How does light affect the presence and antibacterial activity of Phaeobacter in the Ulva ohnoi holobiont?

Abstract

Ulva provides an important niche for biofilm-forming bacteria, such as the Phaeobacter genus, producing antibiotic compounds like TDA (tropodithietic acid) with antagonistic activity towards fish pathogens, including Vibrio anguillarum. It is, thus, interesting from an economic point of view to understand the environmental conditions that promote the presence of Phaeobacter on the surface of Ulva used in Integrated Multi-Trophic Aquaculture (IMTA), including the application as a biofilter of fish effluents. Upon inoculation of U. ohnoi with Phaeobacter sp. 4UAC3, it is maintained when U. ohnoi is kept in the dark, however, Phaeobacter sp. eventually disappears when the algae are growing under light in a regular photoperiod. Our study focused on the effect of light on (i) the metabolome of the Ulva holobiont in the two light regimes, (ii) the production of TDA by Phaeobacter forming a biofilm in U. ohnoi, (iii) the inhibition of V. anguillarum by Phaeobacter biofilms in U. ohnoi. Our results show (i) significant differences in the metabolome produced by the Ulva holobiont in the two light regimes, which may be responsible for the disappearance of Phaeobacter in the Ulva biofilm, (ii) an up-regulation of TDA on U. ohnoi cultivated in the dark, (iii) inhibition of V. anguillarum by Phaeobacter when forming a biofilm on U. ohnoi. Our results highlight the challenge of mastering seaweed physiology from the holobiont perspective for applications in aquaculture