Abstract
The unicellular microalga Haematococcus pluvialis has emerged as a promising biomass feedstock for the ketocarotenoid astaxanthin and neutral lipid triacylglycerol. Motile flagellates, resting palmella cells, and cysts are the major life cycle stages of H. pluvialis. Fast-growing motile cells are usually used to induce astaxanthin and triacylglycerol biosynthesis under stress conditions (high light or nutrient starvation); however, productivity of biomass and bioproducts are compromised due to the susceptibility of motile cells to stress. This study revealed that the Photosystem II (PSII) reaction center D1 protein, the manganese-stabilizing protein PsbO, and several major membrane glycerolipids (particularly for chloroplast membrane lipids monogalactosyldiacylglycerol and phosphatidylglycerol), decreased dramatically in motile cells under high light (HL). In contrast, palmella cells, which are transformed from motile cells after an extended period of time under favorable growth conditions, have developed multiple protective mechanisms—including reduction in chloroplast membrane lipids content, downplay of linear photosynthetic electron transport, and activating nonphotochemical quenching mechanisms—while accumulating triacylglycerol. Consequently, the membrane lipids and PSII proteins (D1 and PsbO) remained relatively stable in palmella cells subjected to HL. Introducing palmella instead of motile cells to stress conditions may greatly increase astaxanthin and lipid production in H. pluvialis culture.
Highlights
Astaxanthin is a superb antioxidant and a natural food coloring agent that has been used in nutraceutical, aquaculture, and poultry industries [1,2]
Under the favorable low light (LL) condition, the motile cells were usually pear-shaped with a pair of flagellae at the anterior end, and the protoplast of the motile cell was enclosed by a swollen, gelatinous extracellular matrix (Fig. 1A)
When H. pluvialis cells were cultivated under favorable growth conditions for an extended period of time (e.g., 3–5 days), the motile cells lost their flagellae and became palmella cells with thickened cell walls
Summary
Astaxanthin is a superb antioxidant and a natural food coloring agent that has been used in nutraceutical, aquaculture, and poultry industries [1,2]. Among the naturally occurring organisms capable of producing astaxanthin, the unicellular microalga Haematococcus pluvialis can accumulate the largest amounts [up to 4% of its dry weight (DW)] under various adverse environmental or culture conditions [3]. Over the past two decades, mass culture of H. pluvialis in photobioreactors has been exploited to produce natural astaxanthin [4,5]. This organism has emerged as a promising cell factory for biofuels because of its ability to produce large amounts of neutral lipids, mainly in the form of triacylglycerol (TAG) [6]. Because of its great growth potential and high photosynthetic efficiency, H. pluvialis is an alternative solution for removing CO2 from fossil-fired power plants [7]
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