Comparing the impact of high CO2 on calcium carbonate structures in different marine organisms

Coastal seas are critical components of the global carbon cycle, yet little research has been conducted on the impact of ocean acidification on coastal benthic organisms. Calcifying marine organisms are predicted to be most vulnerable to a decline in oceanic pH (ocean acidification) based on the assumption that calcification will decrease as a result of changes in seawater carbonate chemistry, particularly reduced carbonate ion concentration (and associated saturation states). Net calcium carbonate production is dependent on an organism's ability to increase calcification sufficiently to counteract an increase in dissolution. Here, a critical appraisal of calcification in five benthic species showed, contrary to popular predictions, the deposition of calcium carbonate can be maintained or even increased in acidified seawater. This study measured changes in the concentration of calcium ions seen in shells taken from living animals exposed to acidified seawater. These data were compared with data from isolated shells that were not associated with living material to determine a species' ability to maintain the physiological process of calcification under high carbon dioxide (CO2) conditions and characterize the importance of dissolution and abiotic influences associated with decreasing pH. Comparison with palaeoecological studies of past high CO2 events presents a similar picture. This conclusion implies that calcification may not be the physiological process that suffers most from ocean acidification; particularly as all species investigated displayed physiological trade-offs including increased metabolism, reduced health, and changes in behavioural responses in association with this calcification upregulation, which poses as great a threat to survival as an inability to calcify.