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Summary Microalgae adapted to near‐zero temperatures and high light levels live on snowfields and glaciers worldwide. Snow algae have red‐colored pigments that darken snow surfaces, lowering its albedo and accelerating snowmelt. Despite their importance to the cryosphere, we know little about controls on snow algal productivity and biomass.Here, we characterize photophysiology from diverse natural field‐collected populations of alpine snow algae from the North Cascades of Washington, USA, where the major red‐bloom producing generaChlainomonas,Sanguina, andRosettawere present. We tested short‐term physiological responses of snow algae to light (0–3000 μmol m⁻² s⁻¹) and CO₂levels (0–1600 ppm), allowing us to determine the saturating light and CO₂levels for snow algal community net photosynthesis.All snow algal communities surveyed were adapted to extremely high light levels (3000 μmol m⁻² s⁻¹). In addition, photosynthesis rates of all the snow algal communities responded strongly to increasing CO₂levels. At current atmospheric CO₂levels (420 ppm), snow algal net photosynthesis rates were onlyc.50% saturated.Together, these results suggest the primary productivity of important bloom‐forming snow algal communities in alpine ecosystems will likely rise as atmospheric CO₂concentrations increase, regardless of potential changes in available light levels. 

Abstract High‐latitude oceans experience strong seasonality where low light limits photosynthetic activity most of the year. This limitation is pronounced for algae within and underlying sea ice, and these algae are uniquely acclimated to low light levels. During spring melt, however, light intensity and daylength increase drastically, triggering blooms of ice algae that play important roles in carbon cycling and ecosystem productivity. How the algae acclimate to this dynamic and heterogeneous environment is poorly understood. Here, we measured¹⁴C‐carbon fixation rates, photophysiology, and ribulose 1,5‐bisphosphate carboxylase oxygenase (Rubisco) content of sea‐ice algae in coastal waters near the western Antarctic Peninsula during spring, ranging from a low‐light‐acclimated, bottom community to a light‐saturated bloom. Carbon fixation rates by sea‐ice algae were similar to other Antarctic sea‐ice measurements (2–49 mg C m⁻²d⁻¹), and there was little phytoplankton biomass in the underlying water at the time of sampling. Net‐to‐gross ratios of carbon fixation were generally high and showed no relationship with ice type. We found algal photophysiology and Rubisco concentrations varied in relation to the different types of ice, altering the balance between the photochemical and biochemical processes that constrain carbon fixation rates. For algae inhabiting the bottom layers of sea ice, rates of carbon fixation were largely constrained by light availability whereas in surface seawater, interior and rotten/brash ice, carbon fixation rates could be calculated with reasonable accuracy from measurements of Rubisco concentrations. This work provides additional insight and means to evaluate carbon fixation rates as sea ice continues to change in future.