Impact of biomineralization processes on the Mg content of foraminiferal shells: A biological perspective

Abstract

[1] The Mg/Ca ratio in foraminiferal shells is widely used as a proxy for paleotemperatures. Nevertheless, it seems that the basic Mg content of foraminifera is determined by biological factors, as can be concluded from the large inter species and intrashell variability and the frequent deviations from inorganic behavior. This paper discusses three possible ways by which foraminifera can control or modify the Mg content in their shell: (1) involvement of organic matrix in the precipitation process that may alter the partition coefficient of Mg in biogenic calcite, (2) controlled conversion of transient amorphous phases to calcite, and (3) modification of the Mg concentration in the parent solution from which the crystals precipitate. The first two mechanisms are probably responsible for the precipitation of high-Mg calcite phases (whole shell or sublayers), while the third mechanism leads to the formation of low-Mg calcite phases. We propose a model adapted from epithelial cells that allows massive Mg 2+ removal from the biomineralization site. This model is especially relevant to the planktonic and deep benthic low-Mg foraminifera that are frequently used for paleotemperature reconstructions. We discuss the possible biological roles of Mg in the shell in terms of the calcite polymorph conservation, the in vivo chemical stability of the shell, the functions of Mg as a stabilizer of transient phases and as a controlling agent of the precipitation process. Several temperature sensitive biological processes that may influence the Mg/Ca ratio of the shell are suggested and a model that combines biogenic and inorganic considerations is presented. The model uses Mg heterogeneity in the shell together with temperature response (biologic and inorganic) of biomineralization processes, to account for the deviation of planktonic foraminifera from inorganic calcite at equilibrium with seawater.

Effect of salinity and seawater calcite saturation state on Mg and Sr incorporation in cultured planktonic foraminifera

“…Although increasing salinity of the seawater does not change the Mg/Ca ratio of the parent solution, absolute concentrations of these two elements do change. According to Bentov and Erez (2006), there is an inward driving force for Mg 2+ from the extracellular environment into the cell. This has only been established in eukaryotic cells but it is probable that this is not significantly different in foraminifera.…”

“…This has only been established in eukaryotic cells but it is probable that this is not significantly different in foraminifera. In order to keep a low intracellular Mg 2+ activity, as other eukaryotes do, foraminifera may use different cellular mechanisms such as Mg protein-based transport systems, cellular buffering and/or sequestration within cellular organelles (Bentov and Erez, 2006). At higher salinities the parent solution from which calcite precipitates has a higher Mg 2+ concentration.…”

Biomineralization in perforate foraminifera

“…1). Most foraminiferal species incorporate Mg with one to two orders of magnitude lower concentration compared to non-biologically precipitated calcium carbonate (Katz, 1973;Bender et al, 1975;Bentov and Erez, 2006). The concentration of barium, on the other hand, is~10 times higher in foraminiferal calcite (Lea and Boyle, 1991;Lea and Spero, 1992) compared to inorganic precipitation results (Pingitore and Eastman, 1984).…”

“…The concentration of barium, on the other hand, is~10 times higher in foraminiferal calcite (Lea and Boyle, 1991;Lea and Spero, 1992) compared to inorganic precipitation results (Pingitore and Eastman, 1984). Additionally, elemental concentrations between foraminiferal species can vary by several orders of magnitude (up to two orders of magnitude for Mg; Bentov and Erez, 2006). The biological controls on element incorporation and isotope fractionation that cause these offsets are often summarized as 'the vital effect' Weiner and Dove, 2003).…”

“…Calcification from seawater can be promoted using different mechanisms. Hence, multiple mechanisms have been proposed to explain test calcification, including endocytosis of seawater, transmembrane ion transporters, ion-specific organic templates, production of a privileged space and mitochondrial activity (Spero, 1988;Erez, 2003;Bentov and Erez, 2006;Bentov et al, 2009).…”