At the arctic archipelago of Svalbard, bare glacier surfaces are populated by microalgae like Ancylonema nordenskiöldii (Zygnematales, Streptophyta). The resulting blooms cause, due to a vacuolar pigmentation, brownish colourations of the glacier surface. This freshwater ice alga has been described from several polar and alpine glaciers; however, these reports lacked data about the ecophysiology or ultrastructure. Considering the harsh environmental conditions of the exceptional habitat, such as permanently low temperatures, exposure to high irradiation or a short vegetation period, the aim of this study was to elucidate cellular adaptations of A. nordenskiöldii. Thus, samples were collected at two glaciers in Spitsbergen. The cytoarchitecture of the cylindrical cells, which are arranged in unbranched filaments, demonstrates active cells with Golgi bodies, mitochondria and rough endoplasmic reticulum close to the nucleus when investigated by transmission electron microscopy (TEM). The cell walls are pore less and only 90 nm thin. A. nordenskiöldii only sporadically produces oblong zygotes when two filaments conjugate. The most remarkable cytological feature is peripheral brownish vacuoles, appearing osmiophil and electron dense by TEM. Aqueous extracts of this pigmentation show a broad absorption in the visible light and in the UV. Consequently, a protection against excessive irradiation is provided. Photosynthesis measurements performed at different temperatures and light levels indicate that the metabolism is adapted to temperatures close to the freezing point as well as to high light conditions. Therefore, A. nordenskiöldii can be regarded as metabolically and cytological well adapted to live on glaciers.