Fixing CO2 via photosynthesis requires ATP and NADPH. Linear electron transfer (LET) supplies both metabolites, yet depending on environmental conditions, additional ATP is required which can be generated by cyclic electron transfer (CET). How the balance between LET and CET is set remains largely unknown. Ferredoxin (Fd)-NADP+ reductase (FNR) has been suggested to act as the switch, channelling photosynthetic electrons to LET when it is bound to photosystem I (PSI) or CET when bound to cytochrome b6f. Testing this hypothesis by direct FNR gene knock-out is prevented by its essential role in LET. Here, using CRISPR-Cas9 gene editing in Chlamydomonas reinhardtii, we circumvented this to create cells expressing a chimeric form of FNR tethered to PSI via PSAF. The results provide fascinating and unexpected insights into the role of FNR in regulating photosynthetic electron transfer.