A conserved photosynthetic cytochrome enhances growth 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 the identification of a ubiquitous and highly conserved homologue, now named cytochrome c6A (c6A), led to a paradigm shift. However, c6A was soon shown to be unable to replace c6 (or plastocyanin) functionally in the photosynthetic electron transport chain, despite their significant structural homology. The function of c6A, and why it is apparently universal in plants and green algae, remain 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 new 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.