Desferrioxamine Sharing among Microalgae-Associated Bacteria Enhances Iron-Limited Inhibition of Vibrio anguillarum

Vibrio anguillarum remains a major problem in marine aquaculture, motivating the search for ecologically grounded microbial strategies for pathogen control. We previously showed that a coculture of Vreelandella alkaliphila D2 and Sulfitobacter pontiacus D3, isolated from Isochrysis galbana, inhibited V. anguillarum more strongly than either strain alone. Here, we demonstrate that this suppression is driven by desferrioxamines (DFO) produced by V. alkaliphila, enabling synergistic iron sequestration through siderophore sharing with S. pontiacus. Mass spectrometry imaging revealed multiple DFO analogues spatially colocalized within inhibition zones and confirmed xenosiderophore uptake of DFO analogues by Sulfitobacter. Bioassays showed iron-chelation activity in V. alkaliphila, while iron supplementation and DFO-B standards validated siderophore-mediated bacteriostatic inhibition. Untargeted metabolomics found no increase in siderophore abundance in coculture, indicating that enhanced inhibition results from shared iron chelation rather than elevated production. Genome mining identified a complete DFO biosynthetic gene cluster in V. alkaliphila and a desferrioxamine receptor in Sulfitobacter. Finally, in axenic I. galbana, Sulfitobacter, alone or with Vreelandella, maintained V. anguillarum at the level of inoculation, highlighting that the substrate has a marked influence on inhibitory activity. Together, these findings show how DFOs act as shared public goods that enable both cooperation and pathogen antagonism.