Figure 2: Experimental responses and variation in leaf CaCO₃ content.

(a) Relative changes (percentage) in the instantaneous calcification rates of T. testudinum leaves with respect to the maximum calcification rate of the control leaf fragments, determined under maximum photosynthesis activity (P_max). Leaf segments were incubated at 28 °C for 30 min under saturating irradiance (500 μmol quanta m⁻² s⁻¹), and in darkness. Leaf segments were infiltrated for 10 min in darkness with the herbicide DCMU and treated with Acetazolamide (+AZ), an inhibitor of the extracellular activity of the enzyme carbonic anhydrase, before incubation under similar light conditions as the control. Each determination was replicated four times. (b) Mean±s.e. (n=9–12) of the percentage of leaf CaCO₃ content (% dry weight, DW) estimated for leaves of the seagrass Thalassia testudinum collected in the reef lagoon of Puerto Morelos (Mexican Caribbean) in May 1995 and in September 2013. Grey background shows values determined for the second youngest leaf collected in 1995, in the back reef, either in the seagrass bed or adjacent to massive coral colonies (shoots next to corals) and in a meadow adjacent to the UNAM pier. Comparisons between values of the CaCO₃ content variation between the second and third leaves of shoots collected in 2013 (#3 is the oldest leaf of the shoot) are also shown.