Microbial interactions in macroalgae with implications in aquaculture

Macroalgae, like Ulva genus (Ulvales, Chlorophyta), provides an important niche for epiphytic biofilm-forming bacteria, including those of the genus Phaeobacter with the ability to antagonize fish pathogens such as Vibrio anguillarum, through the production of tropodithietic acid (TDA). P. gallaeciensis has previously demonstrated its effectiveness as a probiotic in aquaculture by reducing mortality in fish larvae experimentally infected with this pathogen. The bacterial communities associated with Ulva can be conditioned by the surrounding medium and environmental factors. Based on this flexibility in biofilm colonization of Ulva thallus and the probiotic properties of Phaeobacter, it has been shown that experimental colonization of U. ohnoi is possible with a P. gallaeciensis strain with antagonistic activity. This experimental colonization could be used as a pathogen control strategy in multitrophic fish-algae cultures in recirculating water systems (IMTA-RAS), improving the health of the fish. However, the optimal conditions for the culture of U. ohnoi could have a determining influence both on the maintenance of these biofilms and on the production of TDA, especially the intensity of light. The hypothesis is that the light intensity can condition the epiphytic microbial communities of Ulva in terms of diversity and functionality, and, specifically, influence the colonization and TDA production capacity of P. gallaeciensis. Given that both processes are regulated by quorum mechanisms, mediated both by AHLs and by TDA itself, the implication of these mechanisms in alga-bacteria interactions will be studied, in relation to the intensity and light heterogeneity to which the alga is subjected. The results of this research will not only make a relevant contribution to the knowledge of microbial ecology in U. ohnoi and the physiology of P. gallaeciensis, but it will also make it possible to design cultivation strategies that allow optimizing the colonization and effect of P gallaeciensis as a pathogen antagonist in IMTA-RAS systems.