Regulation of carotenogenesis in Haematococcus pluvialis was studied using various carotenoid and fatty acid synthesis inhibitors under high light in nutrient sufficient (NS/HL) and nutrient deficient (ND/HL) condition. The results showed inhibition of astaxanthin by all inhibitors studied except cerulenin in ND/HL Total carotenoids were reduced by inhibitors except DPA in ND/HL Lycopene accumulation was observed with nicotine treatment while beta-carotene accumulation was observed with DPA. Under NS/HL condition, expression of carotenogenic gene PSY (phytoene synthase) was enhanced by all inhibitors studied except DPA while PDS (phytoene desaturase) expression was reduced by all inhibitors tested except nicotine. Expression of LCY (lycopene cyclase) was enhanced by inhibitors. Reduced expression of BKT (beta-carotene ketolase) and CHY (beta-carotene hydroxylase) were observed under DPA and NF treatment while enhanced expression for BKT was observed under nicotine treatment and CHY expression under nicotine and cerulenin. ND/HL exposure enhanced carotenogenic genes expression when compared to respective inhibitors of NS/HL in most cases. Reduced expression of ICY by nicotine resulted in increased lycopene accumulation while PSY and LCY by cerulenin resulted in decreased beta-carotene and lutein. The results indicate the involvement of other regulatory factors besides transcriptional regulation of carotenogenesis in H. pluvialis. (C) 2009 Elsevier Inc. All rights reserved.