Metabolic cost as a determinant of light quality acclimation: a full-PAR characterization of the CA3 cyanobacterium Nostoc sp. CCAP 1453/38

Light quality acclimation is one of the key drivers of cyanobacterial physiology, ecology, and productivity. Although chromatic acclimation type 3 (CA3) is the canonical example of light quality-driven phycobilisome remodeling, full-PAR physiological characterization of CA3 strains has so far been limited. Here, we characterize the spectral acclimation strategies of the cyanobacterium Nostoc sp. CCAP 1453/38 across the full photosynthetically active radiation (PAR) range. Genomic analysis confirmed Nostoc as a CA3 strain capable of dynamically adjusting phycoerythrin (PE) and phycocyanin (PC) content in its phycobilisome (PBS) rods. During cultivation under narrow-band LEDs, PE was upregulated under violet, blue and green light (435-555 nm), optimizing light harvesting primarily in the blue-green part of the PAR spectrum, while PC was upregulated under red light (633-687 nm). Beyond canonical CA3 pigment switching, Nostoc responded to wavelengths poorly absorbed by PBS in two qualitatively different ways. Under growth-constraining blue light (465 nm), the strain upregulated total PBS and photosystem II (PSII) levels and biased phycobilisome coupling toward PSII (an increased PBS-PSII/PBS-PSI ratio). However, this metabolically costly response could not overcome the underlying excitonic imbalance caused by preferential PSI excitation, resulting in a low cell division rate. Under near far-red light (687 nm), PBS-PSII coupling itself was enhanced, yet total PBS content was reduced rather than increased. Specific growth rates remained as high as under red light, suggesting that this PBS-PSII reorganization avoided the metabolic burden of antenna upregulation. These results indicate that the same underlying challenge of PSII under-excitation can trigger qualitatively different acclimation responses, only some of which are energetically affordable. Spectral acclimation thus depends not only on matching pigment composition to incident wavelengths, but also on the metabolic cost of the response. Compared with parallel datasets on CA1 and non-CA strains obtained under identical conditions, our findings extend CA3 characterization beyond the canonical green/red framework, highlight bottlenecks and advantages of light quality acclimation in Nostoc, and provide a physiological basis for optimizing light regimes in controlled cyanobacterial cultivations.