Roles of the transthylakoid proton gradient and xanthophyll cycle in the non-photochemical quenching of the green alga Ulva linza

Non-photochemical quenching (NPQ) is one of the most important photoprotection mechanisms in photosynthetic organisms when they are exposed to excessive irradiation. The basic principle of NPQ, which is the safe dissipation of excessive absorbed light as heat, is identical in all photoautotrophs. However, crucial differences in its regulation and structural mechanisms exist in different phototrophs. Here, we investigated NPQ in the green alga Ulva linza coupled with inhibitors to alter the amplitude of the transthylakoid proton gradient (ΔpH) and/or de-expoxidation of xanthophyll cycle (XC) under high light conditions. The data demonstrates that NPQ started with a rapid initial rise within the first minute of illumination, followed by a decline before a further rise in quenching. During the whole phase, NPQ was triggered and completely controlled by ΔpH, then strengthened and modulated by zeaxanthin. NPQ relaxation was slower in U. linza when compared to plants and other green algae, and it may be mainly caused by the slow conversion of zeaxanthin to violaxanthin. NPQ in U. linza is controlled to a greater extent by XC, which show more similarities to Arabidopsis than to Chlamydomonas and may be an adaptive mechanism for its successful colonization of coastal ecosystems.