Inhibition of astaxanthin synthesis under high irradiance does not abolish triacylglycerol accumulation in the green alga Haematococcus pluvialis (Chlorophyceae)

Under stress conditions, Haematococcus pluvialis Flotow accumulates fatty acid-esterified astaxanthin, in extraplastidial lipid globules. The enhanced accumulation of fatty acids, mainly in triacylglycerols (TAG), among which oleic acid predominates, is linearly correlated with that of astaxanthin. We used inhibitors of either carotenoid or lipid biosynthesis to assess the interrelationship between carotenogenesis and TAG accumulation under high light irradiance as the stress factor. The two carotenogenesis inhibitors used-norflurazon, an inhibitor of phytoene desaturase, and diphenylamine (DPA), an inhibitor of beta-carotene C-4 oxygenase-suppressed the accumulation of astaxanthin in a concentration-dependent manner. Concurrently, the accumulation of neutral lipids was significantly less affected. The lipid biosynthesis inhibitor sethoxydim, which inhibits acetyl-CoA carboxylase, significantly decreased de novo fatty acid synthesis and, in concert, drastically inhibited astaxanthin formation. In the presence of various concentrations of the three inhibitors, the inhibition of astaxanthin was not accompanied by a proportional decrease in oleic acid, which was used as a marker for TAG fatty acids. When astaxanthin synthesis was completely inhibited, the volumetric content of oleic acid was about 60% of the control value when the two carotenogenesis inhibitors (0.05 mu M norflurazon or 20 mu M DPA) were used and 27% of the control when the lipid-synthesis inhibitor (50 mu M) was used. We suggest therefore that TAG accumulation under high irradiance is not tightly coupled with astaxanthin accumulation, although the correlation between these two processes was demonstrated earlier. Furthermore, we propose that the accumulation of a certain amount of TAG is a prerequisite for the initiation of fatty acid-esterified astaxanthin accumulation in lipid globules.