Xiujun Xie

SUMMARY: Global warming causes a serious threat to the productivity and species composition of marine diatoms which dominate marine ecosystems and possess immense ecological significance. However, the molecular basis of their high‐temperature adaptation remains enigmatic. Here, we focused on the Constitutive Photomorphogenesis 9 (COP9), and identified the COP9 signalosome (CSN) complex as a potential regulator of high‐temperature acclimation in diatoms. Meta‐transcriptomic data from the Tara Oceans project revealed that CSN5, the catalytic subunit of CSN, showed increased transcript abundance with rising temperature. In the model marine diatom Phaeodactylum tricornutum , CSN5 expression was induced at high temperature, and CSN5 knockout mutants displayed growth defects and altered morphotypes at 28°C. These phenotypes were rescued by CSN5 complementation, while overexpression enhances growth and biomass at high temperature. Proteomic analysis indicated that CSN5 mediates high‐temperature acclimation by modulating chloroplast, cytoplasmic, and nuclear processes including photosynthesis and energy metabolism. These findings not only help us to elucidate how marine diatoms acclimate to high temperature, but also provide a molecular target for developing high‐temperature‐tolerant P. tricornutum .

Pyropia yezoensis, an economically valuable macroalga, occupies a pivotal position in evolutionary history as a red alga, making it an ideal model organism for investigating the evolution of photosynthesis. However, efficient genetic manipulation in P. yezoensis, particularly the stable expression of exogenous genes, presents substantial challenges. Major obstacles, including gene silencing, co-suppression, and unstable transgenerational inheritance, hinder progress in this field. Emerging evidence suggests that epigenetic modifications, including histone acetylation and deacetylation, play crucial roles in regulating endogenous gene expression. Nevertheless, whether these processes influence the expression of exogenous genes in P. yezoensis remains largely unexplored. In this study, we demonstrate that the expression of an exogenous gene can be enhanced by silencing the gene encoding Sin3-associated protein 30 (PySAP30), a critical component of the histone deacetylase (HDAC) complex. To evaluate exogenous gene expression in P. yezoensis, we employed the GUS reporter gene driven by the strong endogenous polyubiquitin gene promoter (PypUbi1). We designed a dual-ribozyme system, comprising hammerhead and hepatitis delta virus ribozymes, to generate antisense fragments targeting PySAP30 exons. This strategy effectively suppressed PySAP30 expression, resulting in a marked increase in both GUS transcript abundance and enzymatic activity. In contrast, transformants without PySAP30 interference showed low GUS activity and pronounced chimerism. Our findings confirm the repressive effect of epigenetic modification on exogenous gene expression in P. yezoensis and highlight a potential strategy for improving exogenous gene expression in macroalgae.

Previous research indicated that free-living sporangial filament keep hollow morph under high-culture density and form bipartite cells under low-culture density, while the following conchospore release was inhibited by high light. Here, we further explored the molecular bases of these affects caused by light and culture density using a transcriptome analysis. Many differentially expressed genes (DEGs) related to carbon dioxide concentration and fixation, photosynthesis, chlorophyll synthesis and nitrogen absorption were upregulated under high-light conditions compared with low-light conditions, indicating the molecular basis of rapid vegetative growth under the former. The stress response- and ion transport-related DEGs, as well as the gene encoding the vacuole formation–brefeldin A-inhibited guanine nucleotide exchange protein (BIG, py05721), were highly expressed under high-density conditions, indicating the molecular basis of the hollow morph of free-living sporangial filaments under high-culture density conditions. Additionally, the brefeldin A treatment indicated that the hollow morph was directly influenced by vacuole formation-related vesicle traffic. Others DEGs related to cell wall components, zinc-finger proteins, ASPO1527, cell cycle and cytoskeleton were highly expressed in the low density with low-light group, which might be related to the formation and release of conchospores. These results provide a deeper understanding of sporangial filaments in Neopyropia yezoensis and related species.

To adapt to the change of intertidal environment, intertidal macroalgae have evolved complicated Ci utilization mechanism. However, our knowledge regarding the CO2 concentrating mechanism (CCM) of macroalgae is limited. Carbonic anhydrase (CA), a key component of CCM, plays important roles in many physiological reactions in various organisms. While there are a large number of genes encoding CA in the Pyropia yezoensis genome, the exact function of specific CA in P. yezoensis remains elusive. To explore the specific function of chloroplast CA in intertidal macroalgae, we produced chloroplast-localized βCA1 knockdown mutants of P. yezoensis through RNA interference, and Pyca1i mutants showed a notable decrease in leaf area and overall biomass, as well as decreased soluble protein and unsaturated fatty acid content under different DIC conditions. However, Pyca1i mutants showed relatively higher starch content compared to the wild-type. The activity of enzymes involved in Calvin cycle, photorespiration, Pentose-phosphate pathway and floridean starch synthesis of P.yezoensis indicated an effective starch accumulation pathway after interference of βCA1. All results suggest that the decreased activity of PyβCA1 impaired the CCM and development of thalli of P.yezoensis, but stimulated starch accumulation in the cytoplasm through feedback to the photorespiration pathway and PP pathway to replenish intermediates for the Calvin cycle. This study is the first to explore the specific function of chloroplast CA in intertidal macroalgae using genomic technology. The results provide valuable insights into the adaption mechanisms of intertidal macroalgae to their environment.

