A β-carotene ketolase gene ( bkt1 ) promoter regulated by sodium acetate and light in a model green microalga Chlamydomonas reinhardtii

Simultaneous accumulation of astaxanthin and β-carotene in Chlamydomonas reinhardtii by the introduction of foreign β-carotene hydroxylase gene in response to high light stress

Carotenoids are important photosynthetic pigments with many physiological functions, nutritional properties and high commercial value. β-carotene hydroxylase is one of the key enzymes in the carotenoid synthesis pathway of Chlamydomonas reinhardtii for the conversion of β-carotene to astaxanthin. The vector p64DZ containing the β-carotene hydroxylase gene crtZ from Haematococcus pluvialis was transformed into C. reinhardtii CC-503. The transformants were selected by alternate culture in solid-liquid medium containing spectinomycin (100 µg mL^-1). PCR results indicated that the gene crtZ and aadA were integrated into the genome of C. reinhardtii. RT-PCR analysis showed that the gene crtZ was transcribed in Chlamydomonas transformants. HPLC analysis showed that the content of astaxanthin and β-carotene in cells of C. reinhardtii were simultaneously increased. Under medium light intensity cultivation (60 µmol m^-2 s^-1), transgenic C. reinhardtii had an 85.8% increase in β-carotene content compared with the wild type. The content of astaxanthin and β-carotene reached 1.97 ± 0.13 mg g^-1 fresh cell weight (FCW) and 105.94 ± 5.84 µg g^-1 FCW, which were increased 18% and 42.4% than the wild type after 6 h of high light treatment (200 µmol m^-2 s^-1), respectively. Our results indicate the regulatory effect on pigments in C. reinhardtii by β-carotene hydroxylase gene of H. pluvialis, and demonstrate the positive effect of high light stress on pigment accumulation in transgenic C. reinhardtii.

Transcription Factors From Haematococcus pluvialis Involved in the Regulation of Astaxanthin Biosynthesis Under High Light-Sodium Acetate Stress

The microalgae Haematococcus pluvialis attracts attention for its ability to accumulate astaxanthin up to its 4% dry weight under stress conditions, such as high light, salt stress, and nitrogen starvation. Previous researches indicated that the regulation of astaxanthin synthesis might happen at the transcriptional level. However, the transcription regulatory mechanism of astaxanthin synthesis is still unknown in H. pluvialis. Lacking studies on transcription factors (TFs) further hindered from discovering this mechanism. Hence, the transcriptome analysis of H. pluvialis under the high light-sodium acetate stress for 1.5 h was performed in this study, aiming to discover TFs and the regulation on astaxanthin synthesis. In total, 83,869 unigenes were obtained and annotated based on seven databases, including NR, NT, Kyoto Encyclopedia of Genes and Genomes Orthology, SwissProt, Pfam, Eukaryotic Orthologous Groups, and Gene Ontology. Moreover, 476 TFs belonging to 52 families were annotated by blasting against the PlantTFDB database. By comparing with the control group, 4,367 differentially expressed genes composing of 2,050 upregulated unigenes and 2,317 downregulated unigenes were identified. Most of them were involved in metabolic process, catalytic activity, single-organism process, single-organism cellular process, and single-organism metabolic process. Among them, 28 upregulated TFs and 41 downregulated TFs belonging to 27 TF families were found. The transcription analysis showed that TFs had different transcription modules responding to the high light and sodium acetate stress. Interestingly, six TFs belonging to MYB, MYB_related, NF-YC, Nin-like, and C3H families were found to be involved in the transcription regulation of 27 astaxanthin synthesis-related genes according to the regulatory network. Moreover, these TFs might affect astaxanthin synthesis by directly regulating CrtO, showing that CrtO was the hub gene in astaxanthin synthesis. The present study provided new insight into a global view of TFs and their correlations to astaxanthin synthesis in H. pluvialis.

