Exploring the microbiome of the "star" freshwater diatom Asterionella formosa in a laboratory context.

Most of our knowledge on the mechanisms underlying diatom-bacterial interactions has been acquired through studies involving isolation of culturable partners. Here, we established a laboratory model of intermediate complexity between complex natural communities and laboratory pure culture models. We investigated the whole community formed by the freshwater diatom Asterionella formosa and its associated bacteria in a laboratory context, including both culturable and unculturable bacteria. Combining cellular and molecular approaches, we showed that in laboratory cultures, A. formosa microbiome was dynamic and comprised of numerous bacterial species (mainly Proteobacteria and Bacteroidetes). Using metagenomics, we explored several metabolic potentials present within the bacterial community. Our analyses suggested that bacteria were heterotrophic although a third of them (Alpha- and Beta-proteobacteria) could also be phototrophic. About 60% of the bacteria, phylogenetically diverse, could metabolize glycolate. The capacity to synthesize molecules such as B vitamins appeared unevenly distributed among bacteria. Altogether, our results brought insights into the bacterial diversity found in diatom-bacterial communities and hinted at metabolic interdependencies within the community that could result in diatom-bacterial and bacterial-bacterial interactions. The present work allowed us to explore the functional architecture of the bacterial community associated with A. formosa in culture and is complementary to field studies.