PhD student: Maxime Beauvais,Thesis directors : François-Yves Beauvais (LOMIC) and Pierre Galand (LECOB)
Thesis defended the 15th of december 2023

Marine microbial communities drive ocean biogeochemical cycles and harbor a tremendous diversity of organisms and functions, which are linked through a complex web of interactions and metabolic dependencies. Vitamins B1 and B12 are scarce in the ocean and have emerged as essential cofactors for phytoplankton and bacteria, as most cannot produce them de-novo (i.e., auxotrophs). These observations suggest that biotic interactions are essential for vitamin exchanges in ocean, but how auxotrophs interact with producers and how these interactions evolve over time remains unknown. To address this question, we conducted a 7-year metagenomic time-series analysis at a coastal station of the NW Mediterranean Sea (SOLA), coupled with bioassay and microcosm experiments. We showed a seasonal and recurrent succession of different potential vitamin B12 producers, dominated by archaea in winter and bacteria in summer, which could use the anaerobic and aerobic vitamin B12 production pathways, respectively. Functional redundancy was observed in vitamin B12 production pathways, providing an ecological insurance against environmental perturbations. We also observed a strong seasonality in metabolisms related to the use of vitamin B1 precursors in summer (i.e., HMP/cHET), dominated by HMP auxotrophs and to a lesser extent, dual auxotrophs. We reported a higher proportion of vitamin B1 producers and dual auxotrophs at SOLA than previously estimated in other oceanic regions. Finally, a metagenomic- informed network analysis showed recurrent patterns of co-occurrence between organisms which are complementary for their vitamin B1 and B12 needs. These metabolic complementarities changed over the seasons, suggesting different patterns of interactions. Finally, vitamin B12, B1 and precursor amendment experiments conducted in microcosms demonstrated their role in structuring microbial communities and confirmed the existence of changing metabolic complementarities along the year. By exploring the long- term patterns of B-vitamins and precursors seasonality and complementarities, our work constitutes a first step towards understanding the nature of vitamin-based mutualistic interactions and their role in structuring seasonal microbial communities in the ocean.

Keywords: Metagenomic | Time series | Functional communities | Vitamin B1 & B12 | Metabolicinterdependencies