Biological mineral storage in an acidic world?

Question

In a world with acidic oceans, shells and bones made of calcium carbonate or calcium phosphate won't work--they just dissolve. As far as shells or structural bones go, that's not a big deal--there are plenty of other substances that can be used to build hard structures, and which could still be synthesized, like chitin or keratin. However, one theory for why living bones evolved in vertebrates is that early bones served as mineral reservoirs.

If precipitating calcium and magnesium salts is more difficult due to lower pH, what else could organisms use as internal mineral storage systems for calcium and phosphate? If they can also be used structurally, like Earthling bones, that's a bonus, but not required.

(Note that, per The Evolution of Bone, another theory about the reason for vertebrate bone is simply that calcium phosphate, the form of hydroxyapatite, is considerably less soluble than calcite in "acidic extracellular conditions"; however, "acidic conditions" in that context means "as low as 6.5"--not, e.g., 4 or 3. If, however, calcium phosphate does happen to be stable at such low pH values, references indicating such would constitute an acceptable answer.)

Answer 1

Got lemons? Make lemonade!

Calcium sulfates are insoluble in water but also poorly soluble in sulfuric acid.

Thermodynamic Modeling of Calcium Sulfate Hydrates in a CaSO4–H2SO4–H2O System from 273.15 to 473.15 K up to 5 m Sulfuric Acid

Calcium sulfate is one of the most common inorganic salts with a high scaling potential existing in many industrial processes.1 It mainly precipitates as a solid scale on equipment surfaces or piping networks, causing production losses, downtime, and efficiency decrease. Calcium sulfate scaling appears commonly in industrial processes when treating natural ores containing calcium minerals with sulfuric acid, especially in the hydrometallurgical processes of primary tungsten, copper, nickel, and zinc manufacturing.2−5 Despite its negative influences, the small solubility of calcium sulfate is beneficial for recycling aqueous solutions in the processing circuit since it limits the accumulation of calcium and sulfate in the process solutions.

Your acid dwellers sequester calcium as the sulfate.

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the phosphorus is trickier. I am thinking polyphosphate iron chelates? But will need to scratch my head some more...

Answer 2

In some corals, magnesium-rich calcites replace other forms of calcium carbonate to form their skeletons. Skeletons of this material have been shown to experience no structural differences when exposed to pH levels of 7.3 and 7.6.

Calcite can also absorb phosphorous resulting in minerals such as calcium phosphate and hydroxyapatite, at pH levels as low as 6. Magnesium-rich calcite constitutes of magnesium carbonate and calcium carbonate. These materials could all be resorbed by the osteoclast equivalents on your world.

The study I linked only tested magnesium calcites under ocean pH levels as low as 7.3, which is acidic by current standards.

So in conclusion, your marine organisms would need skeletons composed of a solid solution of magnesium carbonate and calcium carbonate. Having a higher ratio of MgCO3 to CaCO3 may increase the acid resistance of the skeleton, which could address your concern about 7.3 pH not being acidic enough.

Information from Octocoral Tissue Provides Protection from Declining Oceanic pH and Origin, Evolution, and Modern Aspects of Biomineralization in Plants and Animals, and Retention of phosphorus on calcite and dolomite: speciation and modeling