3.6.2. Transmission Electron Microscopy

To prepare the samples for TEM, the cell suspension was washed twice with 0.1 M cacodylate buffer (pH 7.2) and fixed for transmission electron microscopic investigation according to a modified method suitable for the cells featuring a tough cell wall [40]. Ultrathin sections were prepared using an LKB 4800 ultramicrotome (LKB Produkter, Broma, Sweden), contrasted with lead citrate [41], and analyzed with a Hitachi HU-11F electron microscope (Hitachi Ltd., Tokyo, Japan) at an accelerating voltage of 80 kV. 

## 3.5.3. Scanning Electron Microscopy

For SEM sample preparation, the cells fixed and dehydrated in ethanol were transferred in absolute acetone and dried at a critical point in the HCP-2 dryer (Hitachi, Hitachi, Japan), coated with gold and palladium on by ion-sputtering IB Ion Coater (Eiko, Ibaraki, Japan) and examined in a JSM-6380LA (JEOL, Tokyo, Japan) microscope at an accelerating voltage of 15 kV. 

## **4. Conclusions**

The present work describes, to the best of our knowledge, the first strain of *H. pluvialis* isolated from an arctic seashore habitat characterized with moderate and highly variable salinity levels [42]. Indeed, the current compendia of White Sea algoflora do not include species from the genus *Haematococcus* other than freshwater representatives [43] which are not yet confirmed rigorously. The new strain was capable of massive accumulation of Ast as a secondary Car, mainly in the form of esters of the FA from the C16 and C18 families. Depending on the cultivation conditions, it turned out to be able to accumulate Ast in the amounts of 3%–5.5% of DW, which compares favorably with most of the currently known strains [2]. Our preliminary studies have shown that *H. pluvilais* BM1 seems to have an elevated salt tolerance, the extent of which, however, remains to be elucidated. Nevertheless, given its growth and biosynthetic capacity and potential salt tolerance, the new BM1 strain is a promising organism for biotechnological production of valuable carotenoids using microalgae. In particular, it merits close attention as a potential producer of Ast potentially capable of growth in brackish water. 

## **Acknowledgments**

The dedicated technical assistance of Ekaterina Lobanova and Ksenia Klokova is greatly appreciated. 

## **Author Contributions**

K.C. performed the cultivation experiments, carried out pigment analysis, molecular identification and wrote the manuscript; E.L. isolated the strain and supervised the cultivation; I.S. and L.S. supervised the culture purification and electron microscopy; R.S. supervised the fatty acid analysis; A.S. conceived the experiments and wrote the manuscript. 

## **Conflicts of Interest**

The authors declare no conflict of interest. 
