Testing the effects of ocean acidification on algal metabolism: considerations for experimental designs

Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO2. Experiments to elucidate the biological effects of ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO2 bubbling and HCl ⁄NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl ⁄NaHCO3 additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification. Key index words: algae; bicarbonate; calcium carbonate; carbon; carbon dioxide; climate change; ocean acidification; phytoplankton; seawater carbonate system; seaweed Abbreviations: AT, total alkalinity; CA, carbonic anhydrase; CCM, carbon-concentrating mechanism; CT, total inorganic carbon; pCO2, partial pressure of CO2(g)