Experimental evidence for early bacterial inheritance in the giant kelp, Macrocystis pyrifera.

Bacteria associated with macroalgae influence the development, health, and ecological interactions of their hosts. Although vertical transmission of bacterial symbionts has been proposed in some algal species, evidence remains limited. In this study, we combined 16S rRNA gene amplicon sequencing, fluorescence in situ hybridization (FISH), and transmission electron microscopy (TEM) to assess bacterial presence and localization throughout early developmental stages of the brown macroalga Macrocystis pyrifera. Amplicon sequencing detected bacterial taxa in reproductive sori, released spores, and gametophytes, even under sterile conditions and using pasteurized, filtered seawater. Certain bacterial taxa, including Balneola alkaliphila and Marinoscillum luteum, were recurrently detected across multiple developmental stages. FISH imaging showed bacterial signal associated with host tissues across life stages, distinguishable from algal autofluorescence, while TEM observations revealed structures consistent with intracellular bacterial cells within the cytoplasm of gametophyte cells and embedded in sporangial cuticles. Together, these observations suggest that bacteria are associated with both internal and external reproductive tissues, pointing to multiple potential transmission routes. Altogether, our findings provide evidence consistent with early-stage bacterial inheritance in M. pyrifera, contributing to the growing evidence that macroalgal holobionts are assembled, at least in part, through vertical transmission of symbionts.