Photoprotection in the diatom Thalassiosira pseudonana: Role of LI818-like proteins in response to high light stress

As an important component of marine phytoplankton, diatoms must be able to cope with large changes in illumination on a daily basis. They have an active xanthophyll cycle and non-photochemical quenching (NPQ), but no homolog has been detected for the gene encoding the PsbS protein required for NPQ in plants. However, diatoms do have a branch of the light-harvesting complex superfamily, the Lhcx clade, which is most closely related to the LI818 (LhcSR) genes of the green alga Chlamydomonas, known to be upregulated in response to a variety of stresses. When cultures of the diatom T. pseudonana grown under low light (40 µmol photons m⁻² s⁻¹) were exposed to high light stress (HL, 700 µmol photons m⁻² s⁻¹), transcripts of three of these genes (Lhcx1, Lhcx4, Lhcx6) were transiently accumulated. The amount of Lhcx6 protein was low under low light, but increased continuously during 10 h of HL exposure, then slowly dropped to background levels in the dark. However, HL had little effect on the Lhcx1 protein, which was present under low light and only doubled after HL exposure. Diatoxanthin levels increased throughout the HL period with no change in diadinoxanthin. The fraction of NPQ attributable to photoinhibitory quenching (qI) also increased throughout the HL exposure. Taken together, the Lhcx6 protein could be associated with diatoxanthin binding and play a direct role in excess energy dissipation via sustained quenching during acclimation to prolonged HL stress, while the Lhcx1 protein may play a more structural role in thylakoid membrane organization under all conditions.