Comparison of two potential strategies of planktonic foraminifera for house building: Mg or H removal?

Marine organisms must possess strategies enabling them to initiate calcite precipitation despite the unfavorable conditions for inorganic precipitation in surface seawater. These strategies are poorly understood. Here we compare two potential strategies of marine calcifyers to manipulate seawater chemistry in order to initiate calcite precipitation: Removal of Mg²⁺ and H⁺ ions from seawater solutions. An experimental setup was used to monitor the onset of inorganic precipitation on seed crystals as a function of the Mg²⁺ concentration and pH in artificial seawater. We focused on precipitation rates typical for biogenic calcification in planktonic foraminifera (∼10⁻³ mol m⁻² h⁻¹) and time scales typical for the initiation of calcification in these organisms (minutes to hours). We find that the carbonate ion concentration has to increase by a factor of ∼13 when Mg²⁺] increases from 0 to 53 mmol kg⁻¹ in order to maintain a typical biogenic precipitation rate. Model calculations for the energy requirement for various scenarios of Mg²⁺ and H⁺ removal including Ca²⁺ exchange and CO₂ diffusion are presented. We conclude that the more cost-effective strategy to initiate calcite precipitation in foraminifera is H⁺ removal, rather than Mg²⁺ removal.