Unlike in terrestrial and freshwater ecosystems, light fields in oceans fluctuate due to both horizontal current and vertical mixing. Diatoms thrive and dominate the phytoplankton community in these fluctuating light fields. However, the molecular mechanisms that regulate diatom acclimation and adaptation to light fluctuations are poorly understood. Here, we performed transcriptome sequencing, metabolome profiling, and ¹³C-tracer labeling on the model diatom Phaeodactylum tricornutum. The diatom acclimated to constant light conditions was transferred to six different light conditions, including constant light (CL5d), short-term (1 h) high light (sHL1h), and short-term (1 h) and long-term (5 days) mild or severe light fluctuation conditions (mFL1h, sFL1h, mFL5d, and sFL5d) that mimicked land and ocean light levels. We identified 2,673 transcripts (25% of the total expressed genes) expressed differentially under different fluctuating light regimes. We also identified 497 transcription factors, 228 not reported previously, which exhibited higher expression under light fluctuations, including 7 with a light-sensitive PAS domain (Per-period circadian protein, Arnt-aryl hydrocarbon receptor nuclear translocator protein, Sim-single-minded protein) and 10 predicted to regulate genes related to light-harvesting complex proteins. Our data showed that prolonged preconditioning in severe light fluctuation enhanced photosynthesis in P. tricornutum under this condition, as evidenced by increased oxygen evolution accompanied by the upregulation of Rubisco and light-harvesting proteins. Furthermore, severe light fluctuation diverted the metabolic flux of assimilated carbon preferentially toward fatty acid storage over sugar and protein. Our results suggest that P. tricornutum use a series of complex and different responsive schemes in photosynthesis and carbon metabolism to optimize their growth under mild and severe light fluctuations. These insights underscore the importance of using more intense conditions when investigating the resilience of phytoplankton to light fluctuations.

Increasing CO2 emissions have resulted in ocean acidification, affecting marine plant photosynthesis and changing the nutrient composition of marine ecosystems. The physiological and biochemical processes of marine phytoplankton in response to ocean acidification have been reported, but have been mainly focused on growth and photosynthetic physiology. To acquire a thorough knowledge of the molecular regulation mechanisms, model species with clear genetic background should be selected for systematic study. Phaeodactylum tricornutum is a pennate diatom with the characteristics of small genome size, short generation cycle, and easy to transform. Furthermore, the genome of P. tricornutum has been completely sequenced. In this study, P. tricornutum was cultured at high and normal CO2 concentrations. Cell composition changes during culture time were investigated. The 13C isotope tracing technique was used to determine fractional labeling enrichments for the main cellular components. The results suggested that when lipid content increased significantly under high CO2 conditions, total protein and soluble sugar contents decreased. The 13C labeling experiment indicated that the C skeleton needed for fatty acid C chain elongation in lipid synthesis under high CO2 conditions is not mainly derived from NaHCO3 (carbon fixed by photosynthesis). This study indicated that breakdown of intracellular protein and soluble sugar provide C skeleton for lipid synthesis under high CO2 concentration.

Iron is a limiting factor that controls the phytoplankton biomass in the modern ocean, and iron fertilization of the ocean could lead to blooms dominated by diatoms. Thus, iron plays an important role in controlling the distribution of diatoms. In this study, we measured the growth rate and photosynthetic activity of the model diatom Phaeodactylum tricornutum cultured under different iron concentrations and found that it grew more rapidly and had a much higher photosynthetic efficiency under higher iron concentrations. In order to explore the unique mechanism of the response of diatoms to iron, a proteomic analysis was carried out, and the results indicated that iron promotes the Calvin cycle of P. tricornutum. Diatoms can tolerate the pressure of iron limitation by replacing iron‐rich proteins with flavodoxin, and so on. Moreover, we found that the photosystem I (PSI) activity of iron‐limited algae that were treated by N’,N’,N’,N’‐tetramethyl‐p‐phenylenediamine (TMPD) was increased significantly. As TMPD plays the role of a cytochrome b₆/f complex that transfers electrons from photosystem II to PSI, the cytochrome b₆/f complex is the key to photosynthesis regulation. Iron could influence the growth of P. tricornutum by regulating its biosynthesis. All of the results suggest that iron might affect the growth of diatoms through the Calvin cycle and the cytochrome b₆/f complex.

Heavy metal pollution in aquatic environments is a significant environmental problem. Marine macroalgae are novel materials for the removal of heavy metals from water, aided by their large biomass and the feature of easy collection. In this study, Ulva prolifera O.F.Müller was used to remove cadmium ions (Cd²⁺) from the aquatic environment. We examined the Cd²⁺ absorption capacity of the thalli in a factorial experiment using fresh thalli cultivated in seawater, freshwater, water containing domestic garbage, water containing industrial waste and elutriant of soil with additional Cd²⁺. In addition, changes in photosynthetic activity of thalli were investigated. A significant decrease of maximum quantum yield and effective photochemical quantum yield of PSII (YII) indicated Cd²⁺ was transported into the cells and seriously inhibited photosynthesis. Furthermore, the Cd²⁺ absorption capacity was relatively high. The results indicated the suitability of U. prolifera for removing Cd²⁺ from aquatic environments of different backgrounds.