Unusual features of the high light acclimation of Chromera velia

In the present study, the high light (HL) acclimation of Chromera velia (Chromerida) was studied. HL-grown cells exhibited an increased cell volume and dry weight compared to cells grown at medium light (ML). The chlorophyll (Chl) a-specific absorption spectra ( \(a_{\text{phy}}^{*}\) ) of the HL cells showed an increased absorption efficiency over a wavelength range from 400 to 750 nm, possibly due to differences in the packaging of Chl a molecules. In HL cells, the size of the violaxanthin (V) cycle pigment pool was strongly increased. Despite a higher concentration of de-epoxidized V cycle pigments, non-photochemical quenching (NPQ) of the HL cells was slightly reduced compared to ML cells. The analysis of NPQ recovery during low light (LL) after a short illumination with excess light showed a fast NPQ relaxation and zeaxanthin epoxidation. Purification of the pigment–protein complexes demonstrated that the HL-synthesized V was associated with the chromera light-harvesting complex (CLH). However, the difference absorption spectrum of HL minus ML CLH, together with the 77 K fluorescence excitation spectra, suggested that the additional V was not protein bound but localized in a lipid phase associated with the CLH. The polypeptide analysis of the pigment–protein complexes showed that one out of three known LHCr proteins was associated in higher concentration with photosystem I in the HL cells, whereas in ML cells, it was enriched in the CLH fraction. In conclusion, the acclimation of C. velia to HL illumination shows features that are comparable to those of diatoms, while other characteristics more closely resemble those of higher plants and green algae.