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Microalgae are a commercially viable route for the production of carotenoids. Chlorella sp. BR2 was treated with plant hormones indole‑3‑acetic acid, salicylic acid, abscisic acid and methyl jasmonate and screened for enhanced carotenoid production. Indole‑3‑acetic acid was the only hormone with an inductive effect on carotenoid accumulation. As such, transcriptome-wide changes following auxin treatment were profiled using RNA-Seq and expressed sequences reconstructed with de novo assembly. This revealed the active pathway components of auxin-induced carotenogenesis. Data analysis specified the differentially expressed genes involved in auxin biosynthesis and signal transduction, which hint at close, yet unique relationship to equivalent pathways in higher plants. Unlike in plants, the ancient ABP1/SCFSKP2A/IBR5-mediated pathways for auxin response likely acted as the primary signaling route in Chlorella. As carotenoids are precursors for abscisic acid and antagonists of reactive oxygen species, the findings suggest a potential link between auxin signaling and abiotic stress tolerance.

Haematococcus pluvialis accumulates large amounts of astaxanthin during its haematocyst stage. Cyst germination is a biological pretreatment method for improved astaxanthin extraction with potential to replace energy-intensive homogenizer-based mechanical cell cracking methods. The present study demonstrates effects of different LED light wavelengths on haematocyst germination and on its astaxanthin biosynthesis pathway. Blue light enhanced the germination efficiency and slowed the nitrogen consumption rate, resulting in significantly higher astaxanthin content and improved extractability compared with white and red lights, that also had lower germination rates. After 5 days, the total astaxanthin extractability under blue light was 4.0 and 6.7 times higher than for white and red lights, respectively. The blue light receptor gene phot was significantly induced which upregulated the biosynthesis pathway genes psy and pds, as well as dgat1 and dgat2d. Hence, blue light triggers germination and astaxanthin biosynthesis, providing a strategy for improved extraction while modulating higher biosynthesis during germination.

Lutein is a metabolite produced by plants or microalgae, and offers various health benefits. Chlorella sorokiniana FZU60 is considered a hyper-lutein producing microalga. In this study, the effects of temperature and light intensity on cell growth and lutein accumulation by FZU60 under mixotrophic conditions were investigated. Maximal lutein productivity and production were obtained at 33 °C, and high light intensity enhanced cell concentration but decreased lutein content when nitrate was replete. Further, two fed-batch strategies with gradient (fed-batch strategy I) or constant (fed-batch strategy II) NaAc feeding concentrations were compared. Results showed that the second strategy was more desirable for lutein production. Moreover, two types of mixotrophy/photoautotrophy two-stage strategies with a light intensity of 750 μmol/m2/s (two-stage strategy I) or 150 μmol/m2/s (two-stage strategy II) in stage 2 were explored. Under the two-stage strategy I, high lutein content (11.22 mg/g), productivity (8.25 mg/L/d), and production (65.96 mg/L) were achieved. The lutein content, productivity, and production obtained in this study are higher than those reported in most related studies, indicating that the proposed strategy can be used in commercial production.

Temperature has been regarded as an important factor for microalgal growth and the accumulation of lutein and fatty acid. In this study, the lutein and fatty acid producing microalga Chlamydomonas sp. JSC4 was cultivated at various temperatures (20, 25, 30, 35, and 40 °C). Maximum biomass productivity (1271 mg/L/d) and lutein productivity (3.27 mg/L/d) were achieved at 35 °C, whereas the highest lutein content (3.82 mg/g) was obtained at 20 °C. Similarly, maximum total fatty acid (TFA) productivity (177.31 mg/L/d) was obtained at 35 °C, yet TFA content (202.56 mg/g) was highest at 20 °C with increased unsaturated fatty acid proportion. It was observed that carotenoid, fatty acid, and protein contents increased, but carbohydrate content decreased at low temperature, which could be due to the modulation of metabolic flux from carbohydrate to carotenoid, fatty acid and protein. Inspection of gene expression profile showed that the lutein biosynthesis genes (psy, pds, and lut1) and fatty acid biosynthesis genes (bccp and bc) were upregulated at 20 °C when nitrate was replete and at the beginning of nitrate depletion. This study will help to develop temperature manipulation strategy and provide molecular basis of genetic engineering for improvement of the co-production of lutein and fatty acid in strain JSC4.