Not All Children Are the Same: Differences in the Microbiome Assembly During Early Development of Seaweeds.

Microbial symbionts play key roles in macroalgal development, yet the processes structuring early-life microbiomes remain poorly understood. Using laboratory outgrowth experiments and 16S rRNA gene amplicon sequencing we compared microbiome acquisition and assembly during the early development of three distinct macroalgae: Ulva australis (Chlorophyta), Hormosira banksii (Phaeophyceae) and Delisea pulchra (Rhodophyta). All species established distinct bacterial communities within the first week of outgrowth, with significant shifts in community composition and structure associated with major developmental stages. Stage-enriched taxa included Phaeobacter, Roseobacter and Maribacter, which include members reported to influence algal growth or morphogenesis. Vertical inheritance contributed unevenly to the microbiome assembly across hosts. U. australis recruited low-abundance environmental bacteria, possibly via strong host filtering. H. banksii selectively retained a small, consistent subset of adult-derived bacteria, whereas D. pulchra retained fewer of its inherited bacteria. These results suggest that macroalgae can employ diverse transmission and recruitment strategies to assemble early microbiomes, combining selective inheritance with stage-specific retention and/or environmental acquisition.