Bacterial microbiota of Ostreobium, the coral-isolated chlorophyte ectosymbiont, at contrasted salinities

Microscopic filaments of the siphonous green algae Ostreobium (Ulvophyceae, Bryopsidales) colonize and dissolve the calcium carbonate skeletons of coral colonies, in shallow-water reef environments of contrasted salinities. Their bacterial composition and plasticity in response to salinity remain unknown. Here, we analyzed the bacteria associated with coral-isolated Ostreobium strains from two distinct rbcL lineages, representative of IndoPacific environmental phylotypes, that had been pre-acclimatized (>9months) to three ecologically-relevant reef salinities: 32.9, 35.1 and 40.2 psu. Bacterial phylotypes were visualized at filament scale by CARD-FISH in algal tissue sections, localized to the surface, within filaments or in the algal mucilage. Ostreobium-associated communities, characterized by bacterial 16S rRNA metabarcoding of cultured thalli and corresponding supernatants, were structured by host genotype more than salinity and partly overlapped with those of environmental (Ostreobium-colonized) coral skeletons. Alphaproteobacteria dominated the thalli communities, enriched in Kiloniellaceae or Rhodospirillaceae depending on algal genotype. A small core microbiota composed of 7 ASVs ([~]1.5% of thalli ASVs, 19%-36% cumulated proportions), shared by multiple cultures of both Ostreobium genotypes and persistent across 3 salinities, included putative intracellular Amoebophilus and Rickettsiales bacteria. This novel knowledge on the taxonomic diversity of Ostreobium bacterial associates paves the way to functional interaction studies within the coral holobiont.