| 英文摘要: | Excessive CO2 in the present-day ocean–atmosphere system is causing ocean acidification, and is likely to cause a severe biodiversity decline in the future1, mirroring effects in many past mass extinctions2, 3, 4. Fossil records demonstrate that organisms surviving such events were often smaller than those before5, 6, a phenomenon called the Lilliput effect7. Here, we show that two gastropod species adapted to acidified seawater at shallow-water CO2 seeps were smaller than those found in normal pH conditions and had higher mass-specific energy consumption but significantly lower whole-animal metabolic energy demand. These physiological changes allowed the animals to maintain calcification and to partially repair shell dissolution. These observations of the long-term chronic effects of increased CO2 levels forewarn of changes we can expect in marine ecosystems as CO2 emissions continue to rise unchecked, and support the hypothesis that ocean acidification contributed to past extinction events. The ability to adapt through dwarfing can confer physiological advantages as the rate of CO2 emissions continues to increase. The present rate of ocean acidification is a global concern because many of the mass extinction events that affected evolution of life on Earth are associated with evidence for elevated CO2 and global warming, triggered by large-scale continental volcanism3. These include the largest known extinction event, which occurred in the late Permian2, 8, where atmospheric CO2 levels are estimated to have increased by a factor of four to six9, 10, and the Late Triassic event that saw a doubling in CO2 levels4 and was the most severe extinction to have affected extant groups such as scleractinian corals11. Evidence that ocean acidification due to volcanism played a significant role in past marine extinctions comes from analyses of physiological selectivity12, and changes in shell mineralogy and lithology13. In the immediate aftermath of the mass extinction events, many of the survivors were smaller than before5 (for example, brachiopods14, gastropods14, bivalves and shelled cephalopods6); a phenomenon termed the ‘Lilliput effect’7. After the most severe Late Permian extinction, gastropod species remained relatively small for millions of years15. One hypothesis is that this dwarfing was an adaptation to ocean acidification to mitigate against the increased energetic cost of carbonate secretion8. Calcifiers use the ion transporter Ca2+ATPase to build shells/skeletons which pumps protons out of the extracellular calcifying medium, increasing the internal pH and favouring calcification. This is an energetically expensive process16, the cost of which increases for animals exposed to high pCO2 conditions. For instance, scleractinian corals have an extra metabolic cost of about 10% per 0.1 unit decrease in seawater pH (ref. 17). It is possible that faced with an increase in calcification costs, some species may adapt by decreasing in size18. Areas with naturally high levels of CO2 provide opportunities to study the adaptation of organisms exposed to chronic hypercapnia19, 20. At such sites, increased CO2 levels cause biodiversity loss on sufficiently large spatial and temporal scales to reveal ocean acidification effects at the ecosystem level21, 22. Off Vulcano Island, Sicily23, seep gas composition is 97–98% CO2, which acidifies the surrounding waters down to pHT 5.64 (where pHT is the pH value based on the total hydrogen ion concentration scale) near the main seeps, rising to ambient levels of pHT 8.2 over a distance of around 400 m (ref. 23). Traces of other hydrothermal gases (H2, CH4 and H2S) are also present near the seeps but become undetectable around 5 m away23. Seawater temperature and oxygen levels reach ambient values a few tens of metres from the main seep area. At about 100 m from the main seeps the nassariid gastropods Nassarius corniculus and Cyclope neritea are abundant on coarse sand and gravel (Fig. 1). These species are widespread in coastal lagoons and salt marshes in the Mediterranean as well as at shallow-water hydrothermal seeps (for example, off Milos24 and Pantelleria25). We know that populations of C. neritea and N. corniculus had developed at the CO2 seeps because their shells had paucispiral protoconches indicating these snails lack a planktotrophic larval stage (see Supplementary Information for more details). Seawater off Vulcano has been acidified since the late Pleistocene epoch23 and a dwarf population of N. corniculus has been present for at least 30 years25, providing an opportunity to study chronic effects of ocean acidification on gastropods submitted to high CO2 levels over multiple generations.
| http://www.nature.com/nclimate/journal/v5/n7/full/nclimate2616.html
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