In aquatic systems, the layer that is suitable for positive net photosynthesis (the euphotic zone) is usually considered to extend from the surface down to the depth of penetration of 1% of the surface irradiance, which corresponds to ~15 µE m-2s-1 at solar noon during late summer in open waters in the high Arctic1. In polar regions, vernal blooms of epontic algae (unicellular algae associated with the lower interface of sea ice whose photosynthetic characteristics are not well known2-4) develop under conditions where they are only rarely or briefly exposed to light levels exceeding 1-2% of that incident at the surface. I report here that epontic algae from the Canadian Arctic show unusually high photosynthetic efficiencies normalized to pigment content, which increase with a decrease in the light levels at which the populations are growing. Photosynthesis was measurable at light intensities well below ambient (0.01% of surface irradiance). In the most shade-adapted populations5 under deeper snow cover, photosynthesis was optimal at light intensities close to the maximum ambient level, and inhibited at higher intensities. Furthermore, even after almost 2 months of exposure to elevated light levels (~3-4% of surface irradiance), biomass and productivity normalized to biomass were still lowest in algae from an area almost free of snow cover. Taken together, these results indicate that epontic algae from the high Arctic can be considered as an obligate shade flora genetically constrained to very low photon fluxes.