Aragonite production in calcite seas: effect of seawater Mg/Ca ratio on the calcification and growth of the calcareous algae Penicillus, Halimeda and Udotea

Stanley and Hardie (1998, 1999) have shown that secular variation in the Mg/Ca ratio of seawater throughout the Phanerozoic would have subjected the aragonite-producing Codiacean algae to three transitions between the so-called calcite (molar Mg/Ca < 2) and aragonite (molar Mg/Ca > 2) seas, since their origin in the Ordovician (Roux, 1991). They assert that major sediment production by Codiacean algae in recent tropical seas is permitted by the molar Mg/Ca ratio of modern seawater ( ∼5.2) remaining within the range of aragonite seas (molar Mg/Ca > 2). To test this hypothesis, three major sediment producing Codiacean algae, Penicillus capitatus, Halimeda monile and Udotea flabellum, were grown in three artificial ancient seawaters, corresponding to "calcite seas" (molar Mg/Ca = 1.0), "aragonite seas" (molar Mg/Ca = 5.2) and a boundary composition (molar Mg/Ca = 2.5). Significantly, the Penicillus and Udotea specimens maintained their aragonitic mineralogy in each of the artificial seawaters, suggesting either that the algae pump cations to create an internal aragonite nucleation field or employ organic templates specifying the nucleation of the aragonite polymorph (Borowitzka 1984). The Halimeda specimens also produced aragonite in the aragonite and boundary seawaters, but failed to grow at all in the calcite seawater. Linear growth rates, primary productivity and calcification decreased with reductions in ambient Mg/Ca. A stress-strain analysis of the Penicillus thalli revealed that their stiffnesses also decreased with Mg/Ca. The reduced calcification of the algae grown in the calcite and boundary seawaters is probably due to the kinetic difficulty of precipitating aragonite from seawater which does not favor its nucleation. The decreased rates of linear growth and primary production were probably caused by reductions in CO₂ available for photosynthesis (CO₂ + H₂O = CH₂O + O₂) due to the reduction in calcification (2HCO₃ + Ca = CaCO₃ + CO₂ + H₂O). The decrease in Penicillus thallus stiffness is probably due to reductions in calcification and primary production. This study suggests that producing aragonite in seawater outside of the aragonite + high-Mg calcite nucleation field would have reduced the competitiveness of these algae, made them more susceptible to predation and decreased their contribution to carbonate sedimentation. These findings support Stanley and Hardie's (1998, 1999) empirical evidence that changing Mg/Ca ratios in the oceans have had a significant impact on the major calcifying reef builders and sediment producers throughout the Phanerozoic.