| Haematococcus pluvialis(H.pluvialis)has been widely recognized for its ability to accumulate astaxanthin in large quantities and is currently also the best established biological source of natural astaxanthin in nature.Astaxanthin has been widely concerned and applied in the pharmaceutical,nutraceutical,food,feed and other industries due to its strong antioxidant capacity.Therefore,how to improve the cultivation method of H.pluvialis and improve the astaxanthin yield is one of the current research hotspots.One of the most critical factors for H.pluvialis growth and astaxanthin accumulation is light conditions.Light is essential for plant and microalgae,both for providing energy and as a signaling molecule regulating growth and development.Current research on the effect of light exposure to H.pluvialis mainly proceeds from light intensity,light wavelength and photoperiod.Although there have been many studies on the promotion of H.pluvialis growth and astaxanthin accumulation by different light modes,it needs to continue to be deeply explored in terms of cultivation strategy optimization and related mechanistic studies,etc.This paper mainly starts with the two culture stages of H.pluvialis,ie,the green vegetative growth stage and the astaxanthin accumulation stage,using red,blue and white light treatments,respectively,and then aims at the problems that arise in each stage,taking a corresponding solution,and do a deep exploration of the mechanisms.The research findings of the thesis are summarized as follows:1.The green vegetative growth stage was used to study the effects of red light on CO2 utilization and biomass accumulation in H.pluvialis.Under appropriate photoautotrophic conditions,biomass can be enhanced by about 50%after conversion of white to red light.Interestingly,red light was found to promote algal cell entry into higher photosystem Ⅱ(PSⅡ)activity and increase gene expression of carbonic anhydrases(CAH1 and CAH3),the MYB transcription factor LCR1(Lcrl)and the Rubisco(rbcL)protein.Further studies showed that genes related to photoreceptor interacting factor(COP1,SPA1)and photosynthetic light harvesting protein(Lhca2)were also upregulated,suggesting that red light promotes carbonic anhydrase(CA)activity to maintain pH stability and increase photosynthetic carbon fixation rates by regulating COP1/SPA1 complexes.2.The effect of blue light on astaxanthin accumulation in the H.pluvialis was investigated during the astaxanthin accumulation stage.It was found that at the initial stage of blue light illumination a large number of algal cells died due to photo oxidative damage,which greatly reduced astaxanthin production.It was also discovered that yaminobutyric acid(GABA)improved the photooxidative damage effect to H.pluvialis in the early stage of blue light exposure and obviously increased astaxanthin production.It was found that peroxidase levels were increased in the algal cells at the initial stage of blue light,but both ROS levels and peroxidase levels were decreased after GABA added,confirming the photoprotective effect of GABA on H.pluvialis.3.To investigate how astaxanthin yield can be increased in a white light culture system.For this purpose,GABA was applied in the culture system and found to significantly enhance the growth of H.pluvialis and produce higher astaxanthin yields(The astaxanthin production of 5 mM and 10 mM GABA treated algal cells reached 214.48 mg/L and 354.09 mg/L).The expression of Rubisco,chlorophyll light harvesting protein(Lhca2),plastid terminal oxidase(PTOX),and the expression of astaxanthin synthesis pathway related genes of the GABA treated group were up-regulated in the algal cells under strong white light conditions,indicating that GABA improved the photosynthesis and had a photoprotective effect on the algal cells under strong white light stress.Photosynthetic evaluation showed that GABA enhanced the heat dissipation capacity and reduced photodamage.Therefore,exogenous GABA can improve the mechanism of stress resistance and photoprotective ability of H.pluvialis in high white light conditions,effectively promote the growth of algal cells and increase biomass and astaxanthin production.4.In addition,the effects and roles of other conditions under high white light were explored.To this end,the effects and mechanisms of astaxanthin production enhancement from H.pluvialis treated with low temperature plasma were investigated,especially from an epigenetic perspective.Measurement of hormonal and transcribed genes revealed a significant increase in strigolactone and abscisic acid levels,accompanied by enhanced expression of astaxanthin synthesis genes.One of the key genes,carotenoid isomerase(CRTISO),in particular,was specifically upregulated.Further experiments by immunofluorescence and chip PCR methods confirmed increased histone H3 lysine 4 trimethylation(H3K4me3)at the promoter region of CRTISO. |
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