Reviews and syntheses: Snow algae on the move – biased motility and snowpack interaction from a biophysics perspective

Snow algae are psychrophilic and psychrotolerant photosynthetic microorganisms found on every continent, predominantly in polar and alpine environments. Along with contributing to terrestrial carbon cycling and food webs, colourful snow algal blooms formed on snow surfaces can substantially reduce albedo and accelerate snowmelt. Despite their ecological importance, the mechanisms governing snow algae motility and migration within snow remain poorly understood. This review synthesises current knowledge of snow algae migration, spanning microscopic cell-level motility to macroscopic population-level redistribution within snowpacks. We consider snow algae as biologically active particles within the framework of active matter physics, exploring their non-equilibrium dynamics and self-propelled motion in response to environmental stimuli. Particular attention is given to directional behaviours in response to light, temperature and chemical gradients, gravity and fluid flow. Where data gaps exist, we draw parallels from studies on a model motile microalga, Chlamydomonas reinhardtii . Finally, we identify key knowledge gaps and highlight future research directions, with implications for understanding cryosphere processes, microswimmer tactic behaviour, and the development of emerging biotechnologies.