CAS Reveals Groundbreaking Euglena gracilis PSI-LHCE Supercomplex Structure and Evolutionary Strategy

Chinese researchers have unveiled the high-resolution structure of the Photosystem I-light-harvesting complex Euglena (PSI-LHCE) supercomplex from the flagellate alga Euglena gracilis, marking a significant milestone in understanding photosynthetic evolution. Published in Science Advances, this breakthrough by scientists from the Institute of Physics of the Chinese Academy of Sciences (CAS) and Hebei Normal University provides atomic-level insights into how this secondary endosymbiotic organism adapts its light-harvesting machinery to diverse aquatic environments.

The study, led by Associate Professor Wang Yumei from CAS's Institute of Physics and Professor Tian Lirong from Hebei Normal University, utilized cryo-electron microscopy (cryo-EM) to achieve a remarkable 2.23 Å resolution. This precision reveals a 'small core, large antenna' architecture unlike typical green-lineage photosystems, highlighting China's growing prowess in structural biology.

Understanding Photosystem I: The Heart of Photosynthesis

Photosystem I (PSI) is a multi-subunit protein-pigment complex embedded in the thylakoid membranes of chloroplasts and cyanobacteria, crucial for oxygenic photosynthesis. It absorbs light energy via chlorophylls and carotenoids, transferring electrons from plastocyanin to ferredoxin, generating NADPH for carbon fixation. In higher plants and green algae, PSI typically features a core of 12-14 subunits surrounded by 4-6 light-harvesting complex I (LHCI) proteins, optimized for moderate light conditions.

In E. gracilis, PSI diverges due to its evolutionary history. This protist acquired its plastid through secondary endosymbiosis—a green alga engulfed by a euglenid host—resulting in a mosaic genome blending genes from multiple algal lineages. This hybrid nature equips E. gracilis for mixotrophy, thriving in variable light and nutrient-poor waters, with applications in biofuel production owing to its high biomass and lipid yields.

• PSI core: Converts light to chemical energy via electron transport chain.
• LHCI antennas: Capture photons, funnel energy to reaction center.
• Key subunits: PsaA/B (core), PsaC/D/E/F/G/J/K (peripheral).

For aspiring researchers in China, such studies underscore opportunities in plant biology. Check research jobs to join cutting-edge teams at institutions like CAS.

The Euglena gracilis Model: A Bridge Between Lineages

Euglena gracilis, a freshwater microalga, exemplifies evolutionary innovation. Its secondary green plastid enables paramylon storage (β-1,3-glucan) and wax esters under stress, positioning it as a biofuel candidate. Chinese studies have optimized its cultivation for biodiesel, achieving high fatty acid content in dark-glucose conditions. Recent advances include co-culturing with bacteria for enhanced lipids and wastewater-based biomass production.

Prior to this CAS work, PSI structures from green algae (e.g., Chlamydomonas) showed uniform LHCI trimers. Euglena's PSI-LHCE, however, integrates red-lineage elements like diadinoxanthin (Ddx), a xanthophyll absent in green plants, enabling far-red absorption in shaded waters.

Hebei Normal University's photosynthesis lab, led by Tian Lirong, complements CAS efforts, fostering collaborations vital for China's higher education ecosystem.

Cryo-EM Breakthrough: Resolving the Supercomplex at Atomic Detail

The CAS team purified PSI-LHCE from E. gracilis strain Z, using sucrose density gradient centrifugation, then imaged via cryo-EM at Beijing National Laboratory for Condensed Matter Physics—a CAS facility pioneering sub-2 Å resolutions. Data processing yielded a 2.23 Å map, visualizing 96 chlorophylls, 28 carotenoids, lipids, and quinones.

China's cryo-EM infrastructure, including cryo-EM galleries at Shanghai and Beijing, has propelled over 10,000 structures since 2017, rivaling global leaders. This positions UCAS (University of Chinese Academy of Sciences) graduates for international impact.

Key Structural Features: Diversity and Stability

The supercomplex comprises an 8-subunit PSI core (lacking PsaG/H/I/K/L/O/N) encircled by 16 LHCE antennas: 12 Lhca (Chl a-binding) and 4 Lhcbm (Chl a/b-binding). Arranged in six heterodimer pairs, antennas form a two-layered ring via back-to-back C-helices with conserved EYWRGN motifs, prioritizing antenna-antenna over core interactions for stability.

• 16 antennas from 16 gene products—unprecedented diversity.
• Tight packing: Transmembrane helices ensure robustness in dynamic environments.
• Bear's paw shape: Compact core maximizes peripheral light capture.

This 'small core, large antenna' suits low-light aquatic niches, contrasting plant PSI's balance.

Photo by Blake Weyland on Unsplash

Red-Green Mosaic Pigments: A Evolutionary Hybrid

All LHCE bind Ddx, a red-algal carotenoid quenching excess energy. Notably, four Ddx occupy core sites typically holding β-carotene in green lineage, confirmed by density maps. Lhcbm4 binds four Chl b, shifting absorption to blue-green, while Lhca favor Chl a for red/far-red.

This mosaic—green Chl b with red Ddx—evidences gene transfer post-endosymbiosis, optimizing spectral coverage. FRET analysis shows ~72 ps trapping time, efficient as in Chlamydomonas.

For bioengineering, engineering Ddx-binding sites in crop PSI could boost shade tolerance. Link: Craft your CV for structural biology roles.

Energy Transfer Dynamics: Efficient Funneling

Time-resolved spectroscopy revealed Chl a603-a609 pairs bridging heterodimers, channeling excitons centrally. Ddx aids dissipation, preventing photodamage. Simulations depict a pigment network rivaling vascular plants, despite diversity.

Co-first authors Tianyu Bai (Hebei Normal) and Zhiyuan Mao/Dapeng Sun (CAS) highlight interdisciplinary cryo-EM/biophysics training in China.

Evolutionary Strategy: Insights from Secondary Endosymbiosis

E. gracilis PSI reflects host-alga gene shuffling: green core with red pigments/antennas. Antenna diversity (12 Lhca types) captures variable wavelengths; internal packing enables modular assembly. This strategy, honed over 1 billion years, informs algal evolution beyond Archaeplastida.

Comparative cryo-EM (e.g., vs. maize PSI) shows Euglena's innovations, paralleling diatom PSI-LHCI.

Implications for Bioengineering and Sustainability

Understanding PSI-LHCE paves bioengineering: Expressing LHCE in Chlamydomonas for far-red PSI could enhance microalgae yields for biofuels. E. gracilis already yields 20-50% lipids; structural data guides paramylon/lipid optimization. Chinese firms eye Euglena for aviation fuel.

In agriculture, mimicking Ddx photoprotection aids crop resilience amid climate change. Read more on AI in higher ed research.

China's Leadership in Photosynthesis and Structural Biology

CAS facilities like Beijing Lab drive China's cryo-EM surge, with 30% global structures by 2025. Universities like Hebei Normal and Shandong contribute, training PhDs via NSFC grants. This positions China as photosynthesis hub, with UCAS offering specialized programs.

Stats: China published 15% global photosynthesis papers (2025), up 20% YoY.

Photo by Blake Weyland on Unsplash

Future Outlook: From Algae to Crops

Future: Single-particle analysis of dynamic states; LHCE engineering for synthetic biology. For students, rate-my-professor, explore higher-ed-jobs, career advice, university-jobs. China's vision: Harnessing PSI for green energy.