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Calcification rates, as well as rates of photosynthetic inorganic carbon (Ci) uptake and relative electron transport (rETR), were measured by alkalinity titrations and pulse-amplitude-modulated (PAM) fluorometry in a simple experimental set up for  Halimeda macroloba  and  Halimeda borneensis  from Tangkhen Bay, Phuket, Thailand. While calcification was saturated at bulk seawater concentrations of ~0.3 and ~0.2 m m  CO 3  2- , respectively, for the two species, photosynthetic Ci uptake was saturated at ~1.0 and 0.6 m m  HCO 3  - , respectively. Thus, the normal seawater concentrations of these two Ci forms (0.3 m m  CO 3  2-  and 1.7 m m  HCO 3  - ) were viewed as saturating for both processes. rETR increased towards the end of the incubations as a function of irradiance despite the decrease in Ci uptake. This implies that part of the photosynthetic electron flow was diverted to processes other than CO 2  reduction under Ci-limiting conditions, and shows that fluorometry is not a good way to estimate photosynthetic efficiencies under such conditions. Based on the responses to pH and Ci reported here, we estimate that the predicted range of ocean acidification for 2100 will still allow for positive rates of calcification in these two  Halimeda  species.

Photosynthesis and calcification in two Halimeda species from Phuket, Thailand