Résumé
Algae serve as a source of nutrients for bacteria in the marine environment. The interactions between algae and bacteria at the phycosphere interface are known to include mutualism, commensalism, competition or antagonism. Here, based on in situ observation and on an in vitro interaction study, we report on a novel form of starvation-induced hunting that the cells of selected Vibrio species exert on the dinoflagellate species. Results showed that Vibrio atlanticus was able to coordinate lightning group attacks then kill the dinoflagellate Alexandrium pacificum ACT03. Briefly, the observed coordinated mechanism of algal-killing consists of first, the ‘immobilization stage’ involving the secretion of algicidal metabolites that disrupt the flagella of the prey. The ‘attack stage’ resembles the ‘wolf-pack attack’ strategy, during which Vibrios surrounds algal cells at high density for a brief period without invading them. Finally, the ‘killing stage’ induces the lysis and degradation of the dinoflagellates. By using a combination of biochemical, proteomic, genetic and fluorescence microscopy approaches, we showed that this relationship is not related to the decomposition of algal organic matter, Vibrio quorum sensing pathways, to the toxicity of the algae or to the pathogenicity of the bacterium but is conditioned by nutrient stress, iron availability and link to the iron-vibrioferrin transport system of V. atlanticus . This is the first evidence of a new mechanism that could be involved in regulating Alexandrium spp. blooms and giving Vibrio a competitive advantage in obtaining nutrients from the environment.