Interactions between polystyrene nanoparticles and Chlamydomonas reinhardtii monitored by infrared spectroscopy combined with molecular biology

For several decades, use of nanoparticles (NP) on a global scale has been generating new potential sources of organism disruption. Recent studies have shown that NP can cause modifications on the biochemical macromolecular composition of microalgae and raised questions on the toxicity of plastic particles, which are widespread in the aquatic environment. Polystyrene (PS) particles are among the most widely used plastics in the world. In our experimentation, a combined approach of infrared spectroscopy and molecular biology (real-time PCR) has been applied in order to better apprehend the consequences of interactions between Chlamydomonas reinhardtii, freshwater microalgae and PS NP. Two references have been used, nitrogen deprivation -a well-documented stressor-, and gold nanoparticles (Au-NP). As regards biochemical composition, our experiments show a differing microalga response, according to the NP to which they have been exposed. Results with infrared spectroscopy and gene expression methods are consistent and illustrate variation among several carbohydrates (galactose…). Furthermore, PS-NP seem to react in the same direction as nitrogen limitation, thereby supporting the hypothesis that PS-NP can induce response mechanisms to environmental changes in microalgae. This study highlighted the interest of combining infrared spectroscopy and gene expression as means of monitoring microalgae response to nanoplastics.

Functional Characterization of a Missing Branch Component in Haematococcus pluvialis for Control of Algal Carotenoid Biosynthesis

Cyclization of acyclic lycopene by cyclases marks an important regulatory point in carotenoid biosynthesis. Though some algal lycopene epsilon cyclases (LCYEs) have been predicted computationally, very few have been functionally identified. Little is known about the regulation mechanisms of algal LCYEs. Recent comparative genomic analysis suggested that Haematococcus pluvialis contained only the β type cyclase (HpLCYB). However, in this study, carotenoid profiling found trace α-carotene in the salt-treated cells, indicating the in vivo activity of HpLCYE, a missing component for α-branch carotenoids. Thus, genes coding for HpLCYB and HpLCYE were isolated and functionally complemented in Escherichia coli. Substrate specificity assays revealed an exclusive cyclization order of HpLCYE to HpLCYB for the biosynthesis of heterocyclic carotenoids. Expression pattern studies and bioinformatic analysis of promoter regions showed that both cyclases were differentially regulated by the regulatory cis-acting elements in promoters to correlate with primary and secondary carotenoid biosynthesis under environmental stresses. Characterization of the branch components in algal carotenoid biosynthesis revealed a mechanism for control of metabolic flux into α- and β-branch by the competition and cooperation between HpLCYE and HpLCYB; and supplied a promising route for molecular breeding of cyclic carotenoid biosynthesis.

Artificial miRNA inhibition of phosphoenolpyruvate carboxylase increases fatty acid production in a green microalga Chlamydomonas reinhardtii

BACKGROUND

Nutrient limitation, such as nitrogen depletion, is the most widely used method for improving microalgae fatty acid production; however, these harsh conditions also inhibit algal growth significantly and even kill cells at all. To avoid these problems, we used artificial microRNA (amiRNA) technology as a useful tool to manipulate metabolic pathways to increase fatty acid contents effectively in the green microalga Chlamydomonas reinhardtii. We down-regulated the expression of phosphoenolpyruvate carboxylase (PEPC), which catalyzes the formation of oxaloacetate from phosphoenolpyruvate and regulates carbon flux.

RESULTS

amiRNAs against two CrPEPC genes were designed and transformed into Chlamydomonas cells and amiRNAs were induced by heat shock treatment. The transcription levels of amiRNAs increased 16-28 times, resulting in the remarkable decreases of the expression of CrPEPCs. In the end, inhibiting the expression of the CrPEPC genes dramatically increased the total fatty acid content in the transgenic algae by 28.7-48.6%, which mostly increased the content of C16-C22 fatty acids. Furthermore, the highest content was that of C18:3n3 with an average increase of 35.75%, while C20-C22 fatty acid content significantly increased by 85-160%.

CONCLUSIONS

Overall our results suggest that heat shock treatment induced the expression of amiRNAs, which can effectively down-regulate the expression of CrPEPCs in C. reinhardtii, resulting in an increase of fatty acid synthesis with the most significant increase occurring for C16 to C22 fatty acids.