The underlying green biciliate morphology of the orange snow alga Sanguina aurantia

Abstract

In the summer, blooms of microalgae appear on alpine and polar snowfields, creating expanses of red snow sometimes called 'watermelon snow'¹. These blooms are attracting research attention because they decrease snow albedo, thereby accelerating the effects of global warming on snowmelt². Currently, meltwater from alpine snowfields provides one-sixth of the world's population with water for drinking, agriculture, and the generation of hydroelectric power³. Each spring, the surface of new snow is colonized by microscopic organisms from unknown sources. One possibility is that when the melt begins, ciliated cells swim up from the substrate below to populate the snow surface. However, Sanguina, a cosmopolitan genus that frequently dominates high-alpine and arctic blooms^4,5, are thick-walled, red or orange in colour, and immotile. Here, we describe a culture of motile green biciliate cells isolated from a sample of red snow. Using cross-referenced Bayesian and maximum-likelihood phylogenetic methods for two genetic markers, ITS2 and rbcL, we establish the green biciliate as belonging to the genus Sanguina. Compensatory-base-change analysis of ITS2 rRNA structure delimits the green culture as S. aurantia, conspecific with individual, thick-walled immotile orange cells, picked from field samples collected in British Columbia and Svalbard. Using single cells was invaluable for comparing sequences derived from thick-walled red and orange Sanguina cells, which do not exist in culture, with the cultured green biciliates.