Structure and function of the pyrenoid in marine diatoms

Abstract

Pyrenoid is a protein body in the chloroplast that sequestrates ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), a CO₂ fixing enzyme. The pyrenoid occurs in many algae and a part of hornworts, but the structure does not have membrane and has not been considered as a decisive organelle, thus the detailed function has not been studied well to date. Recent studies demonstrated that green algal pyrenoid in Chlamydomonas is a liquid-liquid phase separation constituted by a natural disorder protein, EPYC1. On the contrary, marine photosynthesis that plays an enormous role as the global gas-exchange system is also sustained by the pyrenoid-based systems. The marine photoautotrophs, such as diatoms, primarily comprise of red-type secondary chloroplast, whose pyrenoid structure is dissimilar to that of green type. Even though these pyrenoids are the products of hetero-origin convergent evolution, recent studies however reveal structural/functional similarity amongst different pyrenoids. For instance, the pyrenoids in the CCM-operating algae have thylakoid-like membrane which penetrates/invaginates the pyrenoid. This may suggest that algae faced to the low CO₂ environment after the Carboniferous were forced to optimize the usage of light and CO₂ supply into some structural/functional frames with few options. This review will describe implications of the pyrenoid structures from some common aspects observed in the diatom pyrenoid.