Astaxanthin-Producing Green Microalga Haematococcus pluvialis

Light microscopic images of H. pluvialiscells in life cycle. (A) Green vegetative motile cell; (B) Green vegetative palmella cell;(C) Astaxanthin accumulating palmella cell in transition to aplanospore; (D) Astaxanthin accumulated aplanospore cell. ...

Figure 2
Life cycle of H. pluvialis. (A) Fluorescence microscopy images, showing the 1- to 32-cell stages, and the flagellated stage. DIC, differential interference contrast image; SYBR, SYBR Green I-stained cells (green); CHL, chlorophyll autofluorescence (red); ...

Figure 3
Gametogenesis in H. pluvialis. (A) C, cyst; G, gametocyst; S, a sporocyst; (B) vegetative zoospore, F, flagella (indicated by arrow); (C): gametocyst before releasing gametes; (D): release of gametes from gametocyst. Reproduced with permission from Triki ...
Ultrastructural changes of H. pluvialis during the life cycle
In H. pluvialis cells, large amount of ultrastructural changes occurs during their life cycle which is frequently associated with responses to stress conditions in the environment. In the green vegetative palmella cells, a thick cell wall surrounds the cell, and two layers of extracellular matrix present near the cell wall (Figure ​(Figure4A).4A). Nucleus is located in the center of the cell, and highly developed chloroplasts are located at the periphery (Figure ​(Figure4A).4A). Few astaxanthin granules are present which located around the nucleus (Figure ​(Figure4A).4A). Conspicuous pyrenoids with electron-dense matrix can be found in the stroma (Figures 4A,B) (Wayama et al., 2013). During the starting of encystment process, Haematococcus turned into greenish-orange cells (with some astaxanthin accumulation) which can be referred as intermediate stage cells. In this stage, conspicuous pyrenoids with electron-dense matrix located in the stroma, remain surrounded by thick starch capsules and many starch grains are located around the pyrenoids (Figure ​(Figure5).5). Round oil droplets with various sizes, containing astaxanthin located around the nucleus (Figure ​(Figure5).5). At this stage, thylakoids become partial degraded (Wayama et al., 2013). With the gradual accumulation of astaxanthin chloroplast reduce in volume but photosynthetic activity remains. In the aplanosopre or cyst stage, astaxanthin accumulates and cells form cysts. Oil droplets and astaxanthin accumulation patterns may differ among cyst cells. For example, electron-dense astaxanthin granules in oil droplets (Figures 6A,B) occurred in some cells. In other cells, relatively large oil droplets occurred throughout the cell (Figures 6C,D). Chloroplasts are degenerated and localized in the interspace between oil droplets (Wayama et al., 2013).

Figure 4