The carbonate shells of tiny marine plankton, foraminifers, are important archives of geochemical records of past climates. Now researchers have discovered that, contrary to long-standing textbook knowledge, these shells do not form as calcite, but instead, are originally formed as the metastable carbonate vaterite and only later transform into calcite. The presence of vaterite instead of calcite in these abundant organisms also means that foraminifer shells are much more susceptible to ocean acidification than has been previously thought.
