A Sargassum-associated phage slows Vibrio alginolyticus persistence on macroalgal surfaces

Macroalgal surfaces harbour dense and metabolically active microbial consortia in which Vibrio alginolyticus exhibits marked ecological plasticity, spanning from mutualistic to opportunistic lifestyles. In these structured biofilm habitats, phages play a central role in shaping bacterial population structure and metabolic interactions. Here, a lytic phage vB_ValP-R39Z (R39Z) and its V. alginolyticus host were isolated from the surface of Sargassum hemiphyllum . Phage R39Z displays a podovirus morphotype, with a latent period of ~50 min and a burst size of 109 p.f.u. per cell, a narrow host range restricted to its original host, and stability across environmentally relevant conditions (pH 7–9 and 4–35 °C). R39Z harbours a 71.8 kb dsDNA genome (37.7 mol% G+C), encoding 83 ORFs, and is distinct from established genera, representing a previously unclassified genus. Functionally, R39Z significantly reduced biofilm biomass in vitro , and in an ex vivo assay using Sargassum blades, it reduced the viable cell abundance of V. alginolyticus ZS10 on blade surfaces by ~99%, as determined by c.f.u. enumeration. These findings demonstrate that R39Z efficiently disrupts biofilm formation and suppresses Vibrio colonization on macroalgal surfaces, underscoring its ecological relevance and potential for targeted biocontrol in algal-associated microbiomes.