The transition toward nitrogen deprivation in diatoms requires chloroplast stand-by and deep metabolic reshuffling

Microalgae have evolved to face abiotic stresses by accumulating energy storage molecules such as lipids, which are also of interest for industries. Alas, the impairment cell division during the accumulation of these molecules constitutes a major bottleneck for the development of efficient microalga-based biotechnology processes. To contribute significantly to the reduction of the bottleneck a multidisciplinary approach was used to study the mechanisms involved in the transition from nitrogen-replete to nitrogen starvation conditions in the marine diatom Phaeodactylum tricornutum cultured in a turbidostat. Combining data demonstrates that the different steps of nitrogen deficiency clustered together in a single state in which the cells are in equilibrium with their environment. The switch between the nitrogen-repleted equilibrium and the nitrogen-deficiency equilibrium is driven by the intracellular nitrogen availability. The switch induces a major gene expression change, which is reflected in the reorientation of the carbon metabolism toward an energy store-and-save mode while still operating as a metabolic flywheel. Although the photosynthetic activity reduced, the chloroplast status is kept in a stand-by mode allowing a fast resuming upon nitrogen-repletion. Altogether, these results contribute to the understanding of the intricated response of diatoms under stress.