*Received: 12 June 2014; in revised form: 17 July 2014 / Accepted: 4 August 2014 / Published: 11 August 2014*

**Abstract:** We report on a novel arctic strain BM1 of a carotenogenic chlorophyte from a coastal habitat with harsh environmental conditions (wide variations in solar irradiance, temperature, salinity and nutrient availability) identified as *Haematococcus pluvialis* Flotow. Increased (25‰) salinity exerted no adverse effect on the growth of the green BM1 cells. Under stressful conditions (high light, nitrogen and phosphorus deprivation), green vegetative cells of *H. pluvialis* BM1 grown in BG11 medium formed non-motile palmelloid cells and, eventually, hematocysts capable of a massive accumulation of the keto-carotenoid astaxanthin with a high nutraceutical and therapeutic potential. Routinely, astaxanthin was accumulated at the level of 4% of the cell dry weight (DW), reaching, under prolonged stress, 5.5% DW. Astaxanthin was predominantly accumulated in the form of mono- and diesters of fatty acids from C16 and C18 families. The palmelloids and hematocysts were characterized by the formation of red-colored cytoplasmic lipid droplets, increasingly large in size and number. The lipid droplets tended to merge and occupied almost the entire volume of the cell at the advanced stages of stress-induced carotenogenesis. The potential application of the new strain for the production of astaxanthin is discussed in comparison with the *H. pluvialis* strains currently employed  in microalgal biotechnology.

**Keywords:** astaxanthin; carotenogenesis; fatty acids; green microalgae

## **1. Introduction**

The red ketocarotenoid Astaxanthin (Ast) is a potent antioxidant exerting a plethora of health-beneficial effects in human and animal organisms. It is of high demand as an ingredient of cosmetic, medical and dietary formulations [1,2] as well as quality feed for aquaculture. In particular, the red color of the crustacean shells and salmon meat is due to the presence of Ast; the only source of Ast in animals is their diet [3]. At present, most of the feed Ast is chemically synthesized although the synthetic pigment, unlike natural Ast, is a racemic mixture containing a substantial proportion of the stereoisomers lacking the biological activity [3]. 

The richest natural source of Ast is the chlorophyte *Haematococcus pluvialis* Flotow [4] that accumulates the pigment in an amount of up to 3%–6% of cell dry weight (DW) under unfavorable environmental conditions [5]. Essentially a freshwater alga, *H. pluvialis* survives in small rain pools under extremely volatile conditions such as extreme temperatures, low nutrient availability and solar irradiance [6] mainly in form of Ast-rich non-motile coccoid cells with an exceptional tolerance of the adverse conditions [7–10]. The massive accumulation of Ast in *H. pluvialis* depends on and is accompanied by the enhanced biosynthesis of neutral lipids, mainly triacylglycerols (TAG) [11]. The reason for this is that Ast is deposited in cytoplasmic lipid droplets (LD) comprised by TAG, mainly in the form of FA esters. Accordingly, *H. pluvialis* can also be a source of valuable FA e.g., oleic acid [5,12]. 

In spite of its high bioavailability and numerous beneficial effects, natural Ast from microalgae hardly can compete with its synthetic analog due to high production cost and limited productivity of the commercial *H. pluvialis* strains [2,3]. Obviously, at least a two-fold increase in the Ast productivity of current strains (which is at the level of ca. 3% DW) is necessary for natural Ast to outcompete the synthetic pigment [13]. Moreover, mass cultivation of *H. pluvialis* is highly demanding of fresh water, which may not be available at the site of cultivation. Therefore bioprospecting of more stress-tolerant *H. pluvialis* strains is important to reduce the costs of the Ast-enriched biomass production e.g., by the use of brackish water. We paid close attention to White Sea coastal rocks characterized by a particularly harsh environment expecting to obtain microalgal isolates naturally adapted to the adverse conditions. In the present work, we obtained a detailed characteristic of a novel *H. pluvialis* strain from an arctic sea and estimated its suitability for Ast production. 
