How do Arctic microalgae thrive in the chaotic light field of in-ice and under-ice marine habitats?

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Marine polar environments are, for photosynthesising organisms, among the most extreme in terms of variations in light. From the polar night to the midnight sun, and from ice-free to ice-covered conditions, microalgae have evolved to outlive sustained lacks and excesses of photons. How polar microalgae survive prolonged darkness, grow at extremely low levels of light, cope with wide and rapid fluctuations, and tolerate excess irradiance remain poorly understood. My research programme will address the following questions: 1) What is the minimum light level at which ice algae and phytoplankton can achieve net growth during early to mid-winter? 2) How is light distributed inside the sea-ice matrix, together with nutrients and grazers, and how does this distribution relates to sea-ice microphysics? 3) How is the under-ice dynamic light regime shaped by various macroscopic features of sea ice (snow-pack heterogeneity and metamorphism, ridges, leads, ice age, melt ponds)? 4) How do phytoplankton respond to this highly fluctuating light regime in terms of gross primary production? 5) Considering also grazing and other loss processes, which dynamic light regime is most favourable for biomass accumulation (bloom) under sea ice? 6) How to properly account for small- and medium-scale light fluctuations when estimating primary production at large scale using remote-sensing data? 7) What are the consequences, on un-der-ice primary production, of the ongoing modifications of sea ice in the Arctic? This Discovery Grant research program will include laboratory experiments, field activities, and the development of upscaling methods for large-scale remote-sensing applications. The laboratory experiments will aim at determining: 1) the minimum light requirement for microalgae growth in conditions typical of the sea-ice matrix and the upper-water column during late winter and early spring, and 2) how polar microalgae cope with wide and rapid fluctuations in light and how the light variation and limitation ultimately affect growth. During the field expeditions, new miniaturised optical sensors and imaging systems mounted onto an ad hoc endoscope will be used to document the optical, physical and chemical properties that shape the micro-scale environment of ice algae. Also, various autonomous platforms (drones, autonomous underwater vehicles and remotely operating vehicles) will be used to describe the under-ice light field at scales ranging from the metre to several kilometres, and we will determine the space and time response of primary production to the broad spectrum of observed light variations. Finally, new upscaling approaches will be developed to appropriately account for small- and medium-scale features of sea ice that affect light and primary production at large scale during all seasons. The new approaches will be applied on remote-sensing data to determine multiyear trends in pan-Arctic primary production, including the under-ice contribution.