Arctic microalgae experience long periods of continuous darkness during the polar night, when they are unable to photosynthesize. Despite numerous studies on overwintering strategies, such as utilization of stored energy products, formation of resting stages, reduction of metabolic rates and heterotrophic lifestyles, there have been few attempts to assess the in situ physiological state and restoration of the photosynthetic apparatus upon re-illumination. In this study, we found diverse and active marine phytoplankton communities during the polar night at 78°N. Furthermore, we observed rapid changes (≤20 min) in the efficiency of photosynthetic electron transport upon re-illumination. High photosynthetic capacity and net primary production were established after 24 h of re-illumination. Our results suggest that some Arctic autotrophs maintain fully functional photosystem II and downstream electron acceptors during the polar night even though the low in situ net primary production levels measured in January prove that light was not sufficient to support any measurable primary production. Due to low temperatures resulting in low respiratory rates as well as the absence of photodamage during the polar night, maintenance of basic photosynthetic machinery may actually pose relatively low metabolic costs for algal cells. This could allow Arctic microalgae to endure the polar night without the formation of dormant stages, enabling them to recover and take advantage of light immediately upon the suns return during the winter-spring transition.

Arctic R and D Akvaplan niva AS CIENS Gaustadalleen 21 0349 Oslo Norway

Centre Algatech Institute of Microbiology Czech Academy of Sciences 37981 Třeboň Czech Republic

Department of Economics and Business Universitat Pompeu Fabra and Barcelona Graduate School of Economics Ramon Trias Fargas 25 27 08005 Barcelona Spain

Institute of Oceanology Polish Academy of Sciences Powstanców Warszawy 55 81 712 Sopot Poland

Marine Biogeoscience Alfred Wegener Institute Am Handelshafen 12 27570 Bremerhaven Germany

The Department of Arctic Biology Svalbard Science Centre University Centre in Svalbard P O Box 156 N 9171 Longyearbyen Norway

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Linking extreme seasonality and gene expression in Arctic marine protists