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

Having at least one of the small soluble electron carriers cytochrome c6 (c6) and plastocyanin is indispensable for photosynthesis. It was believed that c6 had been lost from plants until a ubiquitous and highly conserved homologue, now named cytochrome c6A (c6A), was found in Arabidopsis thaliana. In spite of an early suggestion that c6A functioned like c6 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 c6A, and why it is apparently universal in plants and green algae, therefore remained unknown. Here, we show that, in the green alga Chlamydomonas reinhardtii, c6A 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 c6A, 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 c6A is important for this adaptation. These findings provide a basis for further mechanistic studies on a hypothesized role for c6A in thiol-based redox regulation in the thylakoid lumen, with implications for photoprotection mechanisms such as state transitions.