Low PSI content broadens the optimal light spectrum for a phycoerythrin-dominated cyanobacterium towards near far-red.

Light quality is an important yet often overlooked driver of cyanobacterial physiology, ecology, and productivity. Here, we characterize the spectral acclimation strategies of cyanobacterium Cyanobium sp. NIVA-CYA 375 across the full photosynthetically active radiation range, using narrow-band LEDs with distinct emission profiles. Genomic analysis confirmed that, despite the presence of cpeA and cpcL genes, Cyanobium lacks chromatic acclimation capacity and functions as a spectral generalist. Biophysical and biochemical measurements revealed that this strain possesses high phycoerythrin content in its phycobilisomes (PBS), and inherently low photosystem I (PSI) content relative to photosystem II (PSII). Although phycoerythrin absorbs green light efficiently, Cyanobium exhibited chronic acceptor-side limitation of PSII, constraining its electron transport capacity under most wavelengths including green. Notably, the highest growth rates occurred under near far-red light, due to simultaneous excitation of PSI via chlorophyll a and PSII via phycobilisomes. Despite attempts to balance electron transport rates by increasing absolute PBS abundance and/or PBS attachment to PSII or PSI, other cultivation wavelengths resulted in insufficient excitation of either PSII or PSI, presumably imbalancing NADPH and ATP formation. Our results suggest that low PSI content is an adaptive trait allowing Cyanobium to thrive in the spectrally variable, low-light conditions of turbid freshwater habitats. These findings reveal how low PSI content can drive spectral niche specialization and shape the wavelength-dependency of growth in cyanobacteria.