Strategies of carbon use and photosynthetic performance of the two seaweeds Gracilaria chouae and Gracilariopsis lemaneiformis under different conditions of the carbonate system

Abstract

The use of Gracilaria chouae and Gracilariopsis lemaneiformis in aquaculture has recently been rapidly expanding owing to their bio- and eco-functions on coastal areas. However, high density cultures of these commercial seaweeds can lead to rapid changes in carbonate system even on a very short-term basis, which, in turn, effectively affects the physiology of these commercial seaweeds. To explore the regulation of mechanism of carbon use of G. chouae and Gp. lemaneiformis in different carbonate systems, this study exposed two seaweeds to natural seawater (control), and treatments of high CO2 and normal HCO3− concentrations (HC-NB), and natural CO2 and low HCO3− concentrations (NC-LB) to determine the net photosynthetic rate, pH drift, rapid light curves and chlorophyll fluorescence measurements. The results showed that net photosynthetic rates, non-photochemical quenching (NPQ), potential mechanism of relative electron transfer (rETRm) and the ability of both seaweeds to tolerate high light (Ik) increased significantly in the HC-NB treatment. Gp. lemaneiformis had significantly higher rates of photosynthesis and more dramatic changes in pH in all the treatments. Thalli were treated with an inhibitor of extracellular carbonic ahhydrase acetazolamide (AZ) and an anion exchange protein inhibitor 4,4′-disocyanostilbene-2,2′-disulfonate (DIDS), AZ was the primary inhibitor of HCO3− used for both seaweeds during first 2 h. However, after 24 h, DIDS and AZ were the primary inhibitors of G. chouae in all the treatments, and DIDS was the main inhibitor of Gp. lemaneiformis in the control and HC-NB treatments. In conclusion, the photosynthetic ability of G. chouae is greater in carbonate system manipulated seawaters and is more effective at metabolizing HCO3− by extracellular carbonic anhydrase at high pH values. The direct HCO3− use by the anion exchange transporter could be induced at high pH values and low [HCO3−] for both seaweeds. This study will provide a theoretical basis for seaweed aquaculture.