Enhanced IsiA accumulation under low light does not expand the functional antenna of photosystem I

Introduction Iron scarcity triggers specific acclimatory responses in cyanobacteria, among which is the synthesis of the IsiA protein that can associate with photosystem I (PSI) and increase its antenna size. However, cellular IsiA also occurs as PSI-free assemblies that do not contribute to PSI excitation, suggesting multiple physiological roles. In natural environments, limiting and stress factors often act simultaneously, causing co-limitation effects. Methods Here we investigate the accumulation and function of IsiA in iron-starved Synechocystis sp. PCC 6803 grown under two light environments, normal (35 µmol photons m −2 s −1 ) and low light (15 µmol photons m −2 s −1 ), using steady-state and time-resolved fluorescence spectroscopy. Results Low-light growth resulted in an approximately two-fold increase in the total cellular IsiA concentration compared with normal light. However, the amount of IsiA functionally coupled to PSI did not change, and the excess IsiA was predominantly found in assemblies exhibiting moderate excitation quenching, not contributing to PSI light harvesting. Discussion The results suggest that the additional IsiA accumulated under low light, in excess of the available PSI docking sites, may serve as a chlorophyll reserve. No evidence was found for IsiA-mediated phycobilisome-to-PSI energy transfer; instead, excitation quenching was observed in phycobilisomes under the measurement conditions, revealing broad, multimodal responses of the photosynthetic apparatus to iron depletion beyond IsiA accumulation.