Colonization history of snow algae on Hawai'i Island.

Red-pigmented snow algae are cold-adapted (including cryophilic) photosynthetic microbes commonly found in polar and alpine snowpacks worldwide, but their dispersal across isolated cryospheres remains poorly understood. We report the occurrence of snow algae on Maunakea, Hawai'i, the most isolated cryosphere in the world, during an unusually prolonged summer snow retention event in 2023 associated with La Niña conditions. Red-pigmented algal cells were observed in snow samples collected during this event. ITS2 amplicon sequencing identified two major Chlorophyta groups: the cosmopolitan Sanguina group and the endemic Chloromonadinia snow group. The cosmopolitan Sanguina group disperses into Hawai'i from other cryospheres under present climate conditions, whereas the endemic Chloromonadinia assemblage shows multiple arrivals, with the largest Hawaiian clade indicating colonization between approximately 253 and 130 ka, overlapping the Pohakuloa glaciation (MIS 6) when Maunakea's summit was ice-capped. This study shows how specific climate conditions, such as glaciation, provided long-term habitats that enabled the establishment of distinct snow algae lineages, highlighting the timing and processes of their dispersal as shaped by glaciation and climate change.