Previous studies have shown that secular variation in the Mg/Ca ratio of seawater throughout the Phanerozoic would have subjected the aragonite-producing codiacean algae to at least three transitions between the low-Mg calcite (molar Mg/Ca <2) and aragonite + high-Mg calcite (molar Mg/Ca >2) nucleation fields in the oceans, since their origin in the Ordovician. These studies have asserted that major sediment production by codiacean algae in Recent tropical seas is permitted by the Mg/Ca ratio of modern seawater (molar Mg/Ca ∼5.2) remaining within the aragonitic/high-Mg calcite nucleation field (molar Mg/Ca >2). Here I present the results of experiments conducted to determine the effects of ambient Mg/Ca on the mineralogy, growth rate, primary productivity, calcification rate, and biomechanics of the codiacean alga Penicillus capitatus.

P. capitatus specimens were grown in three artificial seawater treatments that mimic ancient seawater of differing Mg/Ca ratios, corresponding to the low-Mg calcite nucleation field (molar Mg/ Ca ∼1.0), a “boundary field” (molar Mg/Ca ∼2.5), and the aragonite + high-Mg calcite nucleation field (molar Mg/Ca ∼5.2). Significantly, P. capitatus specimens maintained a mostly aragonitic mineralogy in all three seawater treatments. However, linear growth rates, primary productivity, calcification, and thallus stiffness decreased with reductions in ambient Mg/Ca. That P. capitatus precipitates approximately three-quarters of its CaCO3 as aragonite in the seawater treatment that favors the inorganic precipitation of low-Mg calcite suggests that the alga dictates the precipitation of that polymorph, either by pumping cations to create an internal aragonite nucleation field (molar Mg/Ca >2) or by employing organic templates that specify the nucleation of the aragonite polymorph (Borowitzka 1984). However, the alga's precipitation of one-quarter of its CaCO3 as low-Mg calcite suggests that its mineralogical control is limited and can be partially overridden by the Mg/ Ca ratio of ambient seawater. The reduced calcification of P. capitatus specimens grown in the low-Mg calcite and boundary nucleation fields is probably due to the inherent difficulty of precipitating aragonite from seawater which does not naturally support its nucleation. The decreased rates of linear growth and primary production are probably caused by reductions in CO2 available for photosynthesis due to the reduction in calcification (Borowitzka and Larkum 1977). The observed decrease in thallus stiffness is probably due to the reductions in calcification and primary productivity observed in P. capitatus specimens grown in the low-Mg calcite and boundary nucleation fields.

The present study suggests that aragonitic algae would have been handicapped in oceans characterized by Mg/Ca ratios that did not support their inherent mineralogy. Producing aragonite in seawater outside of the aragonite + high-Mg calcite nucleation field would probably have reduced the competitiveness of these algae, made them more susceptible to predation, and reduced their contribution to carbonate sedimentation. These findings support earlier assertions that the dominant ecological and sedimentological roles of codiacean algae in Recent tropical seas is permitted by a Mg/Ca ratio of seawater that supports the algae's aragonitic mineralogy during this time.

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