Ocean plankton do not tend to be limited by lack of C02, because it is already present in dissolved form in such abundance in the ocean water. In addition, they tend to be severely limited by lack of nutrients, which cuts down how much they could potentially benefit from more C02. However, there may be another insidious effect of high C02 levels that could have far-reaching consequences for ocean ecosystems and perhaps the whole planetary system.
Some of the most abundant ocean plankton, known as coccolithophores, make shells for themselves out of little calcium carbonate plates. Experiments show that if the C02 concentration in the water becomes too high, the calcium carbonate gets eaten away and dissolves as fast as it can be laid down. The result is that the cell collapses in and dies. The studies suggest that at atmospheric C02 levels around three times as high as at present (1,000 ppm), coccolithophores will be exposed to so much C02 dissolved in ocean waters that they will die. Such C02 levels may well be reached in the next century or so as humans continue to burn fossil fuels relentlessly. Corals and seashells, which also lay down calcium carbonate skeletons, seem to be faced with a similar problem. However, it would appear that in the geological past ocean plankton and coral that relied on making calcium carbonate skeletons were able to thrive under C02 concentrations higher than this. Either the past C02 reconstructions are wrong, or there are ways in which these organisms can eventually evolve to tolerate high C02. In the meantime, however, many marine ecosystems could collapse and many extinctions follow from this "other" direct C02 effect.
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