Effects of desiccation and rehydration on carbon fixation and DOC release in Sargassum thunbergii

Although macroalgae is regarded as the emerging fourth category of blue carbon , few studies are available on its carbon sequestration and DOC (dissolved organic carbon) release mechanism at present. In addition, tidal action is an important environmental factor in the intertidal zone, which affects the carbon sequestration and DOC release of macroalgae. Therefore, we set 4-h desiccation and 1-h rehydration to examine the effects of drying-rewetting on the photosynthetic carbon fixation capacity and DOC release mechanism of Sargassum thunbergii. The flowing seawater can provide the nutrients for the growth of marine plants, which plays a vital role in the growth and reproduction of marine plants, so a measurement system that can adjust the flow speed was designed to study the effects of 1 h-rehydration on S. thunbergii. The results showed the desiccation decreased carbon fixation capacity, but increased the protection of PSII by down-regulating F-v/F-m and Phi(PSII) and up regulating NPQ, thereby promoting the recovery of carbon fixation capacity during 1-h rehydration period. It was found that the DOC release rate of S. thunbergii decreased with the carbon fixation rate increase and it was much greater than the carbon fixation rate in the 1-h rehydration period. The DOC release after rehydration was somehow a passive release, which was related to the increase of osmotic pressure, cell membrane damage, and carbon accumulation caused by desiccation. This measurement system can provide a practical reference for the study of carbon sink of macroalgae.

Automated summary

This study investigated the impact of drying-rewetting on the photosynthetic carbon fixation capacity and DOC release mechanism of S. thunbergii. The results showed that desiccation decreased carbon fixation capacity but increased the protection of PSII, promoting recovery during rehydration. It was found that DOC release after rehydration was passive, linked to osmotic pressure increase, cell membrane damage, and carbon accumulation from desiccation. This measurement system provides a practical reference for studying carbon sink in macroalgae.