Complex Relationships between the Blue Pigment Marennine and Marine Bacteria of the Genus .

Marennine, the water-soluble blue pigment produced by the marine diatom , is known to display antibacterial activities. Previous studies have demonstrated a prophylactic effect of marennine on bivalve larvae challenged with a pathogenic , suggesting that the blue is a good candidate for applications in aquaculture as a source of a natural antimicrobial agent. Indeed, the genus is ubiquitous in aquaculture ecosystems, and regular events of pathogenic invasion cause some of the biggest losses worldwide. To better characterize the effects of marennine on , a panel of 30 strains belonging to 10 different species was tested, including bivalve pathogenic species (e.g., and ). strains were first exposed to 10 and 25 µg mL of Blue Water (BW), a concentrated culture supernatant of containing marennine. This screening evidenced a great diversity in responses, from growth stimulation to a total inhibition, at both the interspecific or intraspecific level. In a second series of experiments, 10 strains were exposed to BW at concentrations ranging from 5 to 80 µg mL. The highest concentrations of BW did not systematically result in the highest growth inhibition as hormetic responses-opposite effects regarding the concentration-were occasionally evidenced. The relationships between marennine and strains appear more complex than expected and justify further study-in particular, on the mechanisms of action-before considering applications as a natural prophylactic or antibiotic agent in aquaculture.