(1) Background: Microalgae cultures are greatly facilitated if growers can easily and economically ascertain the quantitative and qualitative status of the culture continuously with satisfactory ac-curacy. (2) Methods: The locally isolated microalgae Dunaliella sp. and Anabaena sp. were cultured in small volumes with 2 intensities of white light (2000 and 8000 lux) and with green, blue and red light and the increase of their biomass and pigments was studied. Pigment analyses, continuous recordings of absorption spectra and calibration curves were performed. (3) Results: The intensity of 8000 lux white light yielded the highest increase in biomass, chlorophylls and carotenoids in Dunaliella sp. while 2000 lux and green light caused the greatest increase in biomass and phy-cocyanin in Anabaena sp. From the examination of the absorption spectra, the evolution of the pigment content can be estimated and both pigments and biomass are correlated very satisfactorily with the wavelength of 750 nm. (4) Conclusions: The use of absorption spectra as an easy, fast and economical method can be a useful tool for a good approximation of the state of the microalgae culture. This is clearly shown when the spectra of the culture under different lighting colors are compared which have a catalytic effect on the level of the pigments leading to the increase in carotenoids and phycocyanin of the green light.

Phytoene is a colourless natural carotenoid that absorbs UV light and provides antioxidant and anti-inflammatory activities as well as protection against photodamage. It is therefore valued for its skin health and aesthetic benefits by the cosmetic industry, as well as by the health food sector. The halotolerant green microalga Dunaliella salina is one of the richest sources of natural carotenoids. We have previously investigated over-production of phytoene in D. salina after cultivation with the well-characterised mitosis inhibitor, chlorpropham. In this study, 15 herbicides with different modes of action were tested for their potential to promote phytoene accumulation. All herbicides showed different levels of capabilities to support phytoene over-production in D. salina. Most significantly, the two mitosis inhibitors tested in this study, propyzamide and chlorpropham, showed similar capacities to support over-production of phytoene by D. salina cultures, as phytoene desaturase inhibitors. The cellular content of phytoene increased by over 10-fold within 48 h after treatment with the mitosis inhibitors compared to untreated cultures. Results indicate a general effect of mitosis inhibitors on phytoene accumulation in D. salina. Furthermore, red light was found to significantly enhance the phytoene yield when used in combination with effective inhibitor treatments. Red light can be applied to maximize the production of phytoene from D. salina.

This study investigated the adaptability of Dunaliella salina to different salinity levels, with emphasis on growth, pigment concentration, and desalination potential. It was found that among the 21 salinity levels, salinity 75 produced consistently favorable results in cell count (13.08 x 10³ ± 1.41 x 10³ cells/mL), dry biomass (2.46 ± 0.06 g/L), pigment content (chlorophyll a = 97.5 ± 0.1 µg/L, chlorophyll b = 123.6 ± 0.3 µg/L), and desalination (9.32 ± 0.47 reduction). Therefore, salinity 75 was selected for the final trial (scale-up), which revealed unanticipatedly high cell counts (58.96 x 10³ ± 535.22 cells/mL), with dry biomass weight being statistically different (higher) than the expected (4.21 ± 0.02 g/L) (p<0.0001), most likely due to high cell count and energy reserve storage for high salinity adaption in the form of bio-compounds. Pigment growth continued (chlorophyll a = 95.4 ± 2.2 µg/L, chlorophyll b = 128.1 ± 5.1 µg/L), indicating pigment production under salt stress. Notably, desalination did not occur in this stage, possibly due to the necessity for a bigger initial inoculate, prolonged exposure or bioaccumulation becoming the prevailing mechanism over desalination. Nevertheless, the trial highlights D. salina's strong adaptation to various salinity levels. This suggests a promising future in halophyte research, particularly in understanding the mechanisms that prevent salt accumulation in cells and how to overcome this barrier. Additionally, these results suggest that microalgae could be a viable resource in saline-rich environments unsuitable for conventional agriculture, promoting industrial adaptation to adverse conditions.

After a 1.5 year screening survey in the lagoons of Western Greece in order to isolate and culture sturdy species of microalgae for aquaculture or other value added uses, as dictated primarily by a satisfactory potential for their mass-culture, five species emerged and their growth was monitored in laboratory conditions. Amphidinium carterae, Nephroselmis sp., Tetraselmis sp. (var. red pappas), Asteromonas gracilis and Dunaliella sp. were batch cultured using low (20 ppt), sea (40 ppt) and high salinity (50 or 60 or 100 ppt) and in combination with a low (2000 lux) and high (8000 lux) intensity of illumination. The results exhibited that all these species can be grown adequately in all salinities and with best growth in terms of maximum cell density, specific growth rate (SGR) and biomass yield (g dry weight/L) at high illumination (8000 lux). The five species examined exhibited different responses in the salinities used, Amphidinium clearly does best in 20 ppt far better than 40 ppt and even more than 50 ppt. Nephroselmis and Tetraselmis grow almost the same in 20 and 40 ppt and less well in 60 ppt. Asteromonas does best in 100 ppt although it can grow quite well in both 40 and 60 ppt. Dunaliella grows equally well in all salinities (20-40-60 ppt). Concerning productivity as maximum biomass yield at the end of the culture period, first rank is occupied by Nephroselmis with ~3.0 g d.w./L, followed by Tetraselmis (2.0 g/L), Dunaliella (1.58 g/L), Amphidinium (1.19 g/L) and Asteromonas (0.7 g/L) with all values recorded at high light (8000 lux).

