Taxonomic profling of microbes co-cultured with protists in the Roscoff Culture Collection

The ocean hosts an enormous diversity of microorganisms. However, most of these are notat present available for culture, which makes them difcult to study. We know that manymarine organisms rely on complex nutrient cycles which are difcult to unravel. Microalgaeproduces soluble carbon-, sulfur-, and nitrogen-containing compounds which are taken up byheterotrophic bacteria. The photosynthesizers, on the other hand, often rely on co-factors ofbacterial origin, such as vitamin B12. Since many ocean bacteria may rely on algal organicproducts to obtain essential nutrients, searching for microbes of interest in algal strain culturescan help pinpoint the method of cultivation. In this study, 16S meta-barcoding was performedon bacteria associated to sixty strains from the Roscoff Culture Collection. The bacterial taxonomic composition differed between the algal divisions. Hierarchical clustering revealed interand intra-division differences. The microbiota of the samples mostly belonged to Bacteroidota,Alpha- and Gammaproteobacteria. This is consistent with previous studies on the cycling ofnutrients in the ocean, as these groups are metabolically flexible and ubiquitous heterotrophs.Among the sequenced reads, there appear members of poorly described or undescribed prokaryotic lineages at high taxonomic levels, which could be subject to further study and cultivationefforts. Obtaining these organisms in pure culture could allow the description of new lineagesand their unique metabolisms, and potentially, the production of their bio-products of interest.