Molecular Interactions between phytoplankton and bacteria in the phycosphere

Symbiotic relationships between microalgae and bacteria play a crucial role in the functioning ofaquatic ecosystems as they influence nutrient cycling and carbon biogeochemistry. Despite theimportance of these relations, the communication mechanisms governing these symbiosesremain poorly understood. Here, we use the 15N-labeled ubiquitous diatom Asterionellopsisglacialis to show that only symbiotic bacteria from its microbiome assimilate dissolved organicnitrogen to decipher communication mechanisms between both organisms.We show that a symbiotic bacterium, Pseudosulfitobacter pseudonitzschiae, assimilates dozensof diatom-derived dissolved organic nitrogen metabolites, including vitamin B6 and derivatives.In exchange, the bacterium converts some of these signals to 4-pyridoxate, a vitamin B6 wasteproduct, which is then taken up by the diatom. Along with bacterial-derivedethanolaminephosphate, which is also secreted by the bacterium, 4-pyridoxate regulates nearlyhalf of the diatom genome to prime it to interact with its symbiont