Link between carbon-concentrating mechanism and regulation of the trans-thylakoidal proton-motive force highlighted in the Chlamydomonas reinhardtii nigericin-insensitive field-isolated strain CC-2936

Chloroplasts’ energy production depends on the photosynthetic electron transport chain activity. The linear electron flow converts light-energy into reducing power (NADPH) while generating a transient proton-motive force (pmf) across the thylakoid membrane. Composed of an electrical (ΔΨ) and a chemiosmotic component (ΔpH), the pmf fuels the synthesis of ATP by chloroplastic CF1FO ATP-synthase. In land plants and green algae, high electron transport rates (ETR) lead to the acidification of the thylakoid lumen triggering in turn a cascade of physiological responses such as changes in ion fluxes, redox regulation of the photosynthetic apparatus and activation of the ΔpH-responsive non-photochemical quenching (NPQ) pathways. While the dynamics of pmf regulation are well understood in land plants, the enzymatic import of HCO3− into the thylakoid lumen makes the topic still controversial in green algae.
In this work, we studied the photosynthetic regulatory mechanisms in the CC-1010 lab strain and in the highly polymorphic field-isolated CC-2936 strain of Chlamydomonas reinhardtii, using in vivo spectro-fluorometric techniques and quantitative reverse transcription PCR. When grown in a minimal medium under high light (HL), CC-2936 displays a lower ETR, a faster induction and slower relaxation kinetics of NPQ than CC-1010, suggesting a different pmf regulation between the two strains. Additionally, CC-2936 shows a nigericin-resistant qE, a lower capacity of state transition (qT), an impaired regulation of the antennae size, a higher affinity for extracellular bicarbonate and higher expression levels of many low-CO2 inducible genes both at low light (LL) and HL as compared to CC-1010. Our data suggest that the nigericin-resistant qE phenotype of CC-2936 could be the result of a strong luminal ionic strength that prevents the ionophore nigericin from dissipating the ΔpH during HL exposure. Moreover, supplementation with exogenous bicarbonate improves ETR and accelerates the kinetics of NPQ relaxation in CC-2936 suggesting that the strain is experiencing a bicarbonate limitation. In addition, bicarbonate supplementation enables the action of nigericin on CC-2936’s NPQ suggesting that bicarbonate influx plays a key role in the regulation of the pmf. We propose CC-2936 as a good complementary model to study the role of the carbon concentrating mechanism (CCM) in the dynamics of pmf regulation in C. reinhardtii.