Environmental occurrence of the algal morphogen thallusin and identification of Algoriphagus species as producers in the Baltic Sea (Kiel Fjord, Germany)

Marine macroalgae rely on interactions with associated bacteria for growth and development. In Ulva species, bacterial morphogens such as thallusin are essential for normal morphogenesis, yet their occurrence in natural coastal environments remains poorly understood. Here, we investigated spatial and seasonal variation in thallusin-associated morphogenetic activity in the Kiel Fjord (southwestern Baltic Sea, Germany) by combining bioassays with environmental analyses, microbiome profiling, UHPLC–HRMS, and bacterial culture experiments. Sterile-filtered seawater collected at 32 stations during spring, summer, and autumn was tested for its ability to induce normal morphogenesis in axenic Ulva compressa germlings. A morphogenetic index (MI), representing thallusin-associated activity, showed pronounced spatial and seasonal variation, with significantly higher activity in summer than in autumn. MI was positively associated with temperature and negatively associated with salinity and nitrite concentration. Targeted UHPLC–HRMS analysis confirmed thallusin in coastal seawater, with a concentration of 0.66 ± 0.21 nmol L - ¹ determined in a sample from a site exhibiting consistently high morphogenetic activity. Exploratory microbiome screening identified several bacterial genera as candidate producers. Subsequent culture-based bioassays and chemical analyses demonstrated thallusin production by four Algoriphagus species, all of which induced the characteristic morphogenetic phenotype, cell wall and rhizoid formation, in U. compressa . Together, these results provide the first direct evidence of thallusin in coastal seawater and expand the known diversity of thallusin-producing bacteria to members of the genus Algoriphagus . By linking environmental bioassays with chemical detection and experimental validation of bacterial producers, this study paves the way for investigating the environmental dynamics and ecological functions of bacterial morphogens in macroalgal–microbial interactions.