Ocean Acidification is sometimes referred to as Global Warming's Equally Evil Twin. The irony is intentional and fair enough as far as it goes... No single mechanism explains all the mass extinctions in the record and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean Acidification played a role in at least 2 of the Big Five Extinctions: the End-Permian and the End-Triassic. And quite possibly it was a major factor in a third, the End-Cretaceous. ...Why is ocean acidification so dangerous? The question is tough to answer only because the list of reasons is so long. Depending on how tightly organisms are able to regulate their internal chemistry, acidification may affect such basic processes as metabolism, enzyme activity, and protein function. Because it will change the makeup of microbial communities, it will alter the availability of key nutrients, like iron and nitrogen. For similar reasons, it will change the amount of light that passes through the water, and for somewhat different reasons, it will alter the way sound propagates. (In general, acidification is expected to make the seas noisier.) It seems likely to promote the growth of toxic algae. It will impact photosynthesis—many plant species are apt to benefit from elevated CO2 levels—and it will alter the compounds formed by dissolved metals, in some cases in ways that could be poisonous.Of the myriad possible impacts, probably the most significant involves the group of creatures known as calcifiers. (The term calcifier applies to any organism that builds a shell or external skeleton or, in the case of plants, a kind of internal scaffolding out of the mineral calcium carbonate.)...Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to organisms that build shells or exoskeletons. Imagine trying to build a house while someone keeps stealing your bricks. The more acidified the water, the greater the energy that’s required to complete the necessary steps. At a certain point, the water becomes positively corrosive, and solid calcium carbonate begins to dissolve. This is why the limpets that wander too close to the vents at Castello Aragonese end up with holes in their shells.According to geologists who work in the area, the vents have been spewing carbon dioxide for at least several hundred years, maybe longer. Any mussel or barnacle or keel worm that can adapt to lower pH in a time frame of centuries presumably already would have done so. “You give them generations on generations to survive in these conditions, and yet they’re not there,” Hall-Spencer observed.