The diel vertical migration of microbes within snowpacks driven by solar radiation and nutrients

Abstract. Seasonal snowpacks are inhabited by various microbes despite their ephemeral and cold environments. Physicochemical conditions in snowpacks drastically change with day-night cycles; however, their effect on the microbial community is largely unknown. This study describes the diel vertical migration (DVM) of microbes within a snowpack in an alpine forest in northern Japan. Microscopy revealed the presence of snow algae, microinvertebrates, and fungi in the snowpack. Periodic sampling across snow depths revealed that the vertical distribution of microbes changed over time. Motile cells of snow algae and microinvertebrates were distributed near the surface at night but migrated to a depth of 10–20 cm during the day. Other microbes, including algal spores and fungi, remained on the surface layer throughout the day. The vertical migration of microbes was synchronized with the intensity of solar radiation, suggesting that the microbes moved downward to avoid intense solar radiation on the snow surface. Soluble nutrients (PO₄^3-, NH₄⁺, and K⁺) were always the highest at the snow surface, suggesting that surface snow provides a favorable environment for microbial growth. These results indicate that motile microbes migrate diurnally within the snowpack to remain under the best conditions for their growth in terms of the intensity of solar radiation and nutrients. Snow core sampling from the surface to bottom revealed that microbes were mostly present above a depth of 30 cm in the snowpack. Therefore, the upper snow layer above this depth, referred to as the Microbial Active Snow Surface (MASS) layer, is important for the life cycles of microbes and biogeochemical cycles within snowpacks.