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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 Chlainomonas(Chlamydomonadales, Chlorophyta) is one of the four genera of snow algae known to produce annual pink or red blooms in alpine snow. NoChlainomonasspecies have been successfully cultured in the laboratory, but diverse cell types have been observed from many field‐collected samples, from multiple species. The diversity of morphologies suggests these algae have complex life cycles with changes in ploidy. Over 7 years (2017–2023), we observed seasonal blooms dominated by aChlainomonasspecies from late spring through the summer months on a snow‐on‐lake habitat in an alpine basin in the North Cascade Mountains of Washington, USA. The Bagley LakeChlainomonasis distinct from previously reported species based on morphology and sequence data. We observed a similar collection of cell types observed in otherChlainomonasspecies, with the addition of swarming biflagellate cells that emerged from sporangia. We present a life cycle hypothesis for this species that links cell morphologies observed in the field to seasonally available habitat. The progression of cell types suggests cells are undergoing both meiosis and fertilization in the life cycle. Since the life cycle is the most fundamental biological feature of an organism, with direct consequences for evolutionary processes, it is critical to understand how snow algal life cycles will influence their responses to changes in their habitat driven by climate warming. For microbial taxa that live in extreme environments and are difficult to culture, temporal field studies, such as we report here, may be key to creating testable hypotheses for life cycles.