Seawater carbonate chemistry, gross photosynthesis and metabolically induced rate of pH change during experiments with macroalgae, 2012

• Main Authors: Cornwall, Christopher Edward, Hepburn, Christopher D, Pritchard, Daniel, Currie, Kim I, McGraw, Christina M, Hunter, Keith A, Hurd, Catriona L
• Format: Dataset
• Language: English
• Published: PANGAEA 2012
• Subjects: Alkalinity; total; standard error; Aragonite saturation state; Benthos; Bicarbonate; Bicarbonate ion; Bottles or small containers/Aquaria (<20 L); Calcite saturation state; Calculated; see reference(s); Calculated using CO2SYS; Calculated using seacarb after Nisumaa et al. (2010); Calculated using SWCO2 (Hunter; 2007); Carbon; inorganic; dissolved; Carbonate ion; Carbonate system computation flag; Carbon dioxide; Chlorophyta; Chromista; Coast and continental shelf; Corallina officinalis; EPOCA; EUR-OCEANS; European network of excellence for Ocean Ecosystems Analysis; European Project on Ocean Acidification; Experimental treatment; Fugacity of carbon dioxide (water) at sea surface temperature (wet air); Gross photosynthesis rate; oxygen; Ocean acidification
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.775819; https://doi.org/10.1594/PANGAEA.775819

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 increase; CO32 decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 uptake, and so HCO3-using macroalgae may benefit in future seawater with elevated CO2.