Secular variation in seawater Mg/Ca would have subjected the aragonite-producing codiacean algae to two intervals of low-Mg calcite seas (mMg/Ca < 2) and two intervals of aragonite + high-Mg calcite seas (mMg/Ca > 2) seas since their origin in the Ordovician. It is asserted that the aragonite + high-Mg calcite affinity of modern and Recent tropical seas (mMg/Ca > 2) has enabled the aragonitic Codiaceae to act as major carbonate sediment producers in these seas. To test this assertion, specimens of the sediment-producing codiacean alga Udotea flabellum were grown in artificial seawaters formulated at mMg/Ca ratios (1.0, 2.5, 5.2) that have occurred in the oceans throughout the geologic history of the Codiaceae.

While the U. flabellum algae produced exclusively aragonite in both the artificial boundary seawater (mMg/Ca = 2.5) and the artificial aragonite + high-Mg calcite seawater (modern seawater; mMg/Ca = 5.2), the algae produced a portion (25 ± 3%) of their CaCO3 as low-Mg calcite (≤ 4 mol% MgCO3) in the artificial calcite seawater (mMg/Ca = 1.0), suggesting that their CaCO3 polymorph control can be partially overridden by ambient seawater chemistry.

Primary productivity, calcification, and linear growth of the algae decreased with reductions in ambient Mg/Ca, despite increases in absolute Ca concentration of the seawater. Reduced calcification in the lower Mg/Ca seawaters is probably due to kinetic difficulties of producing aragonite from seawater that favors the nucleation of calcite. The decreased linear growth and primary production are likely associated with reductions in the ambient CO2 available for photosynthesis, resulting from the reduced calcification.

These findings support the assertion that the dominant ecological and sedimentological roles of codiacean algae in Recent tropical seas is facilitated by a seawater Mg/Ca ratio that favors the nucleation of aragonite over calcite. Furthermore, producing aragonite in seawater that favors the nucleation of low-Mg calcite (mMg/Ca < 2) may have reduced the rates of growth and calcification of codiacean algae, making them less competitive for space on the shallow tropical seafloor and more susceptible to predatory grazing, thereby reducing their contribution to carbonate sedimentation.

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