*2.1. The Habitat of the New Strain*

The carotenogenic microalga designated as BM1 described in this paper was originally discovered as reddish crusts on the northern slope of black granite-gneiss coastal rocks on Kost'yan island (66°29ȝ47Ȟ N, 33°24ȝ22Ȟ E), White Sea (Figure 1). This habitat is characterized by harsh environmental conditions even during the warm season. Thus, during polar days (March to August), the northern slopes of the cliffs are constantly illuminated by sun. As a result, the water filling the rock baths inhabited by the microalga is considerably warmer than the seawater (Supplementary Table S1). Sharp fluctuations of salinity are also typical of this habitat due to enhanced evaporation from sun-heated rocks, especially during windy weather, and regular inflow of seawater from high tides or fresh water from rain and melting snow. 

**Figure 1.** (**a**) Coastal rocks at Kost'yan Island, White Sea; ( **b**) The red crust formed by the astaxanthin-rich hematocysts in a drying rock bath. 

Until the end of June, the microalga dwelled in the baths mainly as motile biflagellate green zoospores or coccoid non-motile cells (Figure 2b). In July, the microalga was represented mainly by large (up to 80 ΐm in diameter) bright redcolored coccoid resting cells (Supplementary Figure S2a) referred to below as "red" cells. By the end of July, the baths usually dried out and the "cells" formed the reddish crusts on the rock surface (Figure 1). In addition to the microalgal cells, thin (1–3 ΐm in diameter) filamentous heterocyst-lacking cyanobacteria (III subsection, presumably Oscillatoriaceae) were encountered in the samples. The surface of the "red" cells was often decorated by numerous bacterial rod-like cells attached by their apical ends (not shown). 

To the best of our knowledge, the literature on isolation of *H. pluvialis* from arctic regions is scarce and limited to the isolates from freshwater habitats. A cold-tolerant strain of *H. pluvialis* capable of growth at a low (4–10 °C) temperature was recently isolated from a freshwater lagoon in Blomstrandhalvøya Island (Svalbard) [14]. In  contrast to BM1, this strain was incapable of sustained growth at temperatures higher than 15 °C since these conditions triggered the formation of the red-colored cysts in the latter. Notably, the enrichment culture of the freshwater arctic strain also contained filamentous heterocyst-lacking cyanobacteria from the III subsection. 
