The photosynthetic electron transport network: occurrence and function of cyclic and side pathways

The current knowledge on cyclic and side pathways of the thylakoidal electron transport system will be discussed. Cyclic or pseudo-cyclic pathways include the PSI-driven cyclic electron transport, the cytochrome b6/f plastoquinone cycle and the oxygen-dependent water/water cycle. Side pathways altogether make up the chlororespiratory pathway, including the non-photochemical, NAD(P)H-dependent plastoquinone reduction by chloroplastic dehydrogenases and plastoquinol oxidation by a putative chloroplastic oxidase. The state of knowledge concerning the components, the importance and function of these pathways will be discussed. Results obtained in Liège using the unicellular green alga Chlamydomonas reinhardtii or higher plant chloroplasts will be presented. The importance of specific pathways in the context of mitochondria-chloroplast metabolic interactions and global cellular bio-energetics will be highlighted. Some of the methodological aspects in this field will be shortly described, in particular the use of fluorescence and absorbance spectroscopy.
In some micro-algae, the electron transport chain can also use protons as terminal electron acceptors under anaerobic conditions, thereby allowing hydrogen evolution in the light. The potential of this fascinating reaction in perspective of the photobiological production of ‘bio-hydrogen’ will be briefly discussed.