What color should glacier algae be? An ecological role for red carbon in the cryosphere
ABSTRACT Red‐colored secondary pigments in glacier algae play an adaptive role in melting snow and ice. We advance this hypothesis using a model of color‐based absorption of irradiance, an experiment with colored particles in snow, and the natural history of glacier algae. Carotenoids and phenols—astaxanthin in snow‐algae and purpurogallin in ice‐algae—shield photosynthetic apparatus by absorbing overabundant visible wavelengths, then dissipating the excess radiant energy as heat. This heat melts proximal ice crystals, providing liquid‐water in a 0°C environment and freeing up nutrients bound in frozen water. We show that purple‐colored particles transfer 87%‐89% of solar energy absorbed by black particles. However, red‐colored particles transfer nearly as much (85%‐87%) by absorbing peak solar wavelengths and reflecting the visible wavelengths most absorbed by nearby ice and snow crystals; this latter process may reduce potential cellular overheating when snow insulates cells. Blue and green particles transfer only 80%‐82% of black particle absorption. In the experiment, red‐colored particles melted 87% as much snow as black particles, while blue particles melted 77%. Green‐colored snow‐algae naturally occupy saturated snow where water is non‐limiting; red‐colored snow‐algae occupy drier, water‐limited snow. In addition to increasing melt, we suggest that esterified astaxanthin in snow‐alga cells increases hydrophobicity to remain surficial.