Ulva spp. are economically important macroalgae with various industrial applications, including as biofiltration agents for fish effluents in integrated multi-trophic aquaculture recirculating systems (IMTA-RAS). Recent works have proposed inoculating U. ohnoi with the probiotic bacterium Phaeobacter sp. strain 4UAC3 to tackle fish pathogens such as Vibrio spp. in IMTA-RAS. However, the disappearance of Phaeobacter sp. 4UAC3 upon inoculation of U. ohnoi under a regular photoperiod presents significant challenges. This study aimed to investigate how different light regimes impact the relationship between the U. ohnoi holobiont and Phaeobacter sp., focusing on how the colonization of Phaeobacter sp. strain 4UAC3 on U. ohnoi surfaces affects the alga's microbiome and metabolome dynamics. We also sought to validate the presence of tropodithietic acid (TDA), which can act as a probiotic. The study revealed the critical role of light in shaping microbial interactions between Phaeobacter sp. and U. ohnoi: The light regime significantly altered the microbial community structure, metabolite production, and physiological responses of both the bacterium and the alga. Phaeobacter sp. strain 4UAC3 thrived in darkness, modulating the microbiome and the exo- and endo-metabolomes of U. ohnoi. TDA was only identified under dark conditions and released into the algal chemosphere, while Phaeobacter antimicrobial properties were most pronounced in close association with the alga. These findings underline the importance of environmental factors, such as light regime, in driving microbial and molecular dynamics in marine holobionts. In addition, our results have direct implications for the application of U. ohnoi and Phaeobacter sp. in aquaculture, providing valuable insights for future research and practical applications in the field

This work was supported by the Deutsche Forschungsgemeinschaft (SFB 1127 ChemBioSys, project number 239748522, to H.H. and T.W.), the Spanish Agencia Estatal de Investigación (UlvaQuo Project RTI2018–095062-B-C21 to J.P. and J.C.), the Fundación Pablo García (BC 3/21, Z.N.Q.M.), Axencia Galega de Innovación, GAIN (IN606A-2021/038 to G.d.O. and IN607B2021/03 to M.P.)

19 pages, 1 table, 10 figures

Peer reviewed