Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers

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Abstract
Photosynthetic carbon uptake of the tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers was studied by several methods. Photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO2 was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO3− uptake. This aspect was further investigated by studying photosynthetic rates in a number of media of varying pH (7.8-8.61) and total inorganic carbon (0.75-13.17 millimolar). In these media, photosynthetic rate was correlated with free CO2 concentration and was independent of the HCO3− concentration in the medium. Short time-course experiments were conducted during equilibration of free CO2 and HCO3− after injection of 14C labeled solution at acid or alkaline pH. High initial photosynthetic rates were observed when acidic solutions (largely free CO2) were used but not with alkaline solutions. The concentration of free CO2 was found to be a limiting factor for photosynthesis in this plant.