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The Southern Ocean is considered a zone of high nutrients and low chlorophyll (HNLC). Despite the year round availability of nutrients, phytoplankton biomass remains low. Key modes of control are now thought to be light, grazing, iron and supply of silicic acid^1^. Physiological photoprotective mechanisms used by phytoplankton to regulate photosynthesis versus rapid light fluctuations have not been considered important. In diatoms and haptophytes, which are the major component of phytoplankton biodiversity in the Southern Ocean, the one step de-epoxidation of diadinoxanthin (Dd) into diatoxanthin (Dt) of the Dd-cycle constitutes the main photoprotective mechanism^2,3^. Here we show that Southern Ocean diatoms and haptophytes have higher concentrations of cellular Dd-cycle pigments than other oceanic regions. The Dd-cycle is activated under low irradiance conditions and the pool of photoprotective pigments increases under intermittent light conditions. The high cellular concentration of Dd-cycle pigments and our observations of an increase of the pool size of Dd-cycle pigments towards the surface implies that the Dd-cycle pigments bind to light harvesting proteins other than with fucoxanthin and chlorophyll c. Within photosystem II, enrichment of Dd and the physiological acclimation to light changes constrains photosynthetic activity, efficiency and potentially growth rates^3-7^. These results raise important questions about the role of photoprotective mechanisms in limiting Southern Ocean primary productivity and how these mechanisms will respond to predicted climate change impacts^8^.

Photoprotection in Southern Ocean phytoplankton: missing key to low primary productivity