Novel Hydrurus species (Chrysophyceae) and their adaptations to high-altitude European and Arctic snowfields

Colored snow caused by green algae (Chlorophyceae) is well known, but melting snowpacks can also harbor golden-brown blooms consisting of Chrysophyceae. We collected 14 samples of cryoflora in the Austrian and Swiss Alps, the High Tatras in Slovakia, and in Arctic Svalbard. Eight laboratory unicellular flagellated strains were established from eight sites and phylogenetic analyses (18S rRNA and rbcL gene sequences) revealed new taxa belonging to the order Hydrurales (Chrysophyceae). Some formed tetrahedral swarmers; others were capsoid or amoeboid forms. Characteristics of vegetative cells and molecular markers, including the ITS2 rRNA region, supported the description of eight species: Hydrurus novisii sp. nov., H. klavenessii sp. nov., H. tatrae sp. nov., H. pulcher sp. nov., H. pascheri sp. nov., H. svalbardensis sp. nov., H. nivalis sp. nov. and H. nemcovae sp. nov. Pulse-amplitude-modulate (PAM) fluorometry indicated that the photosystem II of Arctic populations was adapted to high light conditions. Abundant polyunsaturated fatty acids supported cell survival at temperatures around 0°C, and the composition of these acids differed among species. The cells contained compatible solutes that could act as antifreeze agents. The main carotenoid fucoxanthin caused the overall golden-brown pigmentation. The closest relatives of the new species were reported from snow and cold mountain streams and lakes, indicating that these Hydruralian microalgae prefer low temperatures and elevated irradiation. The large number of new species discovered during this limited sampling campaign suggests the underestimated diversity of phototrophic microbes in melting snow. Consequently, the genus Hydrurus shows a similar high relevance for snow algae blooms as Chloromonas does within the green algae.