Transportation for fuel: Thylakoid membrane bestrophin channels facilitate HCO3− transport to the pyrenoid in diatoms

Marine diatoms are unicellular photosynthetic organisms that are an essential part of the global ocean ecosystem.They are a phenotypically diverse group of phytoplankton, morphologically categorized into the round centric diatoms and elongated pennate diatoms (Alverson et al. 2006).Diatoms formed through secondary endosymbiosis, with their chloroplasts originating from red algae.As such, their chloroplasts have four membranes (Figure 1A): the chloroplast endoplasmic reticulum, periplastidal membrane, outer chloroplast envelope, and inner chloroplast envelope.Thylakoid membranes sit within the chloroplast stroma, consisting of the girdle lamella and stroma thylakoid.At the center lies the pyrenoid, a membraneless organelle providing a CO 2 concentrating mechanism (CCM), crossed by the pyrenoid-penetrating thylakoid (PPT).This CCM helps diatoms perform efficient photosynthesis in a seawater environment where CO 2 levels are low.Most of the carbon here is in the form of HCO 3 -(bicarbonate), and is imported by plasma membrane transporters (Nakajima et al. 2013), after which carbonic anhydrases located in the lumen of the PPT convert HCO 3 -into CO 2 (Kikutani et al. 2016, Nawaly et al. 2023).The converted CO 2 is further concentrated around Rubisco in the pyrenoid to favor carbon fixation.However, it is still not clear how exactly diatoms facilitate the transport of HCO3 -across thylakoid membranes to the pyrenoid.