Probing the electric field across thylakoid membranes in cyanobacteria

Significance

Cyanobacteria were the first organisms to develop oxygenic photosynthesis using water as a source of electrons. Today they remain widespread primary photosynthetic producers and hold a high biotechnological potential. In cyanobacteria, respiration and photosynthesis are interconnected in a complex network of electron fluxes. The study of cyanobacterial physiology is hampered by the lack of techniques, allowing a direct measurement of the transmembrane electric field that develops across their photosynthetic/respiratory membranes. Here, we characterized a probe of the transmembrane electric field, based on the ElectroChromic Shifts of carotenoids, thus opening unprecedented avenues to bioenergetics studies of these major photosynthetic organisms.

Abstract

In plants, algae, and some photosynthetic bacteria, the ElectroChromic Shift (ECS) of photosynthetic pigments, which senses the electric field across photosynthetic membranes, is widely used to quantify the activity of the photosynthetic chain. In cyanobacteria, ECS signals have never been used for physiological studies, although they can provide a unique tool to study the architecture and function of the respiratory and photosynthetic electron transfer chains, entangled in the thylakoid membranes. Here, we identified bona fide ECS signals, likely corresponding to carotenoid band shifts, in the model cyanobacteria Synechococcus elongatus PCC7942 and Synechocystis sp. PCC6803. These band shifts, most likely originating from pigments located in photosystem I, have highly similar spectra in the 2 species and can be best measured as the difference between the absorption changes at 500 to 505 nm and the ones at 480 to 485 nm. These signals respond linearly to the electric field and display the basic kinetic features of ECS as characterized in other organisms. We demonstrate that these probes are an ideal tool to study photosynthetic physiology in vivo, e.g., the fraction of PSI centers that are prebound by plastocyanin/cytochrome c ₆ in darkness (about 60% in both cyanobacteria, in our experiments), the conductivity of the thylakoid membrane (largely reflecting the activity of the ATP synthase), or the steady-state rates of the photosynthetic electron transport pathways.