Dr. Yusuke MATSUDA Photosynthesis of diatoms in seawater – mechanisms based on the unique secondary chloroplast structure
EEB Seminar Series
12h
Le séminaire du Dr. Yusuke MATSUDA (School of Biological & Environmental Sciences, Kwansei Gakuin University, Hyogo, Japan) aura lieu dans la salle Favard, IBENS 46 rue d’Ulm 75005 Paris
Marine diatoms are a vital component of global primary production and carbon cycling. When diatoms grow rapidly, their photosynthetic efficiency is supported by a CO₂-concentrating mechanism (CCM). In seawater under current atmospheric conditions, CO₂ concentration ( 15 μM at 25°C) is insufficient to saturate the CO₂-fixing enzyme Rubisco (Km[CO₂] = 25–68 μM). Diatoms take up abundant bicarbonate (HCO₃⁻ ; 2 mM in seawater), transport it to the chloroplast, and accumulate it in the stroma, where it must be dehydrated to CO₂ to be fixed by Rubisco.
In diatoms, this process is compartmentalized within an inner-chloroplast organelle known as the **pyrenoid**, a Rubisco condensate located at the center of the chloroplast. CO₂ is generated only at the core of this condensate, maximizing fixation efficiency while minimizing leakage. This system is referred to as the **CO₂-evolving machinery**. The diatom chloroplast also possesses an efficient barrier system that prevents CO₂ leakage by recapturing released CO₂ as HCO₃⁻ and recycling it back into the CO₂-evolving machinery. These features of the diatom CCM are tightly linked to the unique structure of the secondary chloroplast and pyrenoid. In this lecture, Dr. Matsuda will present recent molecular models of the structure and function of the diatom CCM and pyrenoid.