Nianjun Xu

This study evaluated the synergistic effects of sodium chloride (NaCl) and cytidine monophosphate (CMP) on enhancing the co-production of astaxanthin and fatty acids while suppressing secondary cell wall (SCW) formation in Haematococcus lacustris (synonym: H. pluvialis) under high-light stress. An orthogonal design identified the optimal combination (0.5 g/L NaCl and 0.5 mM CMP), which significantly increased astaxanthin yield by over 35.6% and total fatty acid yield by 28%, while maintaining 96.8% of cells in motile state (SCW-deficient). Physiological analyses revealed elevated reactive oxygen species levels, concomitant with higher actual photochemical efficiency (Fv′/Fm′) and relative electron transport rates II (rETR(II)) along with enhanced non-photochemical quenching (NPQ) capacity, and metabolic reprogramming characterized by the accumulation of lipids, sugars, and starch alongside decreased protein yield. Metabolomics indicated reduced carbon supply for SCW polysaccharide biosynthesis, coupled with decreased protein yield and altered amino acid profiles characteristic of nitrogen-limited metabolism, which collectively favored the reallocation of carbon resources toward nitrogen-free high-value products. Transcriptomics confirmed the downregulation of SCW component biosynthetic genes and the upregulation of the methylerythritol phosphate (MEP) pathway and astaxanthin biosynthetic pathway. Scale-up experiments validated this strategy for producing astaxanthin-rich motile cells, offering a promising approach for microalgal biorefinery.

Gracilariopsis lemaneiformis is the second most cultivated seaweed in China. In this study, the role of polyamines (PAs) in promoting carpospore release in G. lemaneiformis was investigated. The results showed that 300 μmol/L PAs (putrescine:spermine:spermidine 1:1:1) effectively increased carpospore release in G. lemaneiformis. Exogenous PAs influenced the photosynthetic system of G. lemaneiformis, with the photosynthetic capacity decreasing on the third day but largely recovering on the seventh day. Moreover, the contents of phycoerythrin (PE) and chlorophyll a (Chl a) significantly increased after three days. Additionally, the level of reactive oxygen species (ROS) significantly increased after PAs stimulation, and the activities of antioxidant enzymes and levels of antioxidant substances, such as superoxide dismutase (SOD), peroxidase (POD), malate dehydrogenase (MDH), catalase (CAT), glutathione (GSH), and malondialdehyde (MDA), also changed accordingly. The increase of auxin, cytokinin, and salicylic acid contents, and the decrease in gibberellin content indicate that PAs may promote carpospore release by regulating phytohormones. Transcriptome analysis revealed that after three days’ PAs treatment, the expression of photosynthesis- and antioxidant-related differentially expressed genes (DEGs) was increased. In addition, the MAPK signaling pathway and other pathways initiated by Ca²⁺ were activated, regulating the levels of ROS in G. lemaneiformis. The DEGs related to cell wall degradation and synthesis also indicated that PAs can regulate carpospore release. These findings confirm the role of PAs in promoting carpospore release, reveal the associated physiological changes and potentially involved signaling pathways, and provide new insights for further research and commercial application on G. lemaneiformis reproduction.

Haematococcus pluvialis is a microalga known for producing the red carotenoid, astaxanthin. Key research areas to improve productivity include optimizing vegetative biomass, enhancing astaxanthin content, and controlling secondary cell wall formation. In this study, RNA-seq was conducted on 96 H. pluvialis samples under various treatments and time points, generating 96.7 GB of high-quality data (2,080,511,173 clean reads, quality score > 30). Gene expression was quantified as transcripts per million (TPM), and genes with similar expression patterns were clustered. A co-expression network, constructed with a soft threshold of β = 7 (R² > 0.9), identified 39 co-expression modules, each averaging 732 genes. Additionally, the re-annotation process provided functional descriptions for 8,598 previously uncharacterized genes. Gene functional classification analysis revealed their involvement in metabolic pathways related to genetic information processing and primary/secondary metabolite metabolism. This dataset provides a valuable resource for exploring the molecular mechanisms involved in critical processes such as growth regulation, astaxanthin accumulation, and secondary cell wall synthesis in H. pluvialis.

Gracilariopsis lemaneiformis, as an economically important red alga, often suffers high-temperature stress which poses a threat to algal yield and even survival. It is important to achieve new varieties with stable traits and heat tolerance; hence, research on the discovery and functional analysis of high-temperature responsive genes of G. lemaneiformis is significant. MYB (v-myb avian myeloblastosis viral oncogene homolog) gene family is one of the largest transcription factor superfamilies in eukaryotes and has been proved to regulate multiple environmental stresses. However, the identification and comprehensive analysis of the MYB gene family in G. lemaneiformis has rarely been studied. In this study, we identified 18 MYB genes in G. lemaneiformis at the genome-wide level, including 15 1R-MYB/MYB-related, 2 R2R3-MYB, and 1 R1R2R3-MYB members. Among them, GlMYB3, GlMYB4, GlMYB8, GlMYB9, and GlMYB15 were remarkably induced under high temperature and were also upregulated by salicylic acid, methyl jasmonate, or abscisic acid. GlMYB3, GlMYB4, GlMYB9, and GlMYB15 were localized in the nucleus when transiently expressed in Nicotiana benthamiana plants. GlMYB4, as one of the most strongly induced high temperature-associated genes, showed transactivation activity, and the C-terminal was critical for the transactivation activity. By yeast two-hybrid screening, GlMYB4 may interact with three candidate proteins: calcineurin subunit B (CNB), O-linked N-acetylglucosamine transferase (OGT), and cleavage and polyadenylation specificity factor (CPSF) to modulate high temperature tolerance.

Background: Natural astaxanthin, a commercially valuable carotenoid, is primarily sourced from Haematococcus pluvialis, a microalga known for its remarkable resilience to environmental stress. Methods: In this study, the physiological and transcriptomic responses of H. pluvialis to ND were investigated at various time points under high light conditions. Results: Under high light conditions, nitrogen deprivation (ND) enhances astaxanthin content (33.23 mg g⁻¹) while inhibiting the formation of the secondary cell wall (SCW), increasing astaxanthin content by 29% compared to the nitrogen-replete group (25.64 mg g⁻¹); however, the underlying mechanisms remain unclear. ND reduced chlorophyll fluorescence parameters, elevated reactive oxygen species (ROS) levels, and increased starch and total sugar accumulation while decreasing protein and lipid content. Fatty acid content increased on the first day but had declined by the fifth day. A transcriptomic analysis revealed substantial alterations in gene expression in response to ND. Genes associated with the TCA cycle, glycolysis, astaxanthin biosynthesis, and cell motility were upregulated, while those involved in photosynthesis, lipid synthesis, ribosome biogenesis, amino acid synthesis, and SCW synthesis were downregulated. Additionally, ND modulated the expression of genes involved in ROS scavenging. Conclusions: These findings provide critical insights into the adaptive mechanisms of H. pluvialis in response to ND under high light, contributing to the development of strategies for enhanced production of astaxanthin-rich motile cells.