High salt stress in the upper part of floating mats of Ulva prolifera, a species that causes green tides, enhances non-photochemical quenching.

Abstract

Salt stress is a major abiotic stress factor that can induce many adverse effects on photosynthetic organisms. Plants and algae have developed several mechanisms that help them respond to adverse environments. Non-photochemical quenching (NPQ) is one of these mechanisms. The thalli of algae in the intertidal zone that are attached to rocks can be subjected to salt stress for a short period of time due to the rise and fall of the tide. Ulva prolifera causes green tides and can form floating mats when green tides occur and the upper part of the thalli is subjected to high salt stress for a long period of time. In this study, we compared the Ulvaprolifera photosynthetic activities and NPQ kinetics when it is subjected to different salinities over various periods of time. Thalli exposed to a salinity of 90 for 4d showed enhanced NPQ, and photosynthetic activities decreased from 60min after exposure up to 4d. This indicated that the induction of NPQ in Ulvaprolifera under salt stress was closely related to the stressing extent and stressing time. The enhanced NPQ in the treated samples exposed for 4d may explain why the upper layer ofthe floating mats formed by Ulvaprolifera thalli were able to survive inthe harsh environment. Further inhibitorexperiments demonstrated that theenhanced NPQ was xanthophyll cycle and transthylakoid proton gradient-dependent. However, photosystem II subunit S and light-harvesting complex stress-related protein didn't over accumulate and may not be responsible for the enhanced NPQ.