Elevated light and <scp>CO₂</scp> levels increase photosynthetic rates of diverse snow algal communities from the North Cascades

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 genera Chlainomonas, Sanguina, and Rosetta were present. We tested short-term physiological responses of snow algae to light (0-3000 μmol m^-2 s^-1) and CO₂ levels (0-1600 ppm), allowing us to determine the saturating light and CO₂ levels 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 CO₂ levels. At current atmospheric CO₂ levels (420 ppm), snow algal net photosynthesis rates were only c. 50% saturated. Together, these results suggest the primary productivity of important bloom-forming snow algal communities in alpine ecosystems will likely rise as atmospheric CO₂ concentrations increase, regardless of potential changes in available light levels.