​Different pathways of photosynthetic electron transport take place in the chloroplast. The main pathway is the linear electron transport leading to the generation of NADPH and the formation of a proton gradient required for ATP synthesis. The two minor pathways, cyclic flow and pseudocyclic flow to O₂, lead also to the formation of a proton gradient without generating NADPH. The partition between these three pathways is not well understood. It is generally assumed that the pseudocyclic way is only induced when the final electron acceptor CO₂ is limiting. Chlamydomonas reinhardtii does not generate H₂O₂ by photorespiration and is thus an ideal organism to study the dependence of pseudocyclic flow on acceptor limitation.
In contrast to expectation, it was shown by varying CO₂ availability that pseudocyclic flow occurred in parallel to linear flow and that it was not more active under CO₂ limitation.
Under CO₂ limitation, electron transport is very efficiently down regulated and the reduction of O₂ is suppressed as demonstrated by using several mutants affected in certain regulation mechanisms. This study allows reconsidering the role of pseudocyclic flow also for other organisms like higher plants.