[3S01a] 09. GteX - Part 2

2:30 PM - 3:00 PM JST(5:30 AM - 6:00 AM UTC)

[3GteX-08] Powering Global CO₂ Fixation: Structural and Functional Architecture of the Diatom Pyrenoid

○Onyou Nam¹, Irina Grouneva¹, Sabina Musial¹, Manon Demulder², Caroline McKenzie¹, Adam Dowle¹, Matthew Dowson¹, James Barrett¹, James N. Blaza¹, Benjamin D. Engel², Luke C.M. Mackinder¹ (1. University of York (UK), 2. University of Basel (Switzerland))

Keywords:

Carbon fixation,Pyrenoid,Diatoms,Genome editing,Fluorescent protein tagging

Diatoms are among the most productive photosynthetic organisms on Earth, estimated to be responsible for approximately 20% of global carbon fixation. This productivity is remarkable given that oceanic CO₂ is scarce and diffuses slowly. Diatoms have overcome this limited CO₂ availability by concentrating CO₂ in a specialized chloroplast microcompartment called the pyrenoid, where Rubisco condenses into a tightly packed matrix. However, the molecular assembly of the diatom pyrenoid remains largely unresolved. In this talk, I will describe how improving molecular tools in the centric diatom Thalassiosira pseudonana has advanced our mechanistic understanding of pyrenoid structure and function. We established a versatile molecular toolkit, including fluorescent protein tagging, CRISPR-based gene editing, and endogenous tagging. This enabled us to systematically localize potential pyrenoid proteins in vivo and test their functions within the native cellular context. Leveraging fluorescent protein tagging lines for affinity purification followed by mass spectrometry enabled us to define a protein-protein interaction network of the pyrenoid. From this, we identified previously unknown pyrenoid elements, including Diatom Pyrenoid Component proteins and a family of six Shell proteins that encase the Rubisco matrix. Functional analyses further showed that Shell1/2 and Shell4 are necessary for maintaining normal pyrenoid architecture and for efficient growth under ambient CO₂. This indicates that the pyrenoid depends on specialized structural components and cannot be understood through Rubisco condensation alone. Together, these studies provide mechanistic insight into how a key marine phytoplankton group constructs a unique CO₂-fixing organelle, and raise broader questions about the evolution and design principles of algal carbon fixation.