Short answers, pulled from the story.
A carbonate ion is a polyatomic anion consisting of one carbon atom bonded to three oxygen atoms in a trigonal planar arrangement, with D3h molecular symmetry. It carries a total formal charge of negative two and has a molecular mass of 60.01 g/mol. It is the conjugate base of the bicarbonate ion.
The most common carbonate minerals are calcite (calcium carbonate), which is the chief constituent of limestone and the main component of mollusc shells and coral skeletons; dolomite, a calcium-magnesium carbonate; and siderite, or iron(II) carbonate, which is an important iron ore.
Three reversible reactions involving carbonate, bicarbonate, carbonic acid, and carbon dioxide maintain blood pH within the range of 7.37 to 7.43. When pH shifts, the lungs adjust carbon dioxide exhalation and the kidneys excrete bicarbonate into urine via the urea cycle to restore balance. This is a direct application of Le Chatelier's principle.
Carbonate is central to the long-term carbon cycle because many marine organisms, especially coral, are built from calcium carbonate. Rising ocean temperatures increase carbonate solubility, reducing marine calcite production and releasing more carbon dioxide into the atmosphere, which in turn raises Earth's temperature further.
Small amounts of carbonate deposits have been found on Mars through spectral imaging, and Martian meteorites contain trace quantities. Carbonate evidence has also been detected in the planetary nebula NGC 6302, a setting where liquid water is considered unlikely. Groundwater may once have existed at Gusev and Meridiani Planum on Mars.
Carbonates are used in iron smelting, as a raw material for Portland cement and lime manufacture, and in ceramic glazes. Sodium carbonate and potassium carbonate have been used since antiquity for cleaning, preservation, and glassmaking. Newer applications include thermal energy storage and use as catalysts and electrolytes in molten carbonate fuel cells.