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Fig. 1 | Progress in Earth and Planetary Science

Fig. 1

From: Perspective on the response of marine calcifiers to global warming and ocean acidification—Behavior of corals and foraminifera in a high CO2 world “hot house”

Fig. 1

a Schematic diagram of modern carbon cycle for both global warming and ocean acidification, which are often referred to as the “evil twins” of climate change. The increase in atmospheric CO2 concentration caused by human activities is predicted to promote global warming by ~ 2.5 °C and an ocean pH reduction of ~ 0.3 by 2100 AD. Coral reefs are among the ecosystems most vulnerable to environmental stresses (bleaching and/or ocean acidification). An aragonite saturation state of 3.3 is suggested to be the environmental limit for the existence of tropical coral reefs where calcification exceeds the dissolution of calcium carbonate (Kleypas et al. 2006). The culture experiments reported general negative effects on survival, calcification, growth, and reproduction despite significant variation among taxonomic groups in marine environments. For example, calcification tended to decrease versus pCO2 in both aposymbiotic and symbiotic polyps of Acropora digitifera, although its coral calcification of adult branches was not reduced so much under higher pCO2 conditions. In contrast to the majority of benthic foraminifers dwelling in coral reefs, Calcarina gaudichaudii showed a positive response to increases in pCO2. Since CO2 is more soluble in lower temperature seawater, ocean acidification is more critical in the polar and high-latitude regions. Surface water in the Southern Ocean will be undersaturated with respect to aragonite by 2050 AD (Orr et al. 2005). More severe dissolution of carbonate will occur in deep-sea due to the increase in solubility at higher pressure, which will make the CCD shallow. Most likely, deep-sea benthic foraminifera will be forced to change from calcareous to the agglutinated forms reported at the Paleocene/Eocene boundary. b Schematic diagram of the carbon cycle in coral reefs. The equations involve the following important processes: (1) photosynthesis (CO2 invasion, resulting in slowing acidification), (2) respiration/degradation (CO2 evasion, promoting acidification), (3) calcification (CO2 evasion), and (4) dissolution of carbonate (CO2 invasion, one of the neutralizing processes). Processes (1) and (2) are associated with organic carbon metabolism, whereas (3) and (4) are associated with inorganic carbon processes. Photosynthesis triggers the production of OH, commonly resulting in the deposition of carbonate. In contrast, the reaction of carbonate with CO2 works as a sink of CO2. This reaction is often promoted in deep-sea, which results in the depletion of carbonate on abyssal plains in the Pacific

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