Diurnal pH fluctuations of seawater influence the responses of an economic red macroalga Gracilaria lemaneiformis to future CO2-induced seawater acidification

The diurnal fluctuation of pH in costal seawater in sea-farming areas is larger than that the pH reduction induced by progressive seawater acidification by the end of this century. To best understand how seawater acidification influences the primary productivity of macroalgae in coastal waters under the pH fluctuation conditions, Gracilaria lemaneiformis, an important economical red macroalgae, was selected in our study. pH fluctuation (pH-F, high density cultivation) and pH stabilization (pH-S, low density cultivation) conditions were set to study the effects of seawater acidification on the production of G. lemaneiformis experiencing different pH fluctuation conditions. The results showed that seawater acidification significantly decreased the relative growth rate (RGR) of G. lemaneiformis grown at pH-S condition, but the enhancement of RGR was found in the pH-F-grown thalli. The similar trends were showed in the net photosynthetic rates of G. lemaneiformis. The different responses of RGR and net photosynthetic rates to seawater acidification between pH-S and pH-F conditions might be attributed to different daily pH variations. Under pH-S treatment, pH values varied <0.2 units with all values <8.3, indicating a relative sufficient CO2 supply. Therefore, the effect of seawater acidification was more determined by the negative effect of elevated acidity rather than positive effect of the increased CO2 supply. While under pH-F level, pH values varied >0.6 units and it could reach 9.09 in the LC-grown thalli, suggesting limited CO2 supply as CO2 level decreases about 14.1 mu mol kg(-1) (from 15.8 to 0.7 mu mol kg(-1)) with the increase of pH (from 8.11 to 9.09). Increased CO2 at the seawater acidification condition could significantly relieve this carbon resource limitation, resulting in the enhancement of RGR of HC-grown plants. Statement of relevance: It can be predicted that the carbon resource limitation for high-density cultivation of G. lemaneiformis will be significantly relieved by coastal ocean acidification, resulting in the production increases in the future ocean, which can provide some reference values for the aquaculture of macroalgae. (C) 2017 Elsevier B.V. All rights reserved.