A conserved photosynthetic cytochrome enhances growth of Chlamydomonas reinhardtii in fluctuating light

Having at least one of the small soluble electron carriers cytochrome c 6 ( c 6 ) and plastocyanin is indispensable for photosynthesis. It was believed that c 6 had been lost from plants until a ubiquitous and highly conserved homologue, now named cytochrome c 6A ( c 6A ), was found in Arabidopsis thaliana . In spite of an early suggestion that c 6A functioned like c 6 in the photosynthetic electron transport chain it was soon shown to be unable to do so despite the strong structural homology between the two proteins. The function of c 6A , and why it is apparently universal in plants and green algae, therefore remained unknown. Here, we show that, in the green alga Chlamydomonas reinhardtii , c 6A confers a growth advantage under fluctuating light and is crucial for maintaining the light harvesting balance between photosystems I and II in photomixotrophic conditions. We show that in the absence of c 6A , the light harvesting balance shifts towards PSII, leading to a more reduced plastoquinone pool and increased photooxidative stress. This study provides insights into how photosynthetic organisms acclimatize to stressful light conditions, indicating that c 6A is important for this adaptation. These findings provide a basis for further mechanistic studies on a hypothesized role for c 6A in thiol-based redox regulation in the thylakoid lumen, with implications for photoprotection mechanisms such as state transitions.