Effects of Culture History and Algal Taxonomy on Bacterial Communities in Long-maintained Filamentous Green Algae

Algal cultures are widely used to study algae-bacteria interactions, yet the relative roles of algal taxonomy, culturing environment, and historical contingency in shaping bacterial communities of long-maintained monoalgal cultures remain unclear. In a two-stage experimental design, we used 16 S rRNA metabarcoding to first profile the bacterial communities of 14 filamentous freshwater algal strains from three orders (Chaetophorales, Oedogoniales, Zygnematales) maintained in two distinct culture collections, and then, in a second experiment to test community mixing using a co-culture and re-isolation design with two algal strains (Stigeoclonium sp. and Oedogonium sp.). The 14-strain profiling showed that bacterial communities were broadly similar at higher taxonomic ranks, with many shared orders within Pseudomonadota, whereas composition at the ASV level was largely algal-strain-specific. Ordination of phylogeny-aware UniFrac distances showed that culture collection of origin explained more variation in community composition than algal order, an effect reduced but not eliminated after standardizing the culture medium. The co-culture and re-isolation experiment showed that the two algal strains exchanged many bacterial community members, yet algal strain identity remained detectable after re-isolation. These results support a framework where culture history results in strain-specific ASV-level composition within recurrent higher-rank structure, and where algal-strain-specific effects on bacterial community composition become evident when cultures are exposed to a shared bacterial pool. Co-culture followed by algal re-isolation offers a practical approach to examine the persistence of algal-culture bacterial communities under mixing and may help identify candidate bacterial partners with potentially strong host-specific interactions without requiring axenic hosts.