Organic Controls over Biomineral Ca-Mg Carbonate Compositions and Morphologies

Calcium carbonate minerals, such as aragonite and calcite,arewidespread in biomineral skeletons, shells, exoskeletons, and more.With rapidly increasing pCO(2) levels linkedto anthropogenic climate change, carbonate minerals face the threatof dissolution, especially in an acidifying ocean. Given the rightconditions, Ca-Mg carbonates (especially disordered dolomiteand dolomite) are alternative minerals for organisms to utilize, withthe added benefit of being harder and more resistant to dissolution.Ca-Mg carbonate also holds greater potential for carbon sequestrationdue to both Ca and Mg cations being available to bond with the carbonategroup (CO3 (2-)). However, Mg-bearing carbonatesare relatively rare biominerals because the high kinetic energy barrierfor the dehydration of the Mg2+-water complex severelyrestricts Mg incorporation in carbonates at Earth surface conditions.This work presents the first overview of the effects of the physiochemicalproperties of amino acids and chitins on the mineralogy, composition,and morphology of Ca-Mg carbonates in solutions and on solidsurfaces. We discovered that acidic, negatively charged, hydrophilicamino acids (aspartic and glutamic) and chitins could induce the precipitationof high-magnesium calcite (HMC) and disordered dolomite in solutionand on solid surfaces with these adsorbed biosubstrates via in vitro experiments. Thus, we expect that acidic aminoacids and chitins are among the controlling factors in biomineralizationused in different combinations to control the mineral phases, compositions,and morphologies of Ca-Mg carbonate biomineral crystals. In this study, we show how acidic aminoacids and chitinpromote Ca-Mg carbonate precipitation and control crystal morphologies.