Solanki, J.; Fernandez, M.; Pawitsch, M.; Gehringer, M. M.; Zehner, S.; Frankenberg-Dinkel, N.

Marine Synechococcus cyanobacteria can modify their light-harvesting phycobilisome antennae in response to spectral changes through chromatic acclimation (CA). CA type IV (CA4) involves adjustment of the ratio of the blue-absorbing phycourobilin (PUB) to the green-absorbing phycoerythrobilin (PEB). Although CA4 is associated with a genomic island occurring in two configurations (CA4-A and CA4-B), some strains carrying this island, including Synechococcus sp. WH8109, have been reported to lack CA4 activity. Here, we investigated whether WH8109 undergoes CA4 under blue light (BL) and green light (GL) conditions, using the CA4-positive strain RS9916 as a reference. Whole-cell fluorescence, flow cytometry, isolated phycobilisome analysis, and sequencing of the CA4-B genomic island were used to assess pigment acclimation and identify potential genetic differences. WH8109 showed a stable PUB:PEB ratio of approximately 0.71 under BL and GL, with no detectable changes in PUB content or phycobilisome chromophore composition. In contrast, RS9916 exhibited clear CA4, with PUB:PEB ratios of 1.57 under BL and 0.72 under GL. Sequencing revealed no insertions, deletions, or other sequence anomalies in the WH8109 CA4-B genomic island compared to the public sequence. These results demonstrate that WH8109 does not undergo detectable CA4 under the conditions tested, despite possessing an intact CA4-B genomic island, highlighting a genotype-phenotype discrepancy that warrants further investigation.