The halotolerant photoautotrophic marine microalga Dunaliella salina is one of the richest sources of natural carotenoids. Here we investigated the effects of high intensity blue, red and white light from light emitting diodes (LED) on the production of carotenoids by strains of D. salina under nutrient sufficiency and strict temperature control favouring growth. Growth in high intensity red light was associated with carotenoid accumulation and a high rate of oxygen uptake. On transfer to blue light, a massive drop in carotenoid content was recorded along with very high rates of photo-oxidation. In high intensity blue light, growth was maintained at the same rate as in red or white light, but without carotenoid accumulation; transfer to red light stimulated a small increase in carotenoid content. The data support chlorophyll absorption of red light photons to reduce plastoquinone in photosystem II, coupled to phytoene desaturation by plastoquinol:oxygen oxidoreductase, with oxygen as electron acceptor. Partitioning of electrons between photosynthesis and carotenoid biosynthesis would depend on both red photon flux intensity and phytoene synthase upregulation by the red light photoreceptor, phytochrome. Red light control of carotenoid biosynthesis and accumulation reduces the rate of formation of reactive oxygen species (ROS) as well as increases the pool size of anti-oxidant.

The halotolerant microalga Dunaliella salina has been widely studied for natural β-carotene production. This work shows biochemical characterization of three newly isolated Dunaliella salina strains DF15, DF17 and DF40 compared with D. salina CCAP 19/30 (confirmed to be D. tertiolecta) and D. salina UTEX 2538 (also known as D. bardawil). Although all three new strains have been genetically characterized as Dunaliella salina strains, their ability to accumulate carotenoids and their capacity for photoprotection against high light stress are different. DF15 and UTEX 2538 reveal great potential for producing large amount of β-carotene and maintained a high rate of photosynthesis under light of high intensity; however, DF17, DF40 and CCAP 19/30 showed increasing photoinhibition with increasing light intensity, and reduced contents of carotenoids, in particular b-carotene, suggesting that the capacity of photoprotection is dependent on the cellular content of carotenoids, in particular β-carotene. Strong positive correlations were found between the cellular content of each of all-trans β-carotene, 9-cis β-carotene, all-trans α-carotene and zeaxanthin but not lutein in the D. salina strains. Lutein was strongly correlated with respiration in photosynthetic cells and strongly related to photosynthesis, chlorophyll and respiration, suggesting an important and not hitherto identified role for lutein in co-ordinated control of the cellular functions of photosynthesis and respiration in response to changes in light conditions, which is broadly conserved in Dunaliella strains. Statistical analysis based on biochemical data revealed a different grouping strategy from the genetic classification of the strains. The significance of these data for strain selection for commercial carotenoid production is discussed.

Dunaliella salina is a rich source of 9-cis β-carotene, which has been identified as important in the treatment of retinal dystrophies and other diseases. We previously showed that chlorophyll absorption of red light photons in D. salina is coupled to oxygen reduction and phytoene desaturation and increases the pool size of β-carotene [1]. Here we show for the first time that growth under red light also controls conversion of extant all-trans β-carotene to 9-cis β-carotene by β-carotene isomerases. Cells illuminated with red light from a light emitting diode (LED) during cultivation contained a higher 9-cis β-carotene content compared to cells illuminated with white or blue LED light. The 9-cis/all-trans β-carotene ratio in red light treated cultures reached >2.5:1 within 48 hours and was independent of light intensity. Illumination using red light filters that eliminated blue wavelength light also increased the 9-cis/all-trans β-carotene ratio. With norflurazon, a phytoene desaturase inhibitor which blocked downstream biosynthesis of β-carotene, extant all-trans β-carotene was converted to 9-cis β-carotene during growth with red light and the 9-cis/all-trans β-carotene ratio was ~2:1. With blue light under the same conditions, 9-cis β-carotene was likely destroyed at a greater rate than all-trans β-carotene (9-cis/all-trans ratio 0.5:1). Red light perception by the red light photoreceptor, phytochrome, may increase the pool size of anti-oxidant, specifically 9-cis β-carotene, both by upregulating phytoene synthase to increase the rate of biosynthesis of β-carotene and to reduce the rate of formation of reactive oxygen species (ROS), and by upregulating β-carotene isomerases to convert extant all-trans β-carotene to 9-cis β-carotene.

The role of foods is shifting from focusing on the provision of energy and basic nutrients to also include bioactive compounds that prevent the development of chronic age-related diseases. Mi-croalgae offer a source of nutritional compounds with important health effects, and have the po-tential for production of health-promoting products, without increasing agricultural land use and negatively impacting the environment. Here we investigate the health effects of microalgal ex-tracts with high levels of the colourless carotenoid phytoene. Phytoene is widely available from dietary sources and can be detected across different tissues in the human body. However, it is usually regarded as a precursor for the synthesis of other carotenoids, rather than a bioactive molecule. We utilised the model organism C. elegans to show that phytoene-rich extracts from Chlorella sorokiniana and Dunaliella bardawil have anti-ageing properties. The extracts protect against oxidative damage and amyloid-β42 proteotoxicity (a major pathology of Alzheimer's disease), and extends lifespan. We show that pure phytoene also has these anti-ageing effects, suggesting that phytoene is a bioactive molecule with positive effects on ageing and longevity.