Low-temperature-induced synthesis of alpha-carotene in the microalga Dunaliella salina (chlorophyta)

The microalga Dunaliella salina (Teo.) is well known as an accumulator of b eta-carotene (beta,beta-carotene) when subjected to growth-limiting conditi ons (e.g. exposure to high irradiances). In addition, the carotenoid alpha- carotene (beta,epsilon-carotene) may also be synthesized and subsequently a ccumulated by this alga under specific growth conditions. The main factor i n stimulating the synthesis of this carotene was determined to be exposure to lower than optimum temperatures for algal growth. A 7.5-fold increase in the levels of alpha-carotene was observed when the temperature was decreas ed from 34 to 17 degrees C, whilst levels of beta-carotene were unaltered. The accumulation of or-carotene was unaffected by irradiance, although its isomeric composition was greatly altered by light levels, The proportion of 9-cis alpha-carotene increased from 15% to 45% of total alpha-carotene whe n the irradiance was decreased from 260 to 50 mu mol.m(-2).s(-1). Exposure to higher irradiances had little influence on the isomeric composition of t his carotenoid. A reduction in growth temperature did not influence the iso meric composition of alpha-carotene. Nutrient status (nitrogen and phosphat e) had no effect on either the content or isomeric composition of alpha-car otene accumulated by D. salina.