Snow algae inhabit most of the cold regions worldwide, where long-lasting snow fields are common. The ecophysiology of snow algae has been studied intensively in North America and occasionally in polar regions. In the European Alps, the systematics of snow algae have been studied mainly by light microscopy. We studied temperature and light-dependence of photosynthesis, and plastid and extraplastid red pigment composition of red snow algae (Chlamydomonas nivalis) from snow patches in the high Alps of Austria. Both photosynthetic and respiratory data support the cryophilic adaptation of snow algal cells, but C. nivalis produced oxygen without any inhibition at temperatures up to 20°C and maintained this for 1 h, at irradiances up to 1800 µmol m⁻²s⁻¹. Chlorophyll and primary carotenoid pigment composition was similar to that found in most other Chlorophyta. Additionally, large amounts of free and esterified astaxanthin were located in cytoplasmic lipid globules. Light and electron microscopy showed that the cell walls were frequently covered with tightly bound inorganic particles. Occasionally fungus- or bacteria-like structures were attached to the wall. The typical adult cell contained a single central chloroplast. Cytoplasmic structures were often difficult to resolve optically, as densely packed peripheral lipid globules, containing secondary carotenoids, occupied most of the cell volume. These pigments may shield the chloroplast from high irradiation (thus reducing the risk of photoinhibition) and may also be a potential carbon source during unfavourable climate conditions or the formation of daughter cells.

Snow or glacial algae are found on all continents, and most species are in the Chlamydomonadales (Chlorophyta) and Zygnematales (Streptophyta). Other algal groups include euglenoids, cryptomonads, chrysophytes, dinoflagellates, and cyanobacteria. They may live under extreme conditions of temperatures near 0°C, high irradiance levels in open exposures, low irradiance levels under tree canopies or deep in snow, acidicpH, low conductivity, and desiccation after snow melt. These primary producers may color snow green, golden‐brown, red, pink, orange, or purple‐grey, and they are part of communities that include other eukaryotes, bacteria, archaea, viruses, and fungi. They are an important component of the global biosphere and carbon and water cycles. Life cycles in theChlamydomonas–Chloromonas–Chlainomonascomplex include migration of flagellates in liquid water and formation of resistant cysts, many of which were identified previously as other algae. Species differentiation has been updated through the use of metagenomics, lipidomics, high‐throughput sequencing (HTS), multi‐gene analysis, andITS. Secondary metabolites (astaxanthin in snow algae and purpurogallin in glacial algae) protect chloroplasts and nuclei from damagingPARandUV, and ice binding proteins (IBPs) and polyunsaturated fatty acids (PUFAs) reduce cell damage in subfreezing temperatures. Molecular phylogenies reveal that snow algae in theChlamydomonas–Chloromonascomplex have invaded the snow habitat at least twice, and some species are polyphyletic. Snow and glacial algae reduce albedo, accelerate the melt of snowpacks and glaciers, and are used to monitor climate change. Selected strains of these algae have potential for producing food or fuel products.

Surface ablation of the Greenland ice sheet is amplified by surface darkening caused by light‐absorbing impurities such as mineral dust, black carbon, and pigmented microbial cells. We present the first quantitative assessment of the microbial contribution to the ice sheet surface darkening, based on field measurements of surface reflectance and concentrations of light‐absorbing impurities, including pigmented algae, during the 2014 melt season in the southwestern part of the ice sheet. The impact of algae on bare ice darkening in the study area was greater than that of nonalgal impurities and yielded a net albedo reduction of 0.038 ± 0.0035 for each algal population doubling. We argue that algal growth is a crucial control of bare ice darkening, and incorporating the algal darkening effect will improve mass balance and sea level projections of the Greenland ice sheet and ice masses elsewhere.

Ten algal strains from snow and permafrost substrates were tested for their ability to produce secondary carotenoids and α-tocopherol in response to