Manipulation of the surface-associated bacteria on Asparagopsis taxiformis and its effect on seaweed growth and halogenated natural products

Abstract Epiphytic bacteria can play a crucial role in regulating seaweed traits, yet their specific contributions to both host growth and natural products remain unclear for most seaweeds. Here, we explored the effects of manipulating the surface-associated bacteria on the growth and halogenated compounds of two domesticated strains of Asparagopsis taxiformis sporophytes under intensive culture. Under standard conditions, sporophytes had an average surface bacterial density of 1 per 32 µm 2 . Following five different antibiotic treatments at 10 mg L −1 for 24 h (i.e., penicillin, streptomycin, kanamycin, gentamicin, and vancomycin), bacterial densities were significantly reduced depending on the antibiotic, averaging 1 per 90 µm 2 . Antibiotics treatment led to 22–89% reductions in bacterial density compared to controls after 2 weeks, with gentamicin and streptomycin most effective (86–89% reductions to ~ 1 per 260 µm 2 ), followed by vancomycin (77%), kanamycin (23%) and penicillin (22%). We examined the impacts on growth and the concentration of two major halogenated compounds, bromoform and dibromoacetic acid, with a combined concentration of 1–2% dry weight. Our results highlight that the antibiotic treatments with the lowest bacterial densities (gentamicin and streptomycin) had higher growth rates than the control (> 50% increase). We observed a negative impact on the bromoform and dibromoacetic acid concentrations, with 42% and 35% reductions, respectively, dependent on strain and antibiotic. Overall, our study demonstrates that the epiphytic bacteria in A. taxiformis cultures can be manipulated to enhance seaweed biomass production, but there may be a trade off in natural product levels in some culture lines.