Ocean acidification has further chemical implications: more hydrogen ions mean more bicarbonate ions, and fewer carbonate ions. Carbonate is what corals, the shells of shellfish and the outer layers of many photosynthesising plankton and other microbes are made of. If the level of carbonate ions falls too low the shells can dissolve or might never be made at all. There is evidence that the amount of carbonate in the shells of foraminifera, micro-plankton that are crucial to ocean ecology, has recently dropped by as much as a third.