Ecophysiological characteristics of four intertidal marine macroalgae during emersion along Shantou coast of China, with a special reference to the relationship of photosynthesis and CO₂

Intertidal marine macroalgae experience periodical exposures during low tide due to their zonational distribution. The duration of such emersion leads to different exposures of the plants to light and aerial CO₂, which then affect the physiology of them to different extents.The ecophysiological responses to light and CO₂ were investigated during emersion in two red algae Gloiopeltis furcata and Gigartina intermedia, and two brown algae Petaloniafascia and Sargassum hemiphyllum, growing along the Shantou coast of China. The light-saturated net photosynthesis in G. furcata and P. fascia showed an increase followed by slightly desiccation, whereas that in G.intermedia and S. hemiphyllum exhibited a continuous decrease with water loss. In addition, the upper-zonated G. furcata and P. fascia,exhibited higher photosynthetic tolerance to desiccation and required higher light level to saturate their photosynthesis than the lower-zonated G. intermedia and S. hemiphyllum. Desiccation had less effect on dark respiration in these four algae compared with photosynthesis. The light-saturated net photosynthesis increased with increased CO₂ concentrations, being saturated at CO₂ concentrations higher than the present atmospheric level in G. furcata, G. intermedia and S. hemiphyllum during emersion. It was evident that the relative enhancement of photosynthesis by elevated CO₂ in those three algae increased, though the absolute values of photosynthetic enhancement owing to CO₂ increase were reduced when the desiccation statuses became more severe. However, in the case of desiccated P. fascia (water loss being greater than 20%), light saturated net photosynthesis was saturated with current ambient atmospheric CO₂ level. It is proposed that increasing atmospheric CO₂ will enhance the daily photosynthetic production in intertidal macroalgae by varied extents that were related to the species and zonation.