Optimizing Ulva-Phaeobacter co-culture: A two-phase light intensity approach for integrated multi-trophic aquaculture applications

Abstract Incorporating macroalgae like Ulva species into integrated multi-trophic aquaculture (IMTA) enhances sustainability by filtering effluents and provide epiphytic bacteria, that can contribute to disease prevention. Colonizing Ulva ohnoi with Phaeobacter sp. 4UAC3 is promising for disease control in IMTA systems, but high light intensity impacts Phaeobacter ’s persistence on Ulva . This study investigated the effect of different light intensities and regimes on the ability of Phaeobacter sp. 4UAC3·to colonize U. ohnoi . Experiments assessed algal growth, water physicochemistry, and microbial profiles using culture techniques and 16S rRNA gene sequencing. Light intensities of 143, 75, 45, and 0 μmol photons m −2 s −1 were tested on U. ohnoi and non-living surfaces inoculated with Phaeobacter sp. 4UAC3. Results showed that Phaeobacter sp. 4UAC3 colonization on U. ohnoi decreased in the light but remained stable in the dark. On non-living surfaces, Phaeobacter persisted regardless of light intensity, suggesting light does not directly affect the bacterium. However, light had an impact on microbial community structure on U. ohnoi , with genera including Glaciecola , Maribacter and Roseobacter significantly enriched under higher light conditions. This suggests the disappearance of Phaeobacter sp. 4UAC3 is in part a result of competition with specific bacteria. Additionally, the chemical microenvironment of the algae influenced by light could be important in Phaeobacter disappearance, although further research is required. Finally, optimal co-culture involved alternating dark and low-light (45 μmol photons·m −2 s −1 ) phases, enhancing Phaeobacter sp. 4UAC3 maintenance and optimal algal growth. This knowledge will optimize a fish-algae IMTA-RAS using Ulva colonized with Phaeobacter .