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ABSTRACT
The marine red macroalga Gracilaria lemaneiformis usually experiences a changing carbonate system during mariculture. Thalli of G. lemaneiformis collected from Nanao Island, Shantou, China, were cultured in six treatments, with two inorganic carbon (Ci) levels (ambient-Ci, AC; elevated-Ci, EC) and three pH levels (8.2, 9.0, and 9.4), to examine the influence of increased Ci concentrations and high seawater pH on growth and photosynthesis in this farmed algal species. The capacity to extract HCO3− from seawater was relatively stable, as pH compensation points of G. lemaneiformis remained unchanged (9.54–9.59) in the six treatments. Relative growth rate (RGR) and net photosynthetic rate (Pn) of G. lemaneiformis showed no significant differences between treatments of AC and EC at pH 8.2. However, RGR and Pn were higher at EC than at AC at pH 9.4. In addition, RGR and Pn at AC treatments were 67% ± 12% and 76% ± 6% lower at pH 9.4 than at pH 8.2, respectively. At AC, total pigment content, effective quantum yield of PSII (Yield), and maximum relative electron transport rate (rETRm) were all lower at pH 9.4 than at 8.2. However, reductions of these parameters were significantly ameliorated when thalli of G. lemaneiformis were cultured at EC conditions. We conclude that increasing atmospheric CO2 is conducive to the aquaculture of G. lemaneiformis through the decline of pH and the concurrent increase of Ci in seawater.