Dunaliella salina is an important microalga for β-carotene production. In Thailand, the D. salina has not been commercially cultivated. Aim of this study was to screen high β-carotene producing mutants. D. salina KU11 cells were radiated with 0.4 mmol photons m-2 •s-1 UV-C for 22 min. After 3 weeks, colonies which survived after radiation were allowed to grow on agar plates; fifty colonies out of 19,720 UV-mutagenized strains turned to orange. These colonies were picked up and transfer to modified Johnson medium. Four mutants out of fifty were KU5, KU18, KU20 and KU37 significantly produced high β-carotene at the stationary phase approximately up to 1.62, 2.03, 1.32 and 1.21 folds, respectively, compared to the wild type strain. In all three consecutive cycles of cultivation, the mutant KU18 accumulated 2.4-8.5 folds β-carotene (µg/ml) and 2.1-4.8 folds β-carotene (pg/cell) higher than the wild type, whereas cell density of the mutant KU18 was not different from the wild type. This is the new record of b-carotene producing mutant isolated from saline soil in Thailand.
An isolated <em>Dunaliella salina</em> (<em>D. salina</em>) KU XI from saline soils in northeastern Thailand was cultured in f/2 medium in column photobioreactor. The variations of the growth, chlorophyll and beta-carotene content and the maximum quantum yield of PS II photochemistry (F<sub>v</sub>/F<sub>m</sub>) under different NaH<sub>2</sub>PO<sub>4</sub> concentrations were studied. Based on the results, the growth kinetics of <em>D. salina</em> KU XI was established, which could simulate the algae growth rate under different phosphate concentrations and temperatures. The phosphorus could significantly affect the growth and pigments accumulations of this isolated strain. Increasing NaH<sub>2</sub>PO<sub>4</sub> concentration improved the biomass, the total chlorophyll and beta-carotene content, retarded the decrease of F<sub>v</sub>/F<sub>m</sub> value. The optimal phosphate concentration for the growth of <em>D. salina</em> KU XI was above 72.6 μM. The maximum biomass and beta-carotene were 0.24 g L<sup>-1</sup> and 17.4 mg L<sup>-1</sup> respectively when NaH<sub>2</sub>PO<sub>4</sub> was 290.4 μM. The algae growth was restrained by phosphate or nitrate when NaH<sub>2</sub>PO<sub>4</sub> below 12.1 μM or above 72.6 μM. It indicated that properly supplementing nitrate in the late growth stage with high phosphate concentration was favored for enhancing the growth and biomass production.
Marigold (Tagetes erecta L.) is a perennial edible medicinal plant. In order to test the allelopathic effect of marigold leaf extract on Chlorella vulgaris, the microalgae growth conditions under different concentrations of marigold leaf extract (0 to 50 g/l) were studied. The results showed that microalgal growth was inhibited by marigold leaf extract, and inhibition commonly increased with increasing concentration of the extract and culture time. The maximum inhibition rate (90.1%) appeared in leaf extract of 30 g/l after 14 days of incubation; marigold leaf extract induced chlorophyll degradation in algae cells, increased the permeability of cell membrane, and caused exudation of soluble protein and nucleic acid, leading to the damage of algae cell structure and metabolic dysfunction. The present results confirmed that marigold leaf extract has an allelopathic inhibition effect on C. vulgaris, and the results could be further applied to develop safe and efficient algaecides.
To evaluate the suitability of wastewater on the growth of alga Spirulina, reclaimed wastewaters, fishpond wastewater, industrial wastewater and mariculture water were used to culture Spirulina subsalsa and Spirulina platensis. S. subsalsa showed better adaptability to fishpond wastewater and higher specific growth rate than that of S. platensis. Thus, effects of factors such as the amount of baking soda, inoculation, and sodium nitrate on the growth, biomass and protein content of S. subsalsa were evaluated through the single factors design. Results showed that S. subsalsa had good growth under conditions of 4-12 g/L baking soda, 1-2 g/L NaNO 3 , 25-40°C, and 6000-12000 lux illumination. The maximum dry biomass and protein content were 3.48 g/L and 33.08%, respectively, suggesting that it was feasible to culture S. subsalsa in freshwater aquaculture wastewater.
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