Paradoxical energetics in the polar diatom Fragilariopsis cylindrus exposed to extreme low light

It is generally considered that Arctic diatoms endure total darkness during polar winters by shifting to a hypometabolic state based on internal reserve mobilization. They can nonetheless rapidly initiate under-ice blooms at extreme low-light levels in early spring. The limits and mechanisms underlying this ability remain unknown. To explore the absolute minimum light required for photoautotrophic growth, we investigated the physiological and transcriptional state of the polar diatom Fragilariopsis cylindrus across a winter-to-spring light gradient. While cell division is arrested at 0.18 µmol photons m-2 s-1, photosynthetic electron transport remains operational down to 0.1 µmol photons m-2 s-1, which suggests an uncoupling between photosynthesis and biomass accumulation. Crucially, nondividing cells do not enter hypometabolism but instead maintain a functional photosynthetic metabolism without reserve consumption, redefining the lower limit of photoautotrophic life. While cells optimize photon capture efficiency between 3 and 15 µmol photons m-2 s-1 relative to 30 µmol photons m-2 s-1, this strategy collapses below 1 µmol photons m-2 s-1. In this dim-light regime, cells activate nonphotochemical quenching and a sustained xanthophyll cycle, which indicates a paradoxical requirement for energy dissipation. We propose that at these extremely low light levels, light absorption appears to exceed the minimum energy requirements for cell maintenance. To manage this active imbalance, cells dissipate excess energy as heat through active nonphotochemical quenching and possibly carbohydrate secretion, indicating a paradoxical requirement for energy dissipation despite extreme photon scarcity. This mechanism allows them to maintain a primed photosynthetic metabolism to facilitate rapid growth recovery upon the return of light after the winter solstice.