Phloroglucinol promotes fucoxanthin synthesis by activating the cis-zeatin and brassinolide pathways in Thalassiosira pseudonana

The role of cis-zeatin in enhancing high-temperature resistance and fucoxanthin biosynthesis in Phaeodactylum tricornutum

Phaeodactylum tricornutum a prominent source of industrial fucoxanthin production, faces challenges in its application due to its tolerance to high-temperature environments. This study investigates the physiological responses of P. tricornutum to high-temperature stress and its impact on fucoxanthin content, with a specific focus on the role of cis-zeatin. The results reveal that high-temperature stress inhibits P. tricornutum's growth and photosynthetic activity, leading to a decrease in fucoxanthin content. Transcriptome analysis shows that high temperature suppresses the expression of genes related to photosynthesis (e.g., psbO, psbQ, and OEC) and fucoxanthin biosynthesis (e.g., PYS, PDS1, and PSD2), underscoring the negative effects of high temperature on P. tricornutum. Interestingly, genes associated with cis-zeatin biosynthesis and cytokinesis signaling pathways exhibited increased expression under high-temperature conditions, indicating a potential role of cis-zeatin signaling in response to elevated temperatures. Content measurements confirm that high temperature enhances cis-zeatin content. Furthermore, the exogenous addition of cytokinesis mimetics or inhibitors significantly affected P. tricornutum's high-temperature resistance. Overexpression of the cis-zeatin biosynthetic enzyme gene tRNA DMATase enhanced P. tricornutum's resistance to high-temperature stress, while genetic knockout of tRNA DMATase reduced its resistance to high temperatures. Therefore, this research not only uncovers a novel mechanism for high-temperature resistance in P. tricornutum but also offers a possible alga species that can withstand high temperatures for the industrial production of fucoxanthin, offering valuable insights for practical utilization.IMPORTANCEThis study delves into Phaeodactylum tricornutum's response to high-temperature stress, specifically focusing on cis-zeatin. We uncover inhibited growth, reduced fucoxanthin, and significant cis-zeatin-related gene expression under high temperatures, highlighting potential signaling mechanisms. Crucially, genetic engineering and exogenous addition experiments confirm that the change in cis-zeatin levels could influence P. tricornutum's resistance to high-temperature stress. This breakthrough deepens our understanding of microalgae adaptation to high temperatures and offers an innovative angle for industrial fucoxanthin production. This research is a pivotal step toward developing heat-resistant microalgae for industrial use.

Mediator subunit MED8 interacts with heat shock transcription factor HSF3 to promote fucoxanthin synthesis in the diatom Phaeodactylum tricornutum

Fucoxanthin, a natural carotenoid that has substantial pharmaceutical value due to its anticancer, antioxidant, antiobesity, and antidiabetic properties, is biosynthesized from glyceraldehyde-3-phosphate (G3P) via a series of enzymatic reactions. However, our understanding of the transcriptional mechanisms involved in fucoxanthin biosynthesis remains limited. Using reverse genetics, the med8 mutant was identified based on its phenotype of reduced fucoxanthin content, and the biological functions of MED8 in fucoxanthin synthesis were characterized using approaches such as gene expression, protein subcellular localization, protein-protein interaction and chromatin immunoprecipitation assay. Gene-editing mutants of MED8 exhibited decreased fucoxanthin content as well as reduced expression levels of six key genes involved in fucoxanthin synthesis, namely DXS, PSY1, ZDS-like, CRTISO5, ZEP1, and ZEP3, when compared to the wild-type (WT) strain. Furthermore, we showed that MED8 interacts with HSF3, and genetic analysis revealed their shared involvement in the genetic pathway governing fucoxanthin synthesis. Additionally, HSF3 was required for MED8 association with the promoters of the six fucoxanthin synthesis genes. In conclusion, MED8 and HSF3 are involved in fucoxanthin synthesis by modulating the expression of the fucoxanthin synthesis genes. Our results increase the understanding of the molecular regulation mechanisms underlying fucoxanthin synthesis in the diatom P. tricornutum.

Untargeted metabolomics to expand the chemical space of the marine diatom Skeletonema marinoi

Diatoms (Bacillariophyceae) are aquatic photosynthetic microalgae with an ecological role as primary producers in the aquatic food web. They account substantially for global carbon, nitrogen, and silicon cycling. Elucidating the chemical space of diatoms is crucial to understanding their physiology and ecology. To expand the known chemical space of a cosmopolitan marine diatom, Skeletonema marinoi, we performed High-Resolution Liquid Chromatography-Tandem Mass Spectrometry (LC-MS2) for untargeted metabolomics data acquisition. The spectral data from LC-MS2 was used as input for the Metabolome Annotation Workflow (MAW) to obtain putative annotations for all measured features. A suspect list of metabolites previously identified in the Skeletonema spp. was generated to verify the results. These known metabolites were then added to the putative candidate list from LC-MS2 data to represent an expanded catalog of 1970 metabolites estimated to be produced by S. marinoi. The most prevalent chemical superclasses, based on the ChemONT ontology in this expanded dataset, were organic acids and derivatives, organoheterocyclic compounds, lipids and lipid-like molecules, and organic oxygen compounds. The metabolic profile from this study can aid the bioprospecting of marine microalgae for medicine, biofuel production, agriculture, and environmental conservation. The proposed analysis can be applicable for assessing the chemical space of other microalgae, which can also provide molecular insights into the interaction between marine organisms and their role in the functioning of ecosystems.

Thalassiosira pseudonana (Cyclotella nana) (Hustedt) Hasle et Heimdal (Bacillariophyceae): A genetically tractable model organism for studying diatom biology, including biological silica formation

In 2004, Thalassiosira pseudonana was the first eukaryotic marine alga to have its genome sequenced. Since then, this species has quickly emerged as a valuable model species for investigating the molecular underpinnings of essentially all aspects of diatom life, particularly bio-morphogenesis of the cell wall. An important prerequisite for the model status of T. pseudonana is the ongoing development of increasingly precise tools to study the function of gene networks and their encoded proteins in vivo. Here, we briefly review the current toolbox for genetic manipulation, highlight specific examples of its application in studying diatom metabolism, and provide a peek into the role of diatoms in the emerging field of silica biotechnology.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.