Abstract
The halotolerant chlorophyte microalga, Dunaliella salina, is one of the richest sources of carotenoids and will accumulate up to 10% of the dry biomass as β-carotene, depending on the integrated amount of light to which the alga is exposed during a division cycle. Red light also stimulates β-carotene production, as well as increases the 9-cis β-carotene/all-trans β-carotene ratio. In this paper we investigated the effects of chlorpropham (Iso-propyl-N (3-chlorophenyl) carbamate, CIPC), with and without red light, on carotenoid accumulation. Chlorpropham is a well-known carbamate herbicide and plant growth regulator that inhibits mitosis and cell division. Chlorpropham arrested cell division and induced the massive accumulation of colourless phytoene and phytofluene carotenoids, and, to a much lesser extent, the coloured carotenoids. The chlorophyll content also increased. When phytoene per cell accumulated to approximately the same level with chlorpropham as with norflurazon, a phytoene desaturase inhibitor, coloured carotenoids and chlorophyll increased with chlorpropham but decreased with norflurazon. Cultivation with chlorpropham under red LED light for 2 days did not affect the content of β-carotene, but phytoene was 2.4-fold the amount that was obtained in cultures under white LED light and the 9-cis β-carotene/all-trans β-carotene ratio increased from 1.3 to 1.8. With norflurazon, red LED light boosted the contents of both phytoene and β-carotene, 2-fold and 1.3-fold respectively compared to those under white LED light, and the ratio of 9-cis β-carotene/all-trans β-carotene reached 3.8. The results are discussed in terms of disruption by chlorpropham of synchronised control between nuclear and chloroplast events associated with carotenoid biosynthesis. Since phytoene and phytofluene are colourless carotenoids which are sought after for the development of nutricosmetics and other health/beauty products, the results also present as a new method with low toxicity for production of colourless carotenoids based on the cultivation of D. salina.
Highlights
Carotenoids are conjugated isoprenoids with a long system of con jugated double bonds and are synthesised by all photosynthetic organ isms for light-harvesting and for photo-protection
Norflurazon, at the optimum concentration for phytoene accumulation caused by phytoene desaturase (PDS) inhibition (5 μM determined by prior experiment), by contrast, had no effect on cell division, as ex pected
Since total caroten oids increased when cell division was arrested with chlorpropham (Table 1 and Table 2), and increased with increasing light intensity (Fig. 3), these data confirmed that the amount of phytoene [35] or β-carotene [11,17,23,32,35] that accumulates in D. salina is positively correlated with the integral irradiance to which the cells are exposed during a division cycle
Summary
Carotenoids are conjugated isoprenoids with a long system of con jugated double bonds and are synthesised by all photosynthetic organ isms for light-harvesting and for photo-protection. Phytoene and phytofluene are among the few carotenoids that are colourless and may provide antioxidant activity, anticarcinogenic ac tivity, anti-inflammatory activity, and protection against UV-induced damage, with phytoene absorbing maximally in the UVB region and phytofluene in the UVA region [7,8]. They can be ingested or topically applied and are of great interest in the nutricosmetic field for their skin health and aesthetic benefits. Red light has been shown to promote carotenoid accumulation [21,22]
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