Remote Antarctic Island reveals unique algal dynamics in snow and ice

Snow and glacier algal blooms are increasingly well-documented in the Northern Hemisphere, but their diversity and ecology in Antarctic regions remain poorly understood. Here, we present the outcomes of high-resolution sampling of snow and glacier algal blooms across Signy Island (maritime Antarctica) during the 2023-24 austral summer (February-April 2024). Using light microscopy and metabarcoding (18S V4 region and ITS2) of environmental DNA, we characterised algal diversity across the frozen habitats available on the island (snowpacks, ice cap, and glaciers), with a relatively long-term (6 weeks) period of ecological monitoring on two contrasting blooms. Our data highlight abundant and diverse snow and glacier algal communities, with snowpacks dominated by the snow algal genera Sanguina, Chloromonas, and Chlainomonas, extending the range of several species to this region. Ice surfaces supported mixed snow and glacier algal assemblages, including a novel "pointed" Ancylonema phenotype alongside described species. Phylogenetic analyses highlighted the presence of amplicon sequence variants representing Ancylonema (18S), Chloromonas (18S), and "Scotiella" (ITS2) as potentially unique to Signy Island, identifying both cosmopolitan and endemic snow and glacier algal species. Ecological monitoring revealed disparate communities between habitat types, with an atypical névé-type surface recognised as a potentially new ecological niche where Sanguina spp. cysts dominated in the presence of glacier algae. Neither community showed evidence of macronutrient limitation. Our work adds to the currently restricted knowledge of these key ecosystem engineers in the Southern Hemisphere and underscores the importance of continued in situ studies in these remote environments.