In the present study, eight kinds plant growth regulators—salicylic acid (SA), 1-naphthaleneacetic acid (NAA), gibberellic acid (GA₃), 6-benzylaminopurine (6-BA), 2, 4-epi-brassinolide (EBR), abscisic acid (ABA), ethephon (ETH), and spermidine (SPD)—were used to investigate the impact on microalgal biomass, lipid, total soluble protein, carotenoids, and polyunsaturated fatty acids (PUFAS) production of Chlorella pyrenoidosa ZF strain. The results showed the quickest biomass enhancement was induced by 50 mg·L⁻¹ NAA, with a 6.3-fold increase over the control; the highest protein content was increased by 0.005 mg·L⁻¹ ETH, which produced 3.5-fold over the control; total carotenoids content was induced most effectively by 1 mg·L⁻¹ NAA with 3.6-fold higher production than the control; the most efficient elicitor for lipid production was 5 mg·L⁻¹ GA₃ at 1.9-fold of the control; 0.2 mg·L⁻¹ ETH induced the abundant production of 1.82 ± 0.23% linoleic acid; 0.65 ± 0.01% linolenic acid was induced by 1 mg·L⁻¹ NAA; 2.53 ± 0.15% arachidonic acid and 0.44 ± 0.05% docosahexaenoic acid were induced by 5 mg·L⁻¹ GA₃. Transcriptional expression levels of seven lipid-related genes, including ACP, BC, FAD, FATA, KAS, MCTK, and SAD, were studied by real-time RT-q-PCR. 5 mg·L⁻¹ GA₃ was the most effective regulator for transcriptional expressions of these seven genes, producing 23-fold ACP, 31-fold BC, 25-fold FAD, 6-fold KAS, 12-fold MCTK compared with the controls, respectively.

Microalgae produce a variety of high-value products. Enhancing product contents in microalgal cells is one of the efficient ways to decrease production costs. Improved germplasm and heterotrophic cultivation may enhance microalgae biomass and lipid content. In this study, we investigated the effect of three types of laser irradiation and heterotrophic cultivation on lipid productivity, lipid content, and biomass of two Chlorella strains (i.e., FACHB 9 and FACHB 31). Results showed that the highest biomasses of 4.81 g/L (15.03-fold) and 4.66 g/L (7.32-fold) were obtained in the third generation of FACHB 9 and FACHB 31 induced by a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser for 8 min and 12 min, respectively. The highest lipid contents were 525.6 mg/g (1.67-fold) dry weight (DW) and 780.0 mg/g DW (2.20-fold) in the third and the first generations of FACHB 9 and FACHB 31 induced by Nd:YAG for 8 min and by a helium–neon (He–Ne) laser for 4 min, respectively. The highest lipid productivities of 69.82 ± 3.29 mg/L/d (19.7-fold) and 30.71 ± 3.77 mg/L/d (3.1-fold) were obtained in FACHB 9 and FACHB 31 treated by a semiconductor (SC) laser for 4 min and by a He–Ne laser for 12 min, respectively. Our study suggested that laser mutagenesis is a potential method for screening economically important oleaginous microalgae strains.

Cold stress causes changes in the lipid composition of Mortierella alpina. In order to investigate the molecular mechanisms of M. alpina in response to cold stress, transcriptome analyses were performed on three groups of M. alpina cultured at (1) 25 °C, (2) 15 °C, and (3) first at 25 °C for 2 days and then 15 °C for 7 days, respectively. The results of a gas chromatography–mass spectrometry (GC–MS) analysis suggested that, compared with 25 °C conditions, dry weight and lipid production were significantly decreased in M. alpina grown at 15 °C, with a total of 1552 differentially expressed genes (DEGs) identified in response to cold stress. The quantitative real-time PCR (qRT-PCR) analysis was conducted to verify the expression patterns of six DEGs involved in lipid metabolism. Results of the enrichment analyses of the DEGs based on the Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases showed that fatty acid compositions were regulated by genes involved in the glycolysis pathway, fatty acid synthesis, the pentose phosphate pathway, the glycerolipid pathway, the tricarboxylic acid (TCA) cycle, and the glycerophospholipid pathway. Our study provided solid experimental evidence and novel insights into the metabolic engineering and the molecular mechanisms regulating the response to cold stress in M. alpina.