ULTRAVIOLET-B RADIATION ENHANCES THE GROWTH AND ASTAXANTHIN PRODUCTION IN HAEMATOCOCCUS PLUVIALIS

Haematococcus pluvialis is recognized as the ideal astaxanthin producer since high levels of astaxanthin can be accumulated in its cells under stress conditions. Although appropriate amount of ultraviolet-B (UV-B) can effectively induce astaxanthin biosynthesis, it remains to be elucidated whether the specific light signal perception and transduction mechanisms regulate astaxanthin accumulation in this alga. The present study was conducted to investigate the effects of different intensity of UV-B on the growth, photosynthesis and production of astaxanthin of H. pluvialis under six cultivation conditions (100-500 lux of UV-B was supplemented under LED light culture conditions). A series of physiological and biochemical parameters were examined under these six UV-B treatments designed as CK, U100, U200, U300, U400 and U500, respectively. The results revealed that UV-B radiation decreased the cell density, maximal quantum yield of PSⅡ (F_v/F_m), non-photochemical quenching parameter (NPQ) and chlorophyll (Chl. a and Chl. b) content in H. pluvialis. On the contrary, the astaxanthin content were enhanced with the increase of UV-B radiation intensity (100－400 lx). Compared with the control algal cells without UV-B treatment, astaxanthin content increased by 35.68% and 56.23% in the algal cells treated by 36h and 72h of high-intensity UV-B radiation (U400), with the level up to 5.82 and 7.06 mg/L. Expression analysis by qRT-PCR demonstrated that both 200 and 400 lx UV-B treatments significantly upregulated transcriptional expression of the four carotenoid biosynthesis-related genes (IPI, PSY, BCH and BKT) in H. pluvialis despite their expression profiles were different. UV-B radiation also upregulated the expression of UVR8 and the downstream genes COP1, SPA1, HYH and HY5 which are the core elements in UV-B signal transduction pathway. These findings indicated that the UV-B light signal transduction pathway may function importantly in the regulation of astaxanthin biosynthesis pathway, providing a scientific basis for optimizing culture system to increase astaxanthin accumulation in H. pluvialis by UV-B auxiliary light source. The present data also lay the foundation for further understanding the transcriptional regulation mechanisms of astaxanthin accumulation in this special agal under stress conditions, particularly in response to UV-B radiation.