Atypical absorption response to the trans-thylakoid electric field in the acidothermophilic red algae Cyanidioschyzon merolae and Galdieria partita

Abstract

An absorption change responding to the change in the trans-thylakoid electric field (Δψ), also known as the electrochromic shift (ECS) signal, is widely used to probe multiple photosynthetic processes. The ECS signals either display a linear response of absorption changes to Δψ, independent of the trans-thylakoid electric field preexisting before actinic light (ψ_O), or a quadratic response, dependent on ψ_O. In the acidothermophilic red algae Cyanidioschyzon merolae and Galdieria partita, the absorption changes induced by single turnover saturating light flashes were affected by external pH. The effects of elevated external pH on the flash-induced absorption changes were explained by diminished ψ_O, as shown with the treatment of ionophores. We identified three contributions to the absorption changes: c-type cytochrome oxidized-minus-reduced signal and ECS signals showing both ψ_O-dependent and ψ_O-independent responses. Based on this, we could reveal that the effects of elevated external pH on the flash-induced absorption changes were due to variations of ψ_O, which in turn changed the contribution of the ψ_O-dependent ECS, as shown with the treatment of ionophores. Further analysis revealed that the ψ_O-dependent ECS signal exhibited a quadratic response to Δψ at low ψ_O, but the quadraticity was lost at higher ψ_O, providing insights for comprehending the atypical nature of the ECS signal. Our approach to identifying the ψ_O-dependent and ψ_O-independent ECS signals enables the ECS-based measurements for further investigation of the bioenergetics of electron and proton transport in red algae.