Boom and bust path of Gracilaria: Loss of bacterial "friends" can limit the invasion success of a seaweed host?

Invasive species are one of the principal components of global change along with ocean warming at the global scale, or overfishing and deoxygenation at the regional scale. Seaweeds represent up to 40% of all introduced marine species and some seaweeds can significantly affect the composition and functioning of marine benthic communities. Within ten years of its first discovery in the Kiel Fjord in 2005 the East Asian red seaweed Gracilaria vermiculophylla has spread approximately 100 km eastward and 120 km northward along the German Baltic Sea coast, now inhabiting many lagoons and sheltered bays between the German-Danish border and Neustadt. During the first two years after its discovery Gracilaria vermiculophylla increased its biomass in the Kiel Fjord massively. However, this was followed by a sudden decline in late summer 2008, when the alga decayed in nearly all inhabited parts of the bay within few weeks. Co-cultivation of healthy Gracilaria from unaffected environments with small amounts of decaying material from the Kiel Fjord in laboratory assays demonstrated that the decay was apparently caused by an infectious disease. Thus, 59 different species of epibacteria isolated from Gracilaria were tested for their capacity to induce decay in a bleaching assay. Out of these, three were found to induce the disease, while 19 others significantly reduced the risk of decay and were thus protective. When protectors and pathogens were tested together, the protective strains fully prevented the negative impact of the bleachers, hinting at the presence of an associational defence offered by Gracilaria’s epibacteria. Presence of such an associational resistance was also supported in a follow- up bioassay where surface extract of Gracilaria and its associated microbiome attracted the beneficial strains, but deterred the detrimental ones. Thus, we suggest that the breakdown in 2008 was due to a collapse of such associational resistance provided by bacterial partners