Exploring the associations among halogenated compounds in Asparagopsis taxiformis: temporal shifts and bacterial impacts under intensive cultivation

Bromoform is the key antimethanogenic bioactive in Asparagopsis species, yet its variation in culture and relationship with other halogenated compounds remain poorly understood. Here, the profile of 9 halogenated compounds in A. taxiformis sporophytes cultured at 0.5 g/L over five weeks were quantified, comparing control cultures to those exposed to gentamicin (10 and 20 mg/L) which temporarily reduced surface bacteria by 81–95 %. Chemical analyses ( n = 75) identified three classes of compounds (haloacids, halomethanes and haloacetones) comprising totals of 1–3.3 % dry weight (dw). Haloacids represented half the total; dibromoacetic acid (DBAA, 31.4 %), tribromoacetic acid (TBAA, 10 %) and bromochloroacetic acid (1.3 %). Halomethanes accounted for 39.5 % with bromoform (36.4 %) the single most abundant compound at 0.44–1.38 % dw. DBAA and TBAA concentrations reached 0.47–0.82 % and 0.13–0.38 % dw, respectively. Haloacetones contributed 10.7 % of the total, positively correlating with five other halogenated compounds. However, DBAA and TBAA concentrations correlated more closely to growth, cell morphology and bacterial densities than to other halogenated compounds. Overall, there was a temporal shift in growth rates for both antibiotic treatments – initially suppressing growth by 10–25 % before stabilising with all treatments converging to a specific growth rate of ~8.5 % day −1 by week 5. Extending the cultures for an additional 10 weeks confirmed no long-term impacts of antibiotic treatments on growth, halogenated compounds or the composition of surface bacterial community. This study is the first comprehensive evaluation of halogenated compound loads in cultured Asparagopsis , identifying TBAA as a major component that may contribute to antimethanogenic activity in ruminant feed. • Bromoform was the most abundant compound; haloacids the most abundant class. • Dibromoacetic acid positively correlated with seaweed growth and cell morphology. • Tribromoacetic acid (potentially antimethanogenic) identified and quantified. • Gentamicin initially reduced bacteria, growth and halogenated compounds. • Long-term cultures provide stable growth, compounds and bacterial diversity.