Photoacclimation characteristics of Sargassum thunbergii germlings under different light intensities

Abstract

The photosynthesis and growth responses of Sargassum thunbergii germlings to different light intensities (10, 60, and 300 μmol photons m−2 s−1) were investigated. Maximum photochemical efficiency (Fv/Fm), rapid light curves (RLCs), and photochemical and non-photochemical quenching (qP and NPQ) were estimated by a pulse amplitude-modulated fluorometer. The photosynthesis of S. thunbergii germlings exhibited different properties to optimize light capture and utilization. The excitation pressure (1 − qP) was rapidly increased to approximately 0.27 showing that germlings responded to high light by chronic photoinhibition with an accumulation of closed reaction centers, which ultimately resulted in a slow growth. This was accompanied by a reduced Fv/Fm with time and a development of high capacity for NPQ. Although Fv/Fm in moderate-light germlings did not fully recover overnight, germlings demonstrated a less severe chronic photoinhibition considering the reduced degree of excitation pressure accumulation of approximately 0.15. The relative stability of photosynthetic capacity (rETRmax, Ek, and α) could endow germlings with the highest relative growth rate (RGR) of approximately 9.3 % day−1 in moderate light. By contrast, low-light germlings demonstrated high Fv/Fm and Fo, corresponding high α collectively suggested greater efficiency of light absorption and energy transformation. Sustained increases in electron transport capacity (rETRmax and Ek) occurred in low-light germlings, which resulted in a stable RGR of over 8.2 % day−1. Consequently, S. thunbergii germlings are considered to prefer low light regimes and have a relative capacity of moderate and high light tolerance. However, the light acclimation to oversaturating conditions is at the cost of slow growth to maintain survival.