The first phylogenetic and ecophysiological characterisation of Ankistrodesmus antarcticus CCAP 202/25, an Antarctic green alga isolated from freshwater ice

Glacier ice algae and snow algae are important primary producers in extreme cold environments and significantly impact global processes through their role in carbon cycling and glacier melting by reducing surface albedo. These organisms have evolved unique cold-adaptation mechanisms, making them promising candidates for biotechnological applications, yet few species are characterised in detail. Such studies are also becoming particularly urgent, as polar cryospheric habitats are rapidly changing because of the ongoing climate change. Here we describe the phylogenetic placement and ecophysiological characterisation of the strain Ankistrodesmus antarcticus CCAP 202/25 that was isolated from a sample of green-coloured icicle collected on Sabrina Island (Balleny Islands, Antarctica). Cells maintained the size and shape that were reported in the original description of the strain in 1968. Analysis of the 18S rDNA confirmed its placement in the Selenastraceae family and, more specifically, in the highly supported and ecologically diversified Monoraphidium V clade, suggesting that a revision of its taxonomic assignment to the genus Ankistrodesmus will be necessary. Comparison of secondary structures of ITS2 rDNA uncovered several closely related strains with diverse or unknown origin that should be regarded as conspecific with Ankistrodesmus antarcticus CCAP 202/25. Therefore, more data are necessary to get a detailed insight into the phylogenetic and ecological diversification within this group. The cultivation of the strain in crossed gradients of temperature and light reflected adaptation of the species to cold Antarctic habitats. Similarly to psychrophilic species, Ankistrodesmus antarcticus was able to grow at 1°C, but the highest growth rates were recorded in a wide range of temperature (6–25°C) making its ecophysiological classification difficult. The ability of the species to utilise very low intensity (and simultaneously wide range) of photosynthetically active radiation also indicates its adaptation to a polar cryospheric habitat, where light availability can often represent the limiting factor, but significant fluctuations of irradiance can be common, both short-term and long-term.