Electrochromism: a useful probe to study algal photosynthesis

In photosynthesis, electron transfer along the photosynthetic chain results in a vectorial transfer of protons from the stroma to the lumenal space of the thylakoids. This promotes the generation of an electrochemical proton gradient (Delta mu(+)(H)), which comprises a gradient of electric potential (Delta Psi) and of proton concentration (Delta pH). The Delta mu(+)(H) has a central role in the photosynthetic process, providing the energy source for ATP synthesis. It is also involved in many regulatory mechanisms. The Delta pH modulates the rate of electron transfer and triggers deexcitation of excess energy within the light harvesting complexes. The Delta Psi is required for metabolite and protein transport across the membranes. Its presence also induces a shift in the absorption spectra of some photosynthetic pigments, resulting in the so-called ElectroChromic Shift (ECS). In this review, we discuss the characteristic features of the ECS, and illustrate possible applications for the study of photosynthetic processes in vivo.