Effect of growth temperature on biosynthesis and accumulation of carotenoids in cyanobacterium Anabaena sp. PCC 7120 under diazotrophic conditions

Carotenoid composition has been studied in mesophilic, nitrogen-fixing cyanobacterium Anabaena sp. PCC7120 grown photoautotrophically, under diazotrophic conditions at four different temperatures (15 degrees C, 23 degrees C, 30 degrees C and 37 degrees C). The relative accumulation of chlorophyll, carotenoids and proteins was the highest at temperature of 23 degrees C. At a suboptimal temperature (15 degrees C) (beta-carotene was the dominant carotenoid compound, whereas the increase in temperature caused ketocarotenoids (echinenone, canthaxanthin, keto-myxoxanthophyll) to accumulate. A significant increase in the accumulation of phytoene synthase (CrtB) transcript was observed at both extreme growth temperatures (15 degrees C and 37 degrees C). The relative amount of beta-carotene ketolase (CrtW) transcript directly corresponded to the accumulation of its product (keto-myxoxanthophyll) with a maximum at 30 degrees C and a profound decrease at 37 degrees C, whereas the transcription level of beta-carotene ketolase (CrtO) was significantly decreased only at a suboptimal temperature (15 degrees C). These results show that temperature affects the functioning of the carotenoid biosynthesis pathway in Anabaena cells under photoautotrophic growth. Specifically, the balance between beta-carotene and ketocarotenoids is altered according to temperature conditions. The transcriptional regulation of genes encoding enzymes active both at the early (CrtB) and the final steps (CrtO, CrtW) of the carotenoid biosynthetic pathway may participate in the acclimation mechanism of cyanobacteria to low and high temperatures.