95–99% of the herbivorous sea urchin Diadema antillarum throughout the Caribbean Sea; without these herbivores to control seaweeds, reef-building corals were rapidly overgrown (10–12). Harvell and colleagues (13, 14) proposed that the frequency of disease among marine macroorganisms has increased in recent decades and may continue to increase, because of climate change and human activities that stress hosts, introduce pathogens to new areas, and provide microbes with favorable conditions for